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  • 1.
    Abtahi, Sayyed Mohammad
    et al.
    Isfahan Univ Technol, Dept Min Engn, Esfahan, Iran.
    Pedersen, Laust Börsting
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Kamm, Jochen
    Univ Munster, Dept Geophys, Munster, Germany.
    Kalscheuer, Thomas
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    A new reference model for 3D inversion of airborne magnetic data in hilly terrain: A case study from northern Sweden2018In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 83, no 1, p. B1-B12Article in journal (Refereed)
    Abstract [en]

    The inherent nonuniqueness in modeling magnetic data can be partly reduced by adding prior information, either as mathematical constructs or simply as bounds on magnetization obtained from laboratory measurements. If a good prior model can be used as a reference model, then the quality of estimated models through an inverse approach can be greatly improved. But even though data on magnetic properties of rocks might exist, their distribution may often be quite irregular on local and regional scales, so that it is difficult to define representative classes of rock types suitable for constraining geophysical models of magnetization. We have developed a new way of constructing a reference model that varies only laterally and is confined to the part of the terrain that lies above the lowest topography in the area. To obtain this model, several estimated 2D magnetization distributions were constructed by data inversion as a function of the iteration number. Then, a suitable 2D model of the magnetization in the topography was chosen as a starting point for constructing a 3D reference model by modifying it with a vertical decay such that its average source depth was the same for all horizontal positions. The average source depth of the reference model was chosen to satisfy the average source depth obtained from analyzing the radial power spectrum of the area studied. Finally, the measured magnetic data were inverted in three dimensions using the given reference model. For a selected reference model, shallow structures indicated a better overall correlation with large remanent magnetizations measured on rock samples from the area. Throughout the entire model, the direction of magnetization was allowed to vary freely. We found that the Euclidean norm of the estimated model was reduced compared with the case where the magnetization direction was fixed.

  • 2.
    Abtahi, Sayyed Mohammad
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics. Isfahan Univ Technol, Dept Min Engn, Esfahan, Iran.
    Pedersen, Laust
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Kamm, Jochen
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Kalscheuer, Thomas
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Consistency investigation, vertical gravity estimation and inversion of airborne gravity gradient data – A case study from northern Sweden2016In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 81, no 3, p. B65-B76Article in journal (Refereed)
    Abstract [en]

    For airborne gravity gradient data, it is a challenge to distinguish between high-frequency intrinsic and dynamically produced noise caused by the aircraft and small-scale effects from shallow density variations. To facilitate consistent interpretation, techniques that include all of the measured gravity gradient components are particularly promising. We represented the measurements by a common potential function accounting for lateral and height variations. Thus, it was possible to evaluate the internal consistency between the measured components and to identify components with bias or particularly strong noise. As an extra benefit for data sets that contain terrain-corrected and nonterrain-corrected gravity gradient measurements at flight altitude, we estimated terrain-corrected anomalies on the topographic relief using downward continuation and retrieved nonterrain-corrected gravity gradient data suitable for inversion using upward continuation. For a field data set from northern Sweden, the largest differences (up to 50 eotvos) between the measured and estimated components of the gravity gradient data were found in areas of high topographical relief. But the average residual standard deviations of the individual components were between 3.6 and 7.4 eotvos, indicating that the components were consistent in an average sense. We have determined the successful conversion of terrain-corrected airborne gravity gradient data to Bouguer gravity data on the topographic relief using ground-based vertical gravity data as a reference. A 3D inverse model computed from the nonterrain-corrected data clearly showed the depth extent of the geologic structures observed at the surface, but it only produced a weak representation of the shallow structure. In contrast, a 2D surface density model in which only lateral variations of density in the topographic relief was allowed exhibited more realistic density distributions in fair correlation with geology.

  • 3.
    Abtahi, Sayyed Mohammad
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics. Isfahan Univ Technol, Dept Min Engn, Esfahan, Iran.
    Pedersen, Laust
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Kamm, Jochen
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics. Univ Munster, Dept Geophys, Munster, Germany.
    Kalscheuer, Thomas
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Extracting geoelectrical maps from vintage very-low-frequency airborne data, tipper inversion, and interpretation: A case study from northern Sweden2016In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 81, no 5, p. B135-B147Article in journal (Refereed)
    Abstract [en]

    In 1985, the mining company Luossavaara-Kiirunavaara Aktiebolag collected airborne very-low-frequency (VLF) data in northern Sweden. The operators stored only the vertical component and the total magnetic field, which at that time were believed to be sufficient for qualitative interpretation. Therefore, the data could not be directly used for quantitative tensor VLF processing and inversion. To avoid the costs of resurveying, we have developed a novel technique to estimate the tippers from the measured VLF data by computing anomalous and normal parts of the horizontal components of the magnetic field from two transmitters separately. Retrieval of the normal horizontal components was possible because one component of the horizontal magnetic field was used as the phase reference during the measurements. Additionally, we have determined how the approximate apparent resistivity suitable for data visualization can be computed from the components of the magnetic field assuming an average normal resistivity of the subsurface. Maps of apparent resistivity combined with topography show a clear correlation between high topography and high resistivity, whereas conductive zones are found in valleys in between. More importantly, the 3D model inverted from the calculated tippers shows excellent agreement with a map of the surface geology. Based on this comparison, some less resistive zones can be related to fluids in fractures and others can be related to mineralized contact zones. We suggest to focus further exploration on conductive zones surrounding areas with basaltic composition.

  • 4.
    Ahmadi, Omid
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Juhlin, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Malehmir, Alireza
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Munck, Mie
    Boliden Mines.
    High-resolution 2D seismic imaging and forward modeling of a polymetallic sulfide deposit at Garpenberg, central Sweden2013In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 78, no 6, p. B339-B350Article in journal (Refereed)
    Abstract [en]

    We acquired a high-resolution 2D seismic profile to test the capability of the seismic method in imaging a sulfide ore body at Garpenberg, central Sweden. Delineation of the geologic structures, which surround and host the ore body, is another goal of the survey. Due to the 3D geology of the structures, a cross-dip correction performed to image out-of-the-plane reflections, resulting in a clear high amplitude anomaly at a time and location to that to be expected from near the top of the ore body. Furthermore, DMO processing and migration are applied to the data, providing images of four main reflection groups. The reflections have been interpreted as corresponding to geologic rock units in the area that partly interfere with the potential ore body signal. To further investigate the seismic response of the ore body, forward modeling by ray-tracing is applied using the ore body geometry as mapped by drilling. We use two ray-tracing approaches: standard 3D ray-tracing and an exploding reflector approach. Seven representative samples from the mine area are used to determine P-wave velocities. The measurements show a considerable contrast between the ore body and host rock. By comparing the modeled and observed data, we find that the high amplitude signal in the real seismic section most likely emanates from near the top of one concentrated ore which lies inside the larger mapped ore body that has been modeled as a resource. The base of the ore body is only observed on the synthetic data whereas a signal penetration analysis suggests that the seismic signal penetrated efficiently along the entire survey line. Presence of disseminated ore and lower fold toward the northern end of the profile could be combined reasons that make imaging the base of the ore body difficult.

  • 5.
    Bastani, Mehrdad
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences. Geological Survey of Sweden.
    Hübert, Juliane
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Kalscheuer, Thomas
    Institute of Geophysics, Department of Earth Sciences, ETH Zurich.
    Pedersen, Laust B
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Godio, Alberto
    Politecnico di Torino.
    Bernard, Jean
    IRIS Instruments.
    2D joint inversion of RMT and ERT data versus individual 3D inversion of full tensor RMT data: An example from the Trecate site in Italy2012In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 77, no 4, p. WB233-WB243Article in journal (Refereed)
    Abstract [en]

    Tensor radiomagnetotelluric (RMT) and electrical resistivity tomography (ERT) data were acquired along 10 parallel lines to image electrical resistivity of the vadose and the saturated zone in an area near Trecate, 45 km west of Milan in Italy. In 1994, the area was exposed to an oil contamination caused by a tank explosion and has since been subject to monitoring and remediation programs. For the first time, we have examined a 3D inversion of full tensor RMT data and have compared the results with 2D joint inversion of RMT and ERT data. First, a synthetic 3D resistivity model with similar variations close to those measured at the Trecate site was generated for the comparison. The synthetic tests showed that resistivity models from 2D joint inversion of ERT and RMT data contain more details closer to the surface compared to the models from the 3D inversion of tensor RMT data. High resistivity structures are better resolved by the 2D joint inversion, whereas the more conductive features are better recovered by the 3D inversion. In the next step, the ERT and RMT data collected in the Trecate site were modeled with the same approaches used in the synthetic modeling. Using the measured tensor RMT data, it was possible to carry out full 3D inversion to study the underlying geology. Comparison between the resistivity models from both inversions with the lithological data from the existing boreholes, resistivity models from the inversion of crosshole resistivity data, and water content models from magnetic resonance soundings measurements showed that the electrical resistivity, depth to the top and thickness of the water saturated zone is modeled more accurately With the 3D inversion.

