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  • 1.
    Braun, Martina
    et al.
    Technical University of Berlin, Dept. of Applied Geophysics, Berlin.
    Kamm, Jochen
    Technical University of Berlin, Dept. of Applied Geophysics, Berlin.
    Yaramanci, Ugur
    Technical University of Berlin, Dept. of Applied Geophysics, Berlin; Geozentrum Hannover, Leibniz Institute for Applied Geophysics, Hannover.
    Simultaneous inversion of magnetic resonance sounding in terms of water content, resistivity and decay times2009Ingår i: Near Surface Geophysics, ISSN 1569-4445, E-ISSN 1873-0604, Vol. 7, nr 5-6, s. 589-598Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Magnetic resonance sounding (MRS) or surface nuclear magnetic resonance (SNMR) is used for direct groundwater exploration and for an improved aquifer characterization. Currently, it is the only geophysical method that is capable of directly determining the free water content and estimating the pore sizes of the aquifer in the subsurface. However, MRS is basically an electromagnetic method. Therefore, it is sensitive to the resistivity of the subsurface. The water content is the main target of investigation, therefore first inversion routines focused on the water content. Later on, inversion routines determining water content and decay times became available. Very recently, MRS inversion for water content and resistivity has been realized. We present here a simultaneous inversion of MRS in terms of determining the three inversion parameters - water content, resistivity and decay time - within one single inversion routine. Within the iterative inversion scheme, the extrapolated initial values are determined on the basis of the physical effective decay times in the subsurface, which are estimated within the inversion scheme. Due to an instrumental dead time, the initial values for amplitude and phase, which are related to water content and resistivity, cannot be measured directly. Therefore, the initial amplitude must be extrapolated using the decay time of the signal. The standard approach is a mono-exponential decay curve; implicitly, the phase is assumed to be time-invariant. However, multi-exponential signals are natural when considering relaxation behaviour in the underground. It originates from multi-modal pore size distributions or simply a number of differently relaxing signal contributions from the various lithological units.

  • 2.
    Dehghannejad, Mahdieh
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik. Uppsala Univ, Dept Earth Sci, Uppsala, Sweden..
    Malehmir, Alireza
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik. Uppsala Univ, Dept Earth Sci, Uppsala, Sweden..
    Svensson, Mats
    Tyrens AB, Malmö, Sweden..
    Linden, Mattias
    Tyrens AB, Malmö, Sweden..
    Möller, Henrik
    Geokonsult AB, Malmö, Sweden..
    High-resolution reflection seismic imaging for the planning of a double-train-track tunnel in the city of Varberg, southwest Sweden2017Ingår i: Near Surface Geophysics, ISSN 1569-4445, E-ISSN 1873-0604, Vol. 15, nr 3, s. 226-240Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A newly developed broadband digital-based seismic landstreamer system was employed for the planning of a double-train-track tunnel in the city of Varberg, southwest Sweden. Twenty-five seismic profiles, totalling more than 7.5 km of data, were acquired using a 2-to 4-m receiver and source spacing. At places where it was not possible to move the streamer such as road crossings, wireless recorders connected to 28-Hz geophones were used. In addition to the earlier refraction data analysis and first-break traveltime tomographic modelling, reflection processing of the data was considered in this study, given the realisation of reflections in raw shot gathers and their good quality. Bedrock is strongly reflective in most cases but is not evident in the sections when it gets near the surface. Bedrock undulation is noticeable in most reflection sections, and at one occasion, strong diffraction is observed in the bedrock or near to it. The diffraction is originated, not known during the survey, from a 400-m3 cylindrical (of about 3-m-height and 13-m-diameter) concrete-made fire-protection water tank situated in the bedrock and used in emergency situations. Reflection seismic data greatly complement the tomographic models and support deep bedrock where the excavation of the tunnel is planned in downtown Varberg. This interpretation implies different reinforcements and tunnel construction methods (e.g., roofed concrete) at this section of the tunnel. In addition, weakness zones associated with fracture systems are inferred from the reflection characteristics and in conjunction with the velocity models requiring verification by additional boreholes.

  • 3.
    Malehmir, Alireza
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Bastani, Mehrdad
    Krawczyk, Charlotte M.
    Gurk, Marcus
    Ismail, Nazli
    Polom, Ulrich
    Persson, Lena
    Geophysical assessment and geotechnical investigation of quick-clay landslides - a Swedish case study2013Ingår i: Near Surface Geophysics, ISSN 1569-4445, E-ISSN 1873-0604, Vol. 11, nr 3, s. 341-350Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a preliminary assessment of the potential utility of various geophysical measurements carried out over a quick-clay landslide site in south-west Sweden. The multidisciplinary approach includes active P- and S-wave seismic investigations, including 2D and 3D reflection and refraction surveys, passive single and 3C seismic surveys, electrical resistivity tomography and electromagnetic surveys including controlled-source and radio-magnetotellurics, ground-penetrating radar and potential field studies. The P-wave and particularly S-wave reflection seismic data show a high-resolution image of bedrock topography and the stratigraphy of a 100 m thick sequence of sediments that lies on top, which include lightly consolidated quick-clays. Of particular interest is the identification of a layer of relatively coarse-grained material between 10-20 m below the ground surface. Geotechnical investigations indicate that most but not all quick-clays at the site are located above this layer. Further studies are required to determine the importance of their relationship and whether the coarse-grained layer may have had a role in triggering quick-clay landslides in the region. Geoelectrical and electromagnetic methods provide high-resolution images of the unconsolidated subsurface and particularly the normal and leached clays. Radio-magnetotelluric methods proved valuable near the river where traditional geoelectrical methods failed to provide sufficient depth coverage. The study shows that geophysical data are able to image major subsurface structures associated with quick-clay landslides.