  • 6.
    Bastani, Mehrdad
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Malehmir, Alireza
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Ismail, Nazli
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Pedersen, Laust
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Hedjazi, Farhang
    KAHANROBA Co.
    Delineating hydrothermal stockwork copper deposits using controlled-source and radio-magnetotelluric methods:  A case study from northeast Iran2009In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 74, no 5, p. B167--B181Article in journal (Refereed)
    Abstract [en]

    Radio- and controlled-source-tensor magnetotelluric (RMT and CSTMT)   methods are used to target hydrothermal veins of copper mineralization.   The data were acquired along six eastwest- and three   north-south-trending profiles, covering an area of about 500 x 400   m(2). The tensor RMT data were collected in the 10-250-kHz frequency   band. A double horizontal magnetic dipole transmitter in the 4-12.5-kHz   frequency range allowed us to constrain the deeper parts of the   resistivity models better. To obtain optimum field parameters, ground   magnetic profiling was conducted prior to the RMT and CSTMT surveys.   Although the study area (in Iran) is remote, a number of radio   transmitters with acceptable signal-to-noise ratio were utilized. The   2D inversion of RMT data led to unstable resistivity models with large   data misfits. Thus, the RMT data were used to complement and analyze   the near-surface resistivity anomalies observed in the 2D CSTMT models.   Analyses of strike and dimensionality from the CSTMT data suggests that   the low-resistivity structures are mainly three dimensional; therefore,   2D inversion of determinant data is chosen. Independent 2D inversion   models of the determinant CSTMT data along crossing profiles are in   good agreement. Known copper mineralization is imaged well in the CSTMT   models. The thinning of the conductive overburden correlates very well   with magnetic highs, indicating the bedrock is resistive and magnetic.   In this sense, the magnetic and electromagnetic fields complement each   other. Analysis of the 2D resistivity models indicates the volcanic   rock deepens at the center of the study area. This zone is associated   with a magnetic low and therefore is recommended for detailed   exploration work.

  • 7.
    Bastani, Mehrdad
    et al.
    SGU.
    Persson, Lena
    SGU.
    Mehta, Suman
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Malehmir, Alireza
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Boat-towed radio-magnetotellurics: A new technique and case study from the city of Stockholm2015In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 80, no 6, p. B193-B202Article in journal (Refereed)
    Abstract [en]

    We have developed a new data acquisition system and technique to measure the radio magnetotelluric (RMT) signals from distant radio transmitters with the objective of mapping and modeling electric resistivity structures below a river or lake. The acquisition system is towed by a boat; therefore, we call the technique boat-towed RMT. The data acquisition is fast with a production rate of approximately 1  km/hr using a nominal sampling spacing of 10–15 m. Given the ample number of radio transmitters available in most parts of the world, the method can be used for near-surface studies of various targets. We have developed boat-towed RMT measurements on Lake Mälaren near the city of Stockholm in Sweden to determine the feasibility of the method. Approximately 15 km of RMT data were collected during three days above a planned 60-m-deep bypass tunnel with the goal of providing information on the bedrock depth and possible weak zones within the bedrock. The measured resistivity and phase data were of high quality with errors on the order of a few percent. The resistivity models from 2D inversion of the data showed a good correlation with available geologic data in resolving bedrock depth and also resistivity layering within the lake. Resistivity maps derived from the dense 2D models suggested a northeast–southwest-striking low-resistivity zone at less than a 30-m depth. The zone likely represents fractured crystalline bedrock. The boat-towed RMT technique is well suited for water bodies with moderate electric resistivity such as in brackish and freshwater environments.

  • 8.
    Beiki, Majid
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Analytic signals of gravity gradient tensor and their application to estimate source location2010In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 75, no 6, p. I59-I74Article in journal (Refereed)
    Abstract [en]

    The analytic signal concept can be applied to gravity gradient tensor data in three dimensions. Within the gravity gradient tensor, the horizontal and vertical derivatives of gravity vector components are Hilbert transform pairs. Three analytic signal functions then are introduced along x-, y-, and z-directions. The amplitude of the first vertical derivative of the analytic signals in x- and y-directions enhances the edges of causative bodies. The directional analytic signals are homogenous and satisfy Euler's homogeneity equation. The application of directional analytic signals to Euler deconvolution on generic models demonstrates their ability to locate causative bodies. One of the advantages of this method is that it allows the automatic identification of the structural index from solving three Euler equations derived from the gravity gradient tensor for a collection of data points in a window. The other advantage is a reduction of interference effects from neighboring sources by differentiation of the directional analytic signals in x-, y-, and z-directions. Application of the method is demonstrated on gravity gradient tensor data in the Vredefort impact structure, South Africa.

  • 9.
    Beiki, Majid
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Bastani, Mehrdad
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Pedersen, Laust B.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Leveling HEM and aeromagnetic data using differential polynomial fitting2010In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 75, no 1, p. L13-L23Article in journal (Refereed)
    Abstract [en]

    We introduce a new technique to level aerogeophysical data. Our approach is applicable to flight-line data without any need for tie-line measurements. The technique is based on polynomial fitting of data points in 1D and 2D sliding windows. A polynomial is fitted to data points in a 2D circular window that contains at least three flight lines. Then the same procedure is done inside a 1D window placed at the center of the 2D window. The leveling error is the difference between 1D and 2D polynomial fitted data at the center of the windows. To demonstrate the reliability of the method, it was tested on a synthetic aeromagnetic data set contaminated by some linear artifacts. Using the differential polynomial fitting method, we can remove the linear artifacts from the data. The method then was applied to two real airborne data sets collected in Iran. The leveling errors are removed effectively from the aeromagnetic data using the differential polynomial fitting. In the case of helicopter-towed electromagnetic (HEM) data, the polynomial fitting method is used to level the measured real (in-phase) and imaginary (quadrature) components, as well as the calculated apparent resistivity. The HEM data are sensitive to height variations, so we introduce an average-height scaling method to reduce the height effect before leveling in-phase and quadrature components. The method also is effective in recovering some of the attenuated anomalies. After scaling, the differential polynomial fitting method was applied to the data and effectively removed the remaining line-to-line artifacts.

  • 10.
    Beiki, Majid
    et al.
    CSIRO Earth Science and Resource Engineering, North Ryde, NSW, Australia.
    Pedersen, Laust B
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Deconvolution of gravity gradient tensor data using infinite dike and geological contact models2011In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 76, no 6, p. 159-172Article in journal (Refereed)
    Abstract [en]

    We have developed a constrained inversion technique for interpretation of gravity gradient tensor data. For dike and contact models striking in the y  -direction, the measured g xz   and g zz   components can be jointly inverted for estimating the model parameters horizontal position, depth to the top, thickness, dip angle, and density contrast. For a given measurement point, the strike direction of the gravity gradient tensor caused by a quasi 2D structure can be estimated from the eigenvector corresponding to the smallest eigenvalue. Then, the measured components can be transformed into the strike coordinate system. It is assumed that the maximum of g zz   is approximately located above the causative body. In the case of gridded data, all measurement points enclosed by a square window centered at the maximum of g zz   are used to estimate the source parameters. The number of data points used for estimating source parameters is increased by increasing the size of the window. Solutions with the smallest data-fit error were selected as the most reliable solutions from any set of solutions. The gravity gradient tensor data are deconvolved using both dike and contact models within a set of square windows. Then, the model with the smallest data-fit error is chosen as the best model. We studied the effect of random noise and interfering sources using synthetic examples. The method is applied to a gravity gradient tensor data set from the Vredefort impact structure in South Africa. In this particular case, the dike model provides solutions with smaller data-fit errors than the contact model. This supports the idea that in the central dome area there is a predominance of vertical structures related to the formation of the transient crater and subsequent central uplift of the lower and middle crustal material.

  • 11.
    Beiki, Majid
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Pedersen, Laust B
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Eigenvector analysis of gravity gradient tensor to locate geologic bodies2010In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 75, no 6, p. L37-49Article in journal (Refereed)
    Abstract [en]

    We have developed a new method to locate geologic bodies using the gravity gradient tensor. The eigenvectors of the symmetric gravity gradient tensor can be used to estimate the position of the source body as well as its strike direction. Fora given measurement point, the eigenvector corresponding to the maximumeigenvalue points approximately toward the center of mass of the causative body. For a collection of measurement points, a robust least-squares procedure is used to estimate the source point as the point that has the smallest sum of square distances to the lines defined by the eigenvectors and the measurement positions. It's assumed that the maximum of the first vertical derivative of the vertical component of gravity vector gzz is approximately located above the center of mass. Observation points enclosed in a square window centered at the maximum of gzzare used to estimate the source location. By increasing the size of the window, the number of eigenvectors used in the robust least squares and subsequently the number of solutions increase. As a criterion for selecting the best solution from a set of previously computed solutions, we chose that solution having the minimum relative error (less than a given threshold) of its depth estimate. The strike direction of the source can be estimated from the direction of the eigenvectors correspondingto the smallest eigenvalue for quasi 2D structures. To study the effect of additive random noise and interfering sources, the method was tested on synthetic data sets, and it appears that our method is robust to random noise in the different measurement channels. The method was also tested on gravity gradient tensor data from the Vredefort impact structure, South Africa. The results show a very good agreement with the available geologic information.

  • 12.
    Beiki, Majid
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Pedersen, Laust B.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Eigenvector analysis of gravity gradient tensor to locate geologic bodies2010In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 75, no 6, p. I37-I49Article in journal (Refereed)
    Abstract [en]

    We have developed a new method to locate geologic bodies using the gravity gradient tensor. The eigenvectors of the symmetric gravity gradient tensor can be used to estimate the position of the source body as well as its strike direction. For a given measurement point, the eigenvector corresponding to the maximum eigenvalue points approximately toward the center of mass of the causative body. For a collection of measurement points, a robust least-squares procedure is used to estimate the source point as the point that has the smallest sum of square distances to the lines defined by the eigenvectors and the measurement positions. It's assumed that the maximum of the first vertical derivative of the vertical component of gravity vector g(zz) is approximately located above the center of mass. Observation points enclosed in a square window centered at the maximum of g(zz) are used to estimate the source location. By increasing the size of the window, the number of eigenvectors used in the robust least squares and subsequently the number of solutions increase. As a criterion for selecting the best solution from a set of previously computed solutions, we chose that solution having the minimum relative error (less than a given threshold) of its depth estimate. The strike direction of the source can be estimated from the direction of the eigenvectors corresponding to the smallest eigenvalue for quasi 2D structures. To study the effect of additive random noise and interfering sources, the method was tested on synthetic data sets, and it appears that our method is robust to random noise in the different measurement channels. The method was also tested on gravity gradient tensor data from the Vredefort impact structure, South Africa. The results show a very good agreement with the available geologic information.

  • 13.
    Beiki, Majid
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Pedersen, Laust B
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Hediyeh, Nazi
    Interpretation of aeromagnetic data using eigenvector analysis of pseudogravity gradient tensor2011In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 76, no 3, p. L1-L10Article in journal (Refereed)
    Abstract [en]

    This study has shown that the same properties of the gravity gradient tensor are valid for the pseudogravity gradient tensor derived from magnetic field data, assuming that the magnetization direction is known. Eigenvectors of the pseudogravity gradient tensor are used to estimate depth to the center of mass of geologic bodies. The strike directions of 2D geological structures are estimated from the eigenvectors corresponding to the smallest eigenvalues. For a set of data points enclosed by a square window, a robust least-squares procedure is used to estimate the source point which has the smallest sum of squared distances to the lines passing through the measurement points and parallel to the eigenvectorscorresponding to the maximum eigenvalues. The dimensionality of the pseudogravity field is defined from the dimensionality indicator I, derived from the tensor components. In the case of quasi-2D sources, a rectangular window is used in the robust least-squares procedure to reduce the uncertainty of estimations.Based on synthetic data sets, the method was tested on synthetic models and found to be robust to random noise in magnetic field data. The application of the method was also tested on a pseudogravity gradient tensor derived from total magnetic field data over the Särna area in west-central Sweden. Combined with Euler deconvolution, the method provides useful complementary information for interpretation of aeromagnetic data.

  • 14.
    Beiki, Majid
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Pedersen, Laust
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Nazi, Hediyeh
    Interpretation of aeromagnetic data using eigenvector analysis of pseudogravity gradient tensor2011In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 76, no 3, p. L1-L10Article in journal (Refereed)
    Abstract [en]

    This study has shown that the same properties of the gravity gradient tensor are valid for the pseudogravity gradient tensor derived from magnetic field data, assuming that the magnetization direction is known. Eigenvectors of the pseudogravity gradient tensor are used to estimate depth to the center of mass of geologic bodies. The strike directions of 2D geological structures are estimated from the eigenvectors corresponding to the smallest eigenvalues. For a set of data points enclosed by a square window, a robust least-squares procedure is used to estimate the source point which has the smallest sum of squared distances tothe lines passing through the measurement points and parallel to the eigenvectors corresponding to the maximum eigenvalues. The dimensionality of the pseudogravity field is defined from the dimensionality indicator I, derived from the tensor components. In the case of quasi-2D sources, a rectangular window is used in the robust leastsquares procedure to reduce the uncertainty of estimations. Based on synthetic data sets, the method was tested on synthetic models and found to be robust to random noise in magnetic field data. The application of the method was also tested on a pseudogravity gradient tensor derived from total magnetic field data over the Särna area in west-central Sweden. Combined with Euler deconvolution, the method provides useful complementary information for interpretation of aeromagnetic data.

  • 15. Bellefleur, Gilles
    et al.
    Malehmir, Alireza
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Mueller, Christof
    Elastic finite-difference modeling of volcanic-hosted massive sulfide deposits: A case study from Half Mile Lake, New Brunswick, Canada2012In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 77, no 5, p. WC25-WC36Article in journal (Refereed)
    Abstract [en]

    We present elastic finite-difference modeling results over a geologically realistic 2D representation of the Half Mile Lake volcanic-hosted massive sulfide deposit, New Brunswick, Canada. The model is constrained by geologic information from surface mapping and boreholes, whereas petrophysical properties are provided by wireline logging data acquired in two boreholes intersecting different parts of the deposit. We analyzed the P-P, P-S, S-P, and S-S responses of the lower and deep mineralized zones and assessed some compositional effects by substituting massive sulfides with gabbro properties in the model. Finite-difference modeling results predict complex scattering signature associated with the lower and deep sulfide zones. Both zones scattered back P-P, P-S, S-P, and S-S waves generally having strongest amplitudes in the stratigraphy down-dip direction. The P-S, S-P, and S-S scattered waves, if properly recorded on multicomponent data, represent useful signal that could help the targeting of deep sulfide mineralization. Finite-difference simulations further reveal phase-reversals on P-P wavefields scattered at the lower and deep zones. The phase reversals are not observed for gabbro inclusions, suggesting that this signature could be used to discriminate gabbro units from sulfide mineralization. The finite-difference simulation successfully reproduces many events of the VSP data, in particular P-S and S-S events on the radial component and P-P and S-P events on the vertical component. Comparison with 3D data is rather poor and only shows weak correlation with P-P events from the lower and deep zones. Despite the poor correlation, a prestack time migrated S-P section displays an amplitude anomaly at the location of the deep zone, suggesting that S-P waves were recorded on the 3D data, although this survey was acquired with explosive sources and vertical geophones.

  • 16.
    Bergman, B.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggvason, A.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Juhlin, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    High resolution seismic traveltime tomography incorporating static corrections applied to a till covered bedrock environment2004In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 69, p. 1082-1090Article in journal (Refereed)
    Abstract [en]

    A major obstacle in tomographic inversion is near-surface velocity variations. Such shallow velocity variations need to be known and correctly accounted for to obtain images of deeper structures with high resolution and quality. Bedrock cover in many areas consists of unconsolidated sediments and glacial till. To handle the problems associated with this cover, we present a tomographic method that solves for the 3D velocity structure and receiver static corrections simultaneously. We test the method on first-arrival picks from deep seismic reflection data acquired in the mid- late to 1980s in the Siljan Ring area, central Sweden. To use this data set successfully, one needs to handle a number of problems, including time-varying, near-surface velocities from data recorded in winter and summer, several sources and receivers within each inversion cell, varying thickness of the cover layer in each inversion cell, and complex 3D geology. Simultaneous inversion for static corrections and velocity produces a much better image than standard tomography without statics. The velocity model from the simultaneous inversion is superior to the velocity model produced using refraction statics obtained from standard reflection seismic processing prior to inversion. Best results using the simultaneous inversion are obtained when the initial top velocity layer is set to the near-surface bedrock velocity rather than the velocity of the cover. The resulting static calculations may, in the future, be compared to refraction static corrections in standard reflection seismic processing. The preferred final model shows a good correlation with the mapped geology and the airborne magnetic map.

  • 17.
    Bergman, Björn
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Juhlin, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Seismic tomography studies of cover thickness and near-surface bedrock velocities2006In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 71, no 6, p. U77-U84Article in journal (Refereed)
    Abstract [en]

    Reflection seismic imaging of the uppermost kilometer of crystalline bedrock is an important component in site surveys for locating potential storage sites for nuclear waste in Sweden. To obtain high-quality images, refraction statics are calculated using first-break travel times. These first-break picks may also be used to produce tomographic velocity images of the uppermost bedrock. In an earlier study, we presented a method applicable to data sets where the vast majority of shots are located in the bedrock below the glacial deposits, or cover, typical for northern latitudes. A by-product of this method was an estimate of the cover thickness from the receiver static that was introduced to sharpen the image. We now present a modified version of this method that is applicable for sources located in or on the cover, the general situation for nuclear waste site surveys. This modified method also solves for 3D velocity structure and static corrections simultaneously in the inversion process. The static corrections can then be used to estimate the cover thickness. First, wetest our tomography method on synthetic data with the shot points in the bedrock below the cover. Next, we develop a strategy for the case when the sources are within the cover. The method is then applied to field data from five crooked-line, high-resolution reflection seismic profiles ranging in length from 2 to 5 km. The crooked-line profiles make the study 2.5 dimensional regarding bedrock velocities. The cover thickness along the profiles varies from 0 to 15 m. Estimated thickness of the cover agrees well with data from boreholes drilled near the profiles. Low-velocity zones in the uppermost bedrock generally correlate with locations where reflections from the stacked sections project to the surface. Thus, the method is functional, both for imaging the uppermost bedrock velocities as well as for estimating the cover thickness.

  • 18.
    Bergmann, Peter
    et al.
    GFZ German Research Centre for Geosciences, Centre for Geological Storage, Telegrafenberg, Potsdam, Germany.
    Ivandic, Monika
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Norden, Ben
    GFZ German Research Centre for Geosciences, Reservoir Technologies, Telegrafenberg, Potsdam, Germany..
    Rücker, Carsten
    Technical University Berlin, Department of Applied Geophysics, Berlin, Germany.
    Kiessling, D.
    Lüth, S.
    Schmidt-Hattenberger, Cornelia
    GFZ German Research Centre for Geosciences, Centre for Geological Storage, Telegrafenberg, Potsdam, Germany.
    Juhlin, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Combination of seismic reflection and constrained resistivity inversion with an application to 4D imaging of the CO2 storage site, Ketzin, Germany2014In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 79, no 2, p. B37-B50Article in journal (Refereed)
    Abstract [en]

    A combination of seismic and geoelectric processing was studied by means of a structurally constrained inversion approach. Structural constraints were interpreted from the seismic data and integrated into the geoelectric inversion through a local regularization, which allowed inverted resistivities to behave discontinuously across defined boundaries. This arranged seismic processing and constrained resistivity inversion in a sequential workflow, making the generic assumption that the petrophysical parameters of both methods change across common lithostructural boundaries. We evaluated the approach using a numerical example and a real data example from the Ketzin CO2 pilot storage site, Germany. The latter demonstrated the efficiency of this approach for combining 4D seismic and surface-downhole geoelectric data. In consistence with the synthetic example, the constrained resistivity inversions produced clearer delineated images along the boundary between caprock and reservoir formation. Near the CO2-flooded reservoir, the seismic and geoelectric time-lapse anomalies correlated well. At some distance to the downhole electrodes, however, the geoelectric images conveyed a notably lower resolution in comparison to the corresponding seismic images. Both methods confirm a northwesterly trend for the CO2 migration at the Ketzin site, although a rather northerly direction was initially expected. The results demonstrate the relevance of the presented approach for the combination of both methods for integrated geophysical CO2 storage monitoring.

  • 19.
    Bergmann, Peter
    et al.
    GFZ German Research Centre for Geosciences, Helmholtz Centre Potsdam, Centre for Geological Storage, Potsdam, Germany.
    Kashubin, Artem
    WesternGeco, Schlumberger House, Buckingham Gate, West Sussex, UK .
    Ivandic, Monika
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Lueth, Stefan
    GFZ German Research Centre for Geosciences, Helmholtz Centre Potsdam, Centre for Geological Storage, Potsdam, Germany.
    Juhlin, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Time-lapse difference static correction using prestack crosscorrelations: 4D seismic image enhancement case from Ketzin2014In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 79, no 6, p. B243-B252Article in journal (Refereed)
    Abstract [en]

    A method for static correction of time-lapse differences in reflection arrival times of time-lapse prestack seismic data is presented. These arrival-time differences are typically caused by changes in the near-surface velocities between the acquisitions and had a detrimental impact on time-lapse seismic imaging. Trace-to-trace time shifts of the data sets from different vintages are determined by crosscorrelations. The time shifts are decomposed in a surface-consistent manner, which yields static corrections that tie the repeat data to the baseline data. Hence, this approach implies that new refraction static corrections for the repeat data sets are unnecessary. The approach is demonstrated on a 4D seismic data set from the Ketzin CO2 pilot storage site, Germany, and is compared with the result of an initial processing that was based on separate refraction static corrections. It is shown that the time-lapse difference static correction approach reduces 4D noise more effectively than separate refraction static corrections and is significantly less labor intensive.

  • 20.
    Brodic, Bojan
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Malehmir, Alireza
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Pugin, Andre
    Geological Survey of Canada, Ontario, Canada.
    Maries, Georgiana
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Three-component seismic land streamer study of an esker architecture through S- and surface-wave imaging2018In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 83, no 6, p. B339-B353Article in journal (Refereed)
    Abstract [en]

    We deployed a newly developed 3C microelectromechanical system-based seismic land streamer over porous glacial sediments to delineate water table and bedrock in Southwestern Finland. The seismic source used was a 500 kg vertical impact drop hammer. We analyzed the SH-wave component and interpreted it together with previously analyzed P-wave component data. In addition to this, we examined the land streamer’s potential for multichannel analysis of surface waves and delineated the site’s stratigraphy with surface-wave-derived S-wave velocities and VP∕VS ratios along the entire profile. These S-wave velocities and VP∕VS ratios complement the interpretation conducted previously on P-wave stacked section. Peculiarly, although the seismic source used is of a vertical-type nature, the data inspection indicated clear bedrock reflection on the horizontal components, particularly the transverse component. This observation led us to scrutinize the horizontal component data through side-by-side inspection of the shot records of all the three components and particle motion analysis to confirm the S-wave nature of the reflection. Using the apparent moveout velocity of the reflection, as well as the known depth to bedrock based on drilling, we used finite-difference synthetic modeling to further verify its nature. Compared with the P-wave seismic section, bedrock is relatively well delineated on the transverse component S-wave section. Some structures connected to the kettle holes and other stratigraphic units imaged on the P-wave results were also notable on the S-wave section, and particularly on the surface-wave derived S-wave velocity model and VP∕VS ratios. Our results indicate that P-, SV-, and SH-wave energy is generated simultaneously at the source location itself. This study demonstrates the potential of 3C seismic for characterization and delineation of the near-surface seismics.

  • 21.
    Cheraghi, Saeid
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Malehmir, Alireza
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Bellefleur, Gilles
    3D imaging challenges in steeply dipping mining structures: New lights on acquisition geometry and processing from the Brunswick no. 6 seismic data, Canada2012In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 77, no 5, p. WC109-WC122Article in journal (Refereed)
    Abstract [en]

    We have analyzed and processed a 38-km(2) nonorthogonal 3D surface reflection seismic data in the Brunswick no. 6 area to better understand the effect of acquisition geometry on the resultant image and to provide 3D information about the main geologic structures hosting the mineralization. The 3D data were processed using a conventional prestack dip moveout (DMO) and poststack migration algorithm with special focus on refraction static corrections, velocity analysis, and DMO corrections that are important for the data recorded in crystalline environment. However, the nonorthogonal nature of the 3D data combined with its narrow-azimuth, irregular offset distributions, and 2D nature of midpoint distribution in common depth point bins resulted in a lower quality seismic image than those observed on a series of 2D seismic profiles collected in the area prior to the 3D data acquisition. 2D wavenumber spectrum of the data suggests acquisition footprint associated with the data. Most of the noise associated with the acquisition footprint manifested itself as short-length, high-amplitude shallow reflections but largely were attenuated using a dip filter running in the wavenumber domain. Various bin size and geometries were tested, and the best result was obtained using rectangular bins aligned in the orientation of the shot lines. The processing results indicated that the highly prospective and mineralized Brunswick horizon is part of a continuous reflective package that could guide future deep mineral exploration in this mining camp.

  • 22.
    Dehghannejad, Mahdieh
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Malehmir, Alireza
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Juhlin, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Skytta, Pietari
    Division of Ore Geology, Luleå University of Technology, Luleå, Sweden.
    3D constraints and finite-difference modeling of massive sulfide deposits: The Kristineberg seismic lines revisited, northern Sweden2012In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 77, no 5, p. WC69-WC79Article in journal (Refereed)
    Abstract [en]

    The Kristineberg mining area in the western part of the Skellefte ore district is the largest base metal producer in northern Sweden and currently the subject of extensive geophysical and geologic studies aimed at constructing 3D geologic models. Seismic reflection data form the backbone of the geologic modeling in the study area. A geologic cross section close to the Kristineberg mine was used to generate synthetic seismic data using acoustic and elastic finite-difference algorithms to provide further insight about the nature of reflections and processing challenges when attempting to image the steeply dipping structures within the study area. Synthetic data suggest processing artifacts manifested themselves in the final 2D images as steeply dipping events that could be confused with reflections. Fewer artifacts are observed when the data are processed using prestack time migration. Prestack time migration also was performed on high-resolution seismic data recently collected near the Kristineberg mine and helped to image a high-amplitude, gently dipping reflection occurring stratigraphically above the extension of the deepest Kristineberg deposit. Swath 3D processing was applied to two crossing seismic lines, west of the Kristineberg mine, to provide information on the 3D geometry of an apparently flat-lying reflection observed in both of the profiles. The processing indicated that the reflection dips about 30 degrees to the southwest and is generated at the contact between metasedimentary and metavolcanic rocks, the upper part of the latter unit being the most typical stratigraphic level for the massive sulfide deposits in the Skellefte district.

  • 23.
    García Juanatey, María de los Ángeles
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggavson, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Juhlin, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Bergström, Ulf
    Geological Survey of Sweden.
    Hübert, Juliane
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Pedersen, Laust B
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    MT and reflection seismics in northwestern Skellefte Ore District, Sweden2013In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 78, no 2, p. B65-B76Article in journal (Refereed)
    Abstract [en]

    A seismic reflection and MT survey was carried out along a 27-km long transect in northwestern Skellefte District, as part of a bigger 3D modeling project. The main motivation for the data acquisition is to elucidate the geologic relationship between the known mineralizations in the Adak mining camp to the north and in the well studied Kristineberg area south of the transect. The seismic reflection data were acquired with a VIBSIST system, and show reflectivity down to 3 s. Apart from the conventional processing for crystalline environments, the seismic data was also subject to an azimuthal binning procedure and cross-dip analysis, allowing the orientation of planar reflectors in 3D. Regarding the MT data, it is primarily of good quality along the 17 installed sites. The inversion of the determinant of the impedance tensor yielded a stable 2D resistivity model, dominated by resistors corresponding to the postorogenic intrusions along the transect. Adding the location of the analyzed seismic reflectors in the MT inversion rendered an integrated model that facilitated a preliminary joint interpretation of the data sets. Overall, the results are in good agreement with surface observations and reveal a crude configuration of the geologic units below the transect. The most prominent outcomes are the lateral and depth extent of the large postorogenic intrusions in the area reaching to 5- or 6-km depth, the dimensions of the nearly vertical Brännäs gabbro extending to 6-km depth, and the presence of enhanced conductivities along the transect at about 10 km depth. The latter is probably related to the deep conductor previously identified in the district.

  • 24. Hunkeler, Priska A.
    et al.
    Hendricks, Stefan
    Hoppmann, Mario
    Farquharson, Colin G.
    Kalscheuer, Thomas
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Grab, Melchior
    Kaufmann, Manuela S.
    Rabenstein, Lasse
    Gerdes, Rüdiger
    Improved 1D inversions for sea ice thickness and conductivity from electromagnetic induction data: Inclusion of nonlinearities caused by passive bucking2016In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 81, no 1, p. WA45-WA58Article in journal (Refereed)
    Abstract [en]

    The porosity of sea ice is a fundamental physical parameter that governs the mechanical strength of sea ice and the mobility of gases and nutrients for biological processes and biogeochemical cycles in the sea ice layer. However, little is known about the spatial distribution of the sea ice porosity and its variability between different sea ice types; an efficient and nondestructive method to measure this property is currently missing. Sea ice porosity is linked to the bulk electrical conductivity of sea ice, a parameter routinely used to discriminate between sea ice and seawater by electromagnetic (EM) induction sensors. Here, we have evaluated the prospect of porosity retrieval of sea ice by means of bulk conductivity estimates using 1D multi-frequency EM inversion schemes. We have focused on two inversion algorithms, a smoothness-constrained inversion and a Marquardt-Levenberg inversion, which we modified for the nonlinear signal bias caused by a passive bucking coil operated in such a highly conductive environment. Using synthetic modeling studies, 1D inversion algorithms and multiple frequencies, we found that we can resolve the sea ice conductivity within +/- 0.01 S/m. Using standard assumptions for the conductivity-porosity relation of sea ice, we were able to estimate porosity with an uncertainty of +/- 1.2%, which enables efficient and nondestructive surveys of the internal state of the sea ice cover.

  • 25. Jensen, M.
    et al.
    Kashubin, Artem
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Juhlin, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Elming, S.
    Multidisciplinary study of the hanging wall of the Kiirunavaara iron ore deposit, northern Sweden2012In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 77, no 6, p. B269-B285Article in journal (Refereed)
    Abstract [en]

    Potential weakness zones due to mining-related fracture development under the town of Kiruna, Sweden, have been investigated by integration of seismic, gravity, and petrophysical data. Reflection seismic data were acquired along two subparallel 2D profiles within the residential area of the town. The profiles of ~3.5 km, each oriented approximately east-west, nearly perpendicular to the general strike of the local geology, crossed several contact zones between quartz-bearing porphyries, a sequence of interchanging sedimentary rocks (siltstone, sandstone, conglomerate, and agglomerate), and metabasalt. The resulting reflection seismic sections revealed a strong east-dipping reflectivity that is imaged down to approximately 1.5 km. The location and orientation of major features agree well between the profiles and with the surface geology and known contact zones between the different rock types. Our imaging results, supported by traveltime modelling, indicate that the contact zones dip 40°-50° to the east. The deepest and the weakest reflections are associated with a ~60° dipping structure that is presumably related to the Kiirunavaara iron mineralization. Tomographic inversion of refracted arrivals revealed a more detailed image of the velocity distribution in the upper 100-200 m along the profiles, enabling us to identify near-surface low velocity zones. These could be possible weakness zones developed along the lithological contacts and within the geologic units. The structural image obtained from the seismic data was used to constrain data inversion along a 28 km long east-northeast to west-southwest-oriented gravity profile. The resulting density model indicates that the quartz-bearing porphyry in the hanging wall of the Kiirunavaara mineralization can be separated into two blocks oriented parallel to the ore body. One block has an unexpected low density, which could be an indication of extensive fracturing and deformation.

  • 26.
    Juhlin, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Imaging of fracture zones in the Finnsjon area, central Sweden, using the seismic reflection method1995In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 60, no 1, p. 66-75Article in journal (Refereed)
    Abstract [en]

    In 1987 the Swedish Nuclear Fuel and Waste Management Co. (SKB) funded the shooting of a 1.7-km long, high-resolution seismic profile over the Finnsjon study site using a 60-channel acquisition system with a shotpoint and geophone spacing of 10 m. The site is located about 140 km north of Stockholm and the host rocks are mainly granodioritic. The main objective of the profile was to image a known fracture zone with high hydraulic conductivity dipping gently to the west at depths of 100 to 400 m. The initial processing of the data failed to image this fracture zone. However, a steeply dipping reflector was imaged indicating the field data were of adequate quality and that the problem lay in the processing. These data have now been reprocessed and a clear image of the gently dipping zone has been obtained. In addition, several other reflectors were imaged in the reprocessed section, both gently and steeply dipping ones. Correlations with borehole data indicate that the origin of these reflections are also fracture zones. The improvement over the previous processing is caused mainly by (1) refraction statics, (2) choice of frequency band, (3) F-K filtering, and (4) velocity analyses.In addition to reprocessing the data, some further analyses were done including simulation of acquisition using only the near-offset channels (channels 1-30) and the far-offset channels (channels 31-60), and determining the damping factor Q in the upper few hundred meters based upon the amplitude decay of the first arrivals. The data acquisition simulation shows the far-offset contribution to be significant even for shallow reflectors in this area, contrary to what may be expected. A Q value of 10, determined from observed amplitude decay rates, agrees well with theoretical ones assuming plane wave propagation in an attenuating medium.

  • 27.
    Juhlin, Christopher
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Giese, Rüdiger
    GeoForschungsZentrum Potsdam Germany.
    Zinck-Jørgensen, Kim
    GEUS, Copenhagen, Denmark.
    Cosma, Calin
    Vibrometric Oy,Vantaa, Finland..
    Kazemeini, Sayed Hesammoddin
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Juhojuntti, Niklas
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Lüth, Stefan
    GeoForschungsZentrum Potsdam Germany.
    Norden, Ben
    GeoForschungsZentrum Potsdam Germany.
    Förster, Andrea
    3D baseline seismics at Ketzin, Germany: The CO2SINK project2007In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 72, no 5, p. B121-B132Article in journal (Refereed)
    Abstract [en]

    A 3D 25-fold seismic survey with a bin size of 12 by 12 m and about 12 km(2) of subsurface coverage was acquired in 2005 near a former natural gas storage site west of Berlin, as part of the five-year EU funded CO2SINK project. Main objectives of the seismic survey were to verify earlier geologic interpretations of structure based on vintage 2D seismic and borehole data and to map, if possible, the reservoir pathways in which the CO2 Will be injected at 650 m depth, as well as providing a baseline for future seismic surveys and planning of drilling operations. The uppermost 1000 m are well imaged and show an anticlinal structure with an east-west striking central graben on its top that extendsdown to the target horizon. About 30 m of throw is seen on the bounding faults. No faults are imaged near the planned drill sites. Remnant gas, cushion and residual gas, is present near the top of the anticline in the depth interval of about 250-400 m and has a clear seismic signature; both higher amplitudes in the reservoir horizons and velocity pulldown are observed. Amplitude mapping of these remnant gas horizons shows that they do not extend as far south as the injection site, which is located on the southern flank of the anticline. Amplitude anomalies, gas chimneys along an east-west striking fault, show that the stored or remnant gas either has been or is presently migrating out of the reservoir formations. Summed amplitude mapping of the planned injection horizon indicates that this lithologically heterogeneous formation may be more porous at the injection site.

  • 28.
    Juhlin, Christopher
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Palm, H
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    3-D structure below Ävrö Island from high-resolution reflection seismic studies, southeastern Sweden1999In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 64, no 3, p. 662-667Article in journal (Other academic)
    Abstract [en]

    Two 1-km-long perpendicular seismic reflection lines were acquired on Ävrö Island, southeast Sweden, in October 1996 in order to (1) test the seismic reflection method for future site investigations, (2) map known fracture zones, and (3) add to the Swedish database of reflection seismic studies of the shallow crystalline crust. An east‐west line was shot with 5-m geophone and shot point spacing, and a north‐south line was shot with 10-m geophone and shotpoint spacing. An explosive source with a charge size of 100 g was used along both lines. The data clearly image three major dipping reflectors and one subhorizontal one in the upper 200 ms (600 m). The dipping reflectors (to the south, east, and north‐west) intersect or project to the surface at or close to where surface‐mapped fracture zones exist. The south‐dipping reflector correlates with the top of a heavily fractured interval observed in a borehole (KAV01) at about 400 m. The subhorizontal zone at about 100–200 m correlates with a known fracture zone in the same borehole (KAV01). 3-D effects are apparent in the data, and only where the profiles cross can the true orientation of the reflecting events be determined. To properly orient and locate all events observed on the lines requires acquisition of 3-D data.

  • 29.
    Juhojuntti, Niklas
    et al.
    Formerly Geological Survey of Sweden, Geophysics, Uppsala, Sweden; presently LKAB, Kiruna, Sweden..
    Kamm, Jochen
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Joint inversion of seismic refraction and resistivity data using layered models: Applications to groundwater investigation2015In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 80, no 1, p. EN43-EN55Article in journal (Refereed)
    Abstract [en]

    We developed a method for joint inversion of seismic refraction and resistivity data, using sharp-boundary models with few layers (typically three). We demonstrated the usefulness of the approach via examples from near-surface case studies involving shallow groundwater exploration and geotechnical investigations, although it should also be applicable to other types of layered environments, e.g., sedimentary basins. In our model parameterization, the layer boundaries were common for the resistivity and velocity distributions. Within the layers, only lateral variations in the material parameters (resistivity and velocity) were allowed, and we assumed no correlation between these. The inversion was performed using a nonlinear least-squares algorithm, using lateral smoothing to the layer boundaries and to the materialparameters. Depending on the subsurface conditions, the smoothing can be applied either to the depth of the layer boundaries or to the layer thicknesses. The forward responses and Jacobian for refraction seismics were calculated through ray tracing. The resistivity computations were performed with finite differences and a cell-to-layer transform for the Fréchet derivatives. Our method performed well in synthetic tests, and in the case studies, the layer boundaries were in good agreement with in situ tests and seismic reflection data, although minimum-structure inversion generally has a better data fit due to more freedom to introduce model heterogeneity. We further found that our joint inversion approach can provide more accurate thickness estimates for seismic hidden layers.

  • 30.
    Juhojuntti, Niklas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Wood, Garnet
    Juhlin, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    O'Dowd, Clare
    Dueck, Peter
    Cosma, Calin
    3D seismic survey at the Millennium uranium deposit, Saskatchewan, Canada: Mapping depth to basement and imaging post-Athabasca structure near the orebody2012In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 77, no 5, p. WC245-WC258Article in journal (Refereed)
    Abstract [en]

    Three-dimensional seismic reflection measurements have been used to assist mine planning at the Millennium uranium deposit, Canada. The deposit is located within the crystalline basement, separated from the overlying Athabasca Basin sediments by an unconformity potentially associated with significant fluid flow. The primary objective of the similar to 6.5 km(2) survey was to image the unconformity and possible post-Athabasca deformation structures in and around the deposit. Clear unconformity reflections are observed within most of the survey area, although there are amplitude variations due to complex geology, including intense hydrothermal clay alteration around the deposit. Finite-difference modeling indicates that the wide-angle character of the unconformity reflections is due to a gradual velocity increase at the unconformity. The reflections are obscured by large time delays, due to Quaternary sediments covering the area, making refraction static corrections crucial. The seismic interpretation shows large variations in the unconformity depth (from approximately 430 to 650 m), indicating a pronounced basement depression that coincides with a gravity low. Reflections from the unconformity are vague within the depression, especially in the vicinity of the deposit. Although the orebody is not directly visible in the seismic image, there is a lack of reflectivity coincident with the alteration surrounding the mineralization. We also observed reflections which likely originate at the contact between the altered and fresh basement rock located beneath the deposit. The seismic data further indicate post-Athabasca faults in the vicinity of the orebody. Based on the initial seismic interpretation, the depth of the crown pillar was adjusted and the mine infrastructure moved away from areas interpreted to be affected by the intense hydrothermal alteration surrounding the deposit. The capability to image the unconformity, post-Athabasca structure, and hydrothermal alteration also highlights the potential use of seismic surveys in uranium exploration.

  • 31. Kalscheuer, Thomas
    et al.
    Hübert, Juliane
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Kuvshinov, Alexey
    Lochbuehler, Tobias
    Pedersen, Laust B.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    A hybrid regularization scheme for the inversion of magnetotelluric data from natural and controlled sources to layer and distortion parameters2012In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 77, no 4, p. E301-E315Article in journal (Refereed)
    Abstract [en]

    Magnetotelluric (MT), radiomagnetotelluric (RMT), and, in particular, controlled-source audiomagnetotelluric (CSAMT) data are often heavily distorted by near-surface inhomogeneities. We developed a novel scheme to invert MT, RMT, and CSAMT data in the form of scalar or tensorial impedances and vertical magnetic transfer functions simultaneously for layer resistivities and electric and magnetic galvanic distortion parameters. The inversion scheme uses smoothness constraints to regularize layer resistivities and either Marquardt-Levenberg damping or the minimum-solution length criterion to regularize distortion parameters. A depth of investigation range is estimated by comparing layered model sections derived from first- and second-order smoothness constraints. Synthetic examples demonstrate that earth models are reconstructed properly for distorted and undistorted tensorial CSAMT data. In the inversion of scalar CSAMT data, such as the determinant impedance or individual tensor elements, the reduced number of transfer functions inevitably leads to increased ambiguity for distortion parameters. As a consequence of this ambiguity for scalar data, distortion parameters often grow over the iterations to unrealistic absolute values when regularized with the Marquardt-Levenberg scheme. Essentially, compensating relationships between terms containing electric and/or magnetic distortion are used in this growth. In a regularization with the minimum solution length criterion, the distortion parameters converge into a stable configuration after several iterations and attain reasonable values. The inversion algorithm was applied to a CSAMT field data set collected along a profile over a tunnel construction site at Hallandsasen, Sweden. To avoid erroneous inverse models from strong anthropogenic effects on the data, two scalar transfer functions (one scalar impedance and one scalar vertical magnetic transfer function) were selected for inversion. Compared with a regularization of distortion parameters with the Marquardt-Levenberg method; the minimum-solution length criterion, yielded smaller absolute values of distortion parameters and a horizontally more homogeneous distribution of electrical conductivity.

  • 32.
    Kamm, Jochen
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Antal Lundin, Ildikó
    Geological Survey of Sweden.
    Bastani, Mehrdad
    Geological Survey of Sweden.
    Sadeghi, Martiya
    Geological Survey of Sweden.
    Pedersen, Laust B
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics. Geological Survey of Sweden.
    Joint inversion of gravity, magnetic and petrophysical data - A case study from a gabbro intrusion in Boden, Sweden2015In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 80, no 5, p. B131-B152Article in journal (Refereed)
  • 33.
    Kamm, Jochen
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Becken, Michael
    Pedersen, Laust B.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Inversion of slingram electromagnetic induction data using a Born approximation2013In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 78, no 4, p. E201-E212Article in journal (Refereed)
    Abstract [en]

    We present an efficient approximate inversion scheme for near-surface loop-loop EM induction data (slingram) that can be applied to obtain 2D or 3D models on a normal desktop computer. Our approach is derived from a volume integral equation formulation with an arbitrarily conductive homogeneous half-space as a background model. The measurements are not required to fulfill the low induction number condition (low frequency and conductivity). The high efficiency of the method is achieved by invoking the Born approximation around a half-space background. The Born approximation renders the forward operator linear. The choice of a homogeneous half-space yields closed form expressions for the required electromagnetic normal fields. It also yields a translationally invariant forward operator, i.e., a highly redundant Jacobian. In connection with the application of a matrix-free conjugate gradient method, this allows for very low memory requirements during the inversion, even in three dimensions. As a consequence of the Born approximation, strong conductive deviations from the background model are underestimated. Highly resistive anomalies are in principle overestimated, but at the same time difficult to resolve with induction methods. In the case of extreme contrasts, our forward model may fail in simultaneously explaining all the data collected. We applied the method to EM34 data from a profile that has been extensively studied with other electromagnetic methods and compare the results. Then, we invert three conductivity maps from the same area in a 3D inversion.

  • 34.
    Kamm, Jochen
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Lundin, Ildiko Antal
    Geol Survey Sweden, Mineral Resources Dept, Uppsala, Sweden..
    Bastani, Mehrdad
    Geol Survey Sweden, Mineral Resources Dept, Uppsala, Sweden..
    Sadeghi, Martiya
    Geol Survey Sweden, Mineral Resources Dept, Uppsala, Sweden..
    Pedersen, Laust B.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Joint inversion of gravity, magnetic, and petrophysical data - A case study from a gabbro intrusion in Boden, Sweden2015In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 80, no 5, p. B131-B152Article in journal (Refereed)
    Abstract [en]

    We have studied a gabbro intrusion in northern Sweden, using 3D inversion of airborne magnetic data, ground-based gravity data, and petrophysical measurements on outcrop samples. Gabbro intrusions are of interest because they are potential hosts of Cu-Ni and platinum group element mineralization. We developed a joint inversion algorithm and applied it to both potential-field data sets to obtain spatial distributions of density and magnetic susceptibility. The distributions were coupled through a nonrigidly enforced parameter relationship determined from the petrophysical samples. We managed the problem of balancing the influence of the two data sets by a novel adaptive reweighting scheme that enforced the discrepancy principle for each data set independently. We demonstrated in tests with synthetic data that neither individual nor joint inversions gave reliable estimates for the depth extension of the intrusive body, the near-surface details, or any complex geometrical features. However, the joint inversion improved the image of the interface between the intrusion and the surrounding rocks and revealed that the density and susceptibility models satisfied the observed petrophysical relationship, which, in turn, caused the structures in the models to align. The geometry of the intrusion was an intrinsic result of the inversion, based on the two distinct petrophysical trends for the gabbro and the surrounding rocks. The inferred shape was simple and concise, and was therefore a useful and testable hypothesis about the subsurface geology that was in agreement with both potential-field data sets and the petrophysical information.

  • 35.
    Kazemeini, Sayed Hesammoddin
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Yang, Can
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Juhlin, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Fomel, Sergey
    The University of Texas at Austin.
    Enhancing seismic data resolution using the prestack blueing technique: An example from the Ketzin CO2 injection site, Germany2010In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 75, no 6, p. V101-V110Article in journal (Refereed)
    Abstract [en]

    Restricted resolution imposes limits on the detection of subsurface geologic features using surface seismic data. Despite great improvements in acquisition and processing techniques in recent years, seismic data still suffer from limited resolution. Detailed subsurface information on the geologic conditions at depth cannot always be attained from conventional surface seismic data. For the area of this study, the carbon dioxide (CO2) storage site at Ketzin, Germany, high seismic resolution is required for mapping the internal structure of the main target reservoir. Processing of the 3D surface seismic data using conventional methods did not provide optimum resolution. The spectral blueing technique attempts to enhance the surface seismic data resolution. Well-log data generally show a blue spectrum with higher-amplitudes at the higher frequencies than the commonly assumed white reflection series. By designing and applying one or several operators to poststack data, it has been possible to better match the reflectivity series and improve resolution. Applying the blueing operator to prestack data was investigated to possibly improve seismic resolution, produce more consistent results, and cause fewer ringing artifacts than when applied to poststack data. Prestack blueing, poststack blueing, and no blueing of the Ketzin 3D seismic data were compared with zero-offset vertical seismic profile (VSP) and synthetic seismograms. The comparison shows that prestack spectral blueing can indeed enhance seismic resolution with fewer artifacts associated with it than the poststack technique. The prestack and poststack spectral blueing approaches improve the well to seismic tie.

  • 36. Koivisto, Emilia
    et al.
    Malehmir, Alireza
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Heikkinen, Pekka
    Heinonen, Suvi
    Kukkonen, Ilmo
    2D reflection seismic investigations at the Kevitsa Ni-Cu-PGE deposit, northern Finland2012In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 77, no 5, p. WC149-WC162Article in journal (Refereed)
    Abstract [en]

    In 2007, a 2D reflection seismic survey was conducted at the Kevitsa Ni-Cu-PGE (platinum group elements) deposit in northern Finland. The aims of the survey were to delineate the overall extent of the ore-bearing Kevitsa ultramafic intrusive complex, to study the seismic response of the disseminated ore deposit, to potentially find indications for new ore deposits, and to extract structural information at depth that may be associated with mineralization. In the processing sequence, specific focus was given to finding optimal CDP-line geometries for the crooked-line survey profiles and, due to highly variable bedrock velocities, to detailed velocity analysis. Our conventional processing sequence, involving prestack DMO corrections followed by poststack migration, resulted in high-quality images of the subsurface. First, the data were used to establish the shape and extent of the Kevitsa intrusion, thus providing an overall framework for future exploration in the area. In particular, the data suggest deeper, up to about 1.5 km depth, continuation of the intrusion than previously thought. Furthermore, the images reveal variable reflectivity characteristics within the intrusion from nonreflective to internally reflective. The Kevitsa deposit is located within a part of the intrusion which is associated with distinct, gently dipping reflectivity fabric down to a depth of about 1 km, spatially constrained within a restricted zone internal to the intrusion. This zone can be used as a guideline for the near-mine exploration efforts, and the reflectivity is dominantly associated with magmatic layering controlling the extent of the bulk of economic mineralization. The seismic data also reveal a complex pattern of faults, in particular a series of major fault and shear zones bracketing and crosscutting the Kevitsa intrusion as a whole. Additionally, our interpretation of the data indicates a possible shared origin of the Kevitsa intrusion and the nearby Satovaara intrusion.

  • 37. Linde, Niklas
    et al.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Peterson, John E.
    Hubbard, Susan S.
    Joint inversion of crosshole radar and seismic traveltimes acquired at the South Oyster Bacterial Transport Site2008In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 73, no 4, p. G29-G37Article in journal (Refereed)
    Abstract [en]

    The structural approach to joint inversion, entailing common boundaries or gradients, offers a flexible and effective way to invert diverse types of surface-based and/or crosshole geophysical data. The cross-gradients function has been introduced as a means to construct models in which spatial changes in two distinct physical-property models are parallel or antiparallel. Inversion methods that use such structural constraints also provide estimates of nonlinear and nonunique field-scale relationships between model parameters. Here, we jointly invert crosshole radar and seismic traveltimes for structurally similar models using an iterative nonlinear traveltime tomography algorithm. Application of the inversion scheme to synthetic data demonstrates that it better resolves lithologic boundaries than the individual inversions alone. Tests of the scheme on GPR and seismic data acquired within a shallow aquifer illustrate that the resultant models have improved correlations with flowmeter data in comparison with models based on individual inversions. The highest correlation with the flowmeter data is obtained when the joint inversion is combined with a stochastic regularization operator and the vertical integral scale is estimated from the flowmeter data. Point-spread functions show that the most significant resolution improvements offered by the joint inversion are in the horizontal direction.

  • 38.
    Lundberg, Emil
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Malehmir, Alireza
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Juhlin, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Bastani, Mehrdad
    Andersson, Magnus
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    High-resolution 3D reflection seismic investigation over a quick-clay landslide scar in southwest Sweden2014In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 79, no 2, p. B97-B107Article in journal (Refereed)
    Abstract [en]

    Quick-clay landslides often occur in the northern hemisphere in areas that were covered by Pleistocene glaciation. They are particularly common along the shorelines of the Göta River in southwestern Sweden. Characterization of potential landslide areas and identification of features that indicate high risk are necessary to better understand the triggering mechanisms of these events. Therefore, an intensive characterization project has been initiated at the location of the Fråstad landslide in Sweden. Part of the characterization program included the acquisition of 3D reflection seismic data to image structures in the normally consolidated sediments, as well as the bedrock topography below the landslide scar. Two seismic horizons within the glacial and post-glacial sediments were observed. The shallowest seismic horizon (here referred to as S1) corresponds to a coarse-grained layer that was previously detected by eight geotechnical boreholes located within the 3D survey area. Discontinuities in S1, mapped by the 3D reflection seismic data, occur across a zone that correlates with the landslide scar boundary, suggesting that this zone may have played a role in triggering and/or in limiting the extension of the landslide. If S1 is truncated by or mixed with clays in this zone, then the outflow of water from the permeable S1 into the clays above may have increased the amount of quick-clays above this zone. The increased outflow of water may also have caused a higher pore-water pressure south of the zone, which in turn could have acted as a trigger for the landslide. This study shows the potential of using the 3D reflection seismic method as a complement to drilling and other geophysical methods when performing landslide site investigations. It also demonstrates the importance of further investigating the relationship between 3D subsurface geometries and landslide development.

  • 39. Madonna, Claudio
    et al.
    Quintal, Beatriz
    Frehner, Marcel
    Almqvist, Bjarne
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tisato, Nicola
    Pistone, Mattia
    Marone, Federica
    Saenger, Erik
    Synchrotron-based X-ray tomographic microscopy for rock physics investigations2013In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 78, p. D53-D64Article in journal (Refereed)
  • 40.
    Malehmir, Alireza
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Bellefleur, Gilles
    Geological Survey of Canada.
    3D seismic reflection imaging of volcanic-hosted massive sulfide deposits: Insights from reprocessing Halfmile Lake data, New Brunswick, Canada2009In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 74, no 6, p. B209-B219Article in journal (Refereed)
    Abstract [en]

    Three-dimensional seismic reflection data from the Halfmile Lake area, New Brunswick, Canada, was reprocessed over an 18-km(2) grid to improve the seismic signatures of a 5-million-ton volcanic-hosted massive sulfide (VHMS) deposit located at 1200-m depth, known as the deep zone, as well as key host-rock structures. We chose a prestack dip moveout (DMO) and poststack migration processing sequence to preserve the possible diffraction signature of the deep VHMS zone. Despite the high level of source-generated noise and large statics caused by near-surface conditions, our processing results revealed improved 3D seismic images for shallow and deep structures. Many of the imaged structures were easily correlated with known lithological contacts constrained by boreholes and petrophysical measurements. Ashort, flat-lying segment of high-amplitude reflection at about 800-m depth in the unmigrated cube was interpreted to originate from a small portion of the lower VHMS zone. The DMO stack was characterized by a large, high-amplitude asymmetric diffraction signature originating from the deep VHMS zone. The asymmetry of the diffraction hyperbola relative to the location of the deep zone was interpreted as resulting from a shape effect from the zone, with the strongest amplitudes along the diffraction hyperbola found north-northwest of the apex. This indicated that the deep VHMS zone dips in a similar direction. This diagnostic diffraction signature was not preserved with the prestack migration approach previously implemented for processing Halfmile Lake data.

  • 41.
    Malehmir, Alireza
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Durrheim, Raymond
    Bellefleur, Gilles
    Urosevic, Milovan
    Juhlin, Christopher
    White, Donald John
    Milkereit, Bernd
    Campbell, Geoff
    Seismic methods in mineral exploration and mine planning: A general overview of past and present case histories and a look into the future2012In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 77, no 5, p. WC173-WC190Article in journal (Refereed)
    Abstract [en]

    Due to high metal prices and increased difficulties in finding shallower deposits, the exploration for and exploitation of mineral resources is expected to move to greater depths. Consequently, seismic methods will become a more important tool to help unravel structures hosting mineral deposits at great depth for mine planning and exploration. These methods also can be used with varying degrees of success to directly target mineral deposits at depth. We review important contributions that have been made in developing these techniques for the mining industry with focus on four main regions: Australia, Europe, Canada, and South Africa. A wide range of case studies are covered, including some that are published in the special issue accompanying this article, from surface to borehole seismic methods, as well as petrophysical data and seismic modeling of mineral deposits. At present, high-resolution 2D surveys mostly are performed in mining areas, but there is a general increasing trend in the use of 3D seismic methods, especially in mature mining camps.

  • 42.
    Malehmir, Alireza
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Heinonen, Suvi
    Geol Survey Finland GTK, Espoo, Finland..
    Dehghannejad, Mahdieh
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Heino, Pasi
    Yara Suomi Oy, Siilinjarvi, Finland..
    Maries, Georgiana
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Karell, Fredrik
    Geol Survey Finland GTK, Espoo, Finland..
    Suikkanen, Mikko
    Yara Suomi Oy, Siilinjarvi, Finland..
    Salo, Aleksi
    Yara Suomi Oy, Siilinjarvi, Finland..
    Landstreamer seismics and physical property measurements in the Siilinjarvi open-pit apatite (phosphate) mine, central Finland2017In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 82, no 2, p. B29-B48Article in journal (Refereed)
    Abstract [en]

    We tested the applicability of a newly developed broadband (0-800 Hz) digital-based seismic landstreamer for open-pit mine planning in the apatite-bearing Siilinjarvi mine in central Finland. Four seismic profiles, in total approximately 2.5 km long (2-4 m source and landstreamer receiver spacing), two inside the pit and two on its margins, were acquired in combination with wireless recorders connected to 10 Hz geophones and fixed at every 10 m spacing along the seismic profiles while the streamer data were being acquired. Downhole logging and laboratory physical property measurements on core and rock samples were carried out to not only support the seismic interpretations but also to provide information about the possible geophysical signature of these unique types of deposits. In spite of a highly noisy mining environment, seismic data of high quality were acquired; however, reflection processing and interpretations were challenged by the geologic complexities of several generations of basic and carbonatite dikes. To complement the reflection data imaging and to account for the steep elevation changes and crookedness of some of the seismic profiles, 3D first-arrival traveltime tomography and 3D swath reflection imaging were also carried out. Clear refracted arrivals from the open-pit profiles suggest the possibility of low-velocity zones associated with either blasting or several shear zones intersecting the seismic profiles. In terms of reflectivity, reflections have a different appearance from short and flat to longer and steep ones. The downhole-and borehole logging data suggest that some of these reflections are associated with diabase dikes and some are likely from zones of weaknesses in the alkaline- carbonatite complex. We determine the potential of using seismic streamers for cost-and time-effective open-pit mine planning and encourage further testing in simpler geologic settings to be established.

  • 43.
    Malehmir, Alireza
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Juhlin, Christopher
    Wijns, Chris
    Urosevic, Milovan
    Valasti, Petri
    Koivisto, Emilia
    3D reflection seismic imaging for open-pit mine planning and deep exploration in the Kevitsa Ni-Cu-PGE deposit, northern Finland2012In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 77, no 5, p. WC95-WC108Article in journal (Refereed)
    Abstract [en]

    A 3D reflection seismic survey was conducted over an area of about 9 km(2) at the Kevitsa Ni-Cu-PGE (platinum group elements) orebody, northern Finland, where open-pit mining started in mid-2012. The principal objective of the survey was to image major fault and fracture zones at depth that may have an impact on the mine stability and safety. Mine planning would then take into account the geometry of these zones at Kevitsa. Processing results, using conventional prestack DMO and poststack migration methods, show gently dipping and steeply dipping reflections from depths of approximately 2 km to as shallow as 150-200 m. Many of the reflections are interpreted to originate from either fault systems or internal magmatic layering within the Kevitsa main intrusion. Further correlation between the surface seismic data and VSP data suggests that numerous faults are present in the imaged volume based upon time shifts or phase changes along horizontal to gently dipping reflections. Some of these faults cross the planned open-pit mine at depths of about 300-500 m, and are therefore critical for geotechnical planning. In terms of in-pit and near-mine exploration, the magmatic layering internal to the intrusion controls the distribution of the bulk of economic mineralization. The ability to image this magmatic layering could therefore guide future drilling, particularly by constraining the presumed lateral extents of the resource area. Exploration also will target discrete reflectors that potentially represent higher-grade sulfide mineralization.

  • 44.
    Malehmir, Alireza
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Thunehed, Hans
    GeoVista, Luleå.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    The Paleoproterozoic Kristineberg mining area, northern Sweden: Results from integrated 3D geophysical and geologic modeling, and implications for targeting ore deposits2009In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 74, no 1, p. B9-B22Article in journal (Refereed)
    Abstract [en]

    The Kristineberg mining area in the western part of the Paleoproterozoic Skellefte Ore District, northern Sweden, is well known for its base-metal and recent gold discoveries. A pilot 3D geologic model has been constructed on a crustal scale, covering an area of 30×30  km to depths of 10  km. Constrained 3D inverse and forward gravity modeling have been performed to confirm and refine previous modeling along seismic profiles using mainly 2.5D techniques. The 3D inverse gravity modeling was geared to generating isodensity surfaces that enclose regions within the model of anomalous density contrast. The 3D forward gravity modeling was conducted to include faulting and folding systems that are difficult to include in the inversion. The 3D geologic model supports many previous interpretations but also reveals new features of the regional geology that are important for future targeting of base-metal and gold deposits. The margins of a thick granite in the south dip steeply inward, suggesting the possibility of room to accommodate another large base-metal deposit if the granitic rocks are juxtaposed with volcanic rocks at depth. Gravity modeling also suggests the observed Bouguer gravity high within the western metasediments can be explained by a large mafic intrusion that has dioritic to tonalitic composition and no significant magnetic signature. Because mafic-ultramafic intrusions within metasediments can indicate gold, this interpretation suggests the western metasediments have a high gold potential.

  • 45.
    Malehmir, Alireza
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Wijns, Chris
    First Quantum Minerals Ltd, Perth, WA, Australia.
    Koivisto, Emilia
    Univ Helsinki, Helsinki, Finland.
    Lindqvist, Teemu
    Univ Helsinki, Helsinki, Finland.
    Skyttä, Pietari
    Univ Turku, Turku, Finland.
    Montonen, Markku
    Boliden FinnEx Oy, Kevitsa, Finland.
    Why 3D seismic data are an asset for exploration and mine planning?: Velocity tomography of weakness zones in the Kevitsa Ni-Cu-PGE mine, northern Finland2018In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 83, no 2, p. B33-B46Article in journal (Refereed)
    Abstract [en]

    Kevitsa is a disseminated Ni-Cu-PGE (platinum group elements) ore body in northern Finland, hosted by an extremely high-velocity (6.5-8.5 km/s) ultramafic intrusion. It is currently being mined at a depth of approximately 100 m with open-pit mining. The estimated mine life is 20 years, with the final pit reaching a depth of 500-600 m. Based on a series of 2D seismic surveys and given the expected mine life, a high-resolution 3D seismic survey was justified and conducted in the winter of 2010. We evaluate earlier 3D reflection data processing results and complement that by the results of 3D first-arrival traveltime tomography. The combined results provide insights on the nature of some of the reflectors within the intrusion. In particular, a major discontinuity, a weakness zone, is delineated in the tomography results on the northern side of the planned pit. Supported by the reflection data, we estimate the discontinuity, likely a thrust sheet, to extend down approximately 600 m and laterally 1000 m. The weakness zone terminates prominent internal reflectivity of the Kevitsa intrusion, and it is associated with the extent of the economic mineralization. Together with other weakness zones, a couple of which are also revealed by the tomography study, the discontinuity forms a major wedge block that influences the mine bench stability on the northern side of the open pit and likely will cause more issues during the extraction of the ore in this part of the mine. We argue that 3D seismic data should routinely be acquired prior to commencement of mining activities to maximize exploration efficiency at depth and also to optimize mining as it continues toward depth. Three-dimensional seismic data over mineral exploration areas are valuable and can be revisited for different purposes but are difficult to impossible to acquire after mining has commenced.

  • 46.
    Malehmir, Alireza
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Urosevic, Milovan
    Bellefleur, Gilles
    Juhlin, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Milkereit, Bernd
    Seismic methods in mineral exploration and mine planning - Introduction2012In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 77, no 5, p. WC1-WC2Article in journal (Other academic)
  • 47.
    Malehmir, Alireza
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Zhang, Fengjiao
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics. Jilin Univ, Coll Geoexplorat, Changchun 130023, Peoples R China..
    Dehghannejad, Mahdieh
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Lundberg, Emil
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Dose, Christin
    Tyrens AB, Malmo, Sweden..
    Friberg, Olof
    Tyrens AB, Malmo, Sweden..
    Brodic, Bojan
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Place, Joachim
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Svensson, Mats
    Tyrens AB, Malmo, Sweden..
    Moller, Henrik
    Tyrens AB, Malmo, Sweden..
    Planning of urban underground infrastructure using a broadband seismic landstreamer - Tomography results and uncertainty quantifications from a case study in southwestern Sweden2015In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 80, no 6, p. B177-B192Article in journal (Refereed)
    Abstract [en]

    We have developed a multicomponent broadband seismic landstreamer system based on digital sensors and particularly suitable for noisy environments and areas in which high-resolution images of the subsurface are desired. We have evaluated results, interpretations, and approaches using the streamer in the planning of an approximately 3-km-long underground tunnel in the city of Varberg in the southwestern Sweden. Prospective targets were imaging of the shallow (<20 m) bedrock surface and weak zones, such as fracture and shear zones. Over the course of three weeks, 25 profiles were acquired with a total length of approximately 7.5 km using a source and receiver spacing of 2-4 m. A novel approach of the data acquisition was to integrate the landstreamer with wireless sensors in areas in which the accessibility was restricted by roads and also to increase the source-receiver distances (offsets). Although the area was highly noisy, the seismic data, in conjunction with available boreholes, successfully led to delineation of the bedrock surface, its undulations, and areas of poor rock quality. To overcome challenges due to geologic complexities and crooked-line data acquisition, 3D tomographic inversion of first breaks was carried out. Comparisons of the results with the existing boreholes indicated that in most places, the bedrock surface was well resolved by the method, which supported the indication of weak zones in the bedrock, represented by low-velocity structures in the tomographic results. We also evaluated the effect of poor geodetic surveying, particularly regarding elevation data, which adulterated the tomography results toward undulating bedrock surfaces or zones of low velocities.

  • 48.
    Mehta, Suman
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Bastani, Mehrdad
    SGU.
    Kamm, Jochen
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Malehmir, Alireza
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Preserving the identity of VLF and LF transmitters for enhanced resolution of geoelectric models of RMT dataIn: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156Article in journal (Refereed)
  • 49. Meier, P.
    et al.
    Kalscheuer, Thomas
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Podgorski, J.
    Kgotlhang, L.
    Green, A.
    Greenhalgh, S.
    Rabenstein, L.
    Doetsch, J.
    Kinzelbach, W.
    Auken, E.
    Mikkelsen, P.
    Foged, N.
    Jaba, B.
    Tshoso, G.
    Ntibinyane, O.
    Hydrogeophysical investigations in the western and north-central Okavango Delta (Botswana) based on helicopter and ground-based transient electromagnetic data and electrical resistance tomography2014In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 79, no 5, p. B201-B211Article in journal (Refereed)
  • 50.
    Pedersen, Laust B.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Bastini, M.
    Dynesius, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Some characteristics of the electromagnetic field from radio transmitters in Europe2006In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 71, no 6, p. G279-G284Article in journal (Refereed)
    Abstract [en]

    The radiomagnetotelluric (RMT) method utilizes man-made signals generated by distant transmitters or by dedicated local transmitters. Man-made electromagnetic (EM) signals in the 1-250-kHz frequency band come mainly from two sources: (1) distant radio transmitters operating in the 15-250-kHz band and (2) nearby or distant industrial sources emitting either transients or higher harmonics of 50 Hz. The natural or background EM signals form a kind of noise floor. In Europe for the 15-250-kHz band, there generally are sufficient transmitters available to estimate the EM transfer functions completely, i.e., the full impedance tensor and the tipper vector. We show examples of the variability of power spectra and azimuthal distributions of transmitters from sites in Sweden, Hungary, The Netherlands, and Spain. We also show that the estimated transfer functions are stable as a function of time; even under typical noisy conditions, simple stacking of spectra in narrow frequency bands provides good-quality estimates of transfer functions.

12 1 - 50 of 67
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