  • 4.
    Sharifi Brojerdi, Fatemeh
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Zhang, Fengjiao
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Juhlin, Christopher
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Malehmir, Alireza
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Lehtimäki, Tomas
    Mattsson, Håkan
    Curtis, Philip
    High resolution seismic imaging at the planned tunnel entrance to the Forsmark repository for spent nuclear fuel, central Sweden2014Ingår i: Near Surface Geophysics, ISSN 1569-4445, E-ISSN 1873-0604, Vol. 12, s. 709-719Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The Swedish Nuclear Waste Management Company (SKB) plans to build a repository for storage of high-level radioactive spent nuclear fuel at the Forsmark site in central Sweden at a depth of about 470 m. The planned repository will cover an area of about 3.6 km(2) at this depth. Prior to beginning excavation and tunneling, some detailed geophysical surveys are being performed at the planned site. One of these was a refraction seismic survey to determine depth to bedrock in the vicinity of the planned access ramp. Two lines, each about 300 m long and spaced about 35 m apart, were acquired in August 2011. Since the bedrock topography is known to be highly variable, a close receiver (2 m) and source (6 m) spacing was required to map it. This close spacing allowed the data also to be treated as reflection seismic data and some adjustments to the acquisition procedure were made in the field with this in mind to aid in the later processing. The main adjustment was that seismic data were recorded on all geophone stations simultaneously. That is, as shots were fired along one line, data were recorded along both that line and the other one. Likewise, when shots were fired along the other line, data were recorded along that line and the first line. This adjustment allowed semi-3D coverage between the lines. Results from first break traveltime tomography along the lines indicate a depth to bedrock that is greater than that found from geotechnical observations along the lines. This discrepancy is attributed to the uppermost bedrock being highly fractured and having a velocity significantly below that expected from the intact bedrock deeper down. Reflection seismic processing of the data shows a reflection at about 20 ms (about 60 m). The reflection is interpreted to have a gentle northwesterly dip component to it. Comparison with core data in the area suggests that the reflection is from a thin (a few metres thick) fracture zone, although none of the boreholes actually penetrate the reflector where it is mapped by the seismic data. This fracture zone may be part of a larger fracture zone mapped by core drilling further to the east. The newly mapped reflector may be crossed by the ramp when excavation begins. Further seismic surveying towards the west is required to verify if this will be the case.

  • 5. Shirzad, Taghi
    et al.
    Shomali, Zaher Hossein
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik. Institute of Geophysics, University of Tehran, Tehran, Iran .
    Naghavi, Mojtaba
    Norouzi, Rahim
    Near-surface VS structure by inversion of surface wave estimated from ambient seismic noise2015Ingår i: Near Surface Geophysics, ISSN 1569-4445, E-ISSN 1873-0604, Vol. 13, nr 5, s. 447-453Artikel i tidskrift (Refereegranskat)
  • 6. Srivastava, Shalivahan
    et al.
    Datta, Debanjan
    Agarwal, B. N. P.
    Mehta, Suman
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Applications of Ant Colony Optimization in determination of source parameters from total gradient of potential fields2014Ingår i: Near Surface Geophysics, ISSN 1569-4445, E-ISSN 1873-0604, Vol. 12, nr 3, s. 373-389Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A unified interpretation technique, based on the computation of total gradient from first order horizontal and vertical derivatives of a measured anomaly, namely, magnetic, gravity or self-potential, can be used to determine horizontal location, depth, and the nature of the causative source geometry. The total gradient, in general, may exhibit several symmetrical bell-shaped functions of different magnitudes and with different decay rates beta-termed as 'source geometry factor' to describe the nature of the causative sources. beta-values have been presented for idealized source geometries producing three different types of measurements (anomalies). The non-linear inversion of the total gradient with multiple sources of different geometries is accomplished through Ant Colony Optimization (ACO) - a technique with only a few demonstrated applications in geophysical problems. We have also implemented the ACO which, in its original form, is suitable for discrete space or combinatorial optimization in a continuous model space using decimal coding. Moreover, unlike conventional techniques based on pre-specified moving/fixed data length windows, the proposed technique uses much of the total gradient data points except at the ends of the profile (where error in the total gradient can be large). The presence of singularity in the Euler technique in the analysis of a gravity anomaly over a thick faulted slab has been overcome by the total gradient ACO method. A procedure has also been formulated for analysing gravity and reduced to pole (RTP) total field magnetic anomalies over spherical and vertical cylindrical source geometries. To evaluate the efficacy of the proposed technique, ACO results are compared with other modern techniques, namely, Particle Swarm Optimization (PSO), Enhanced Local Wavenumber (ELW) and the Euler method through simulated data contaminated with random noise and three field examples. This study has revealed that the total gradient ACO technique leads to better convergence towards the global minimum and also better inversion results than those obtained by the PSO and other techniques mentioned above. Unlike the ELW and Euler techniques, the total gradient analysis provides enhanced resolution for interfering sources.

  • 7.
    Yang, Can
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Juhlin, Christopher
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Enescu, Nicoleta
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Cosma, Calin
    Lueth, Stefan
    Moving source profile data processing, modelling and comparison with 3D surface seismic data at the CO2SINK project site, Ketzin, Germany2010Ingår i: Near Surface Geophysics, ISSN 1569-4445, E-ISSN 1873-0604, ISSN 1569-4445, Vol. 8, nr 6, s. 601-610Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Baseline moving source profiling data were acquired in borehole Ketzin 202/2007 along seven lines at the Ketzin CO2 injection site in 2007. The data were recorded on eight three-component receivers spaced 10 m apart over a depth interval of 470-540 m. The main objective of the moving source profile survey was to generate high-resolution seismic images around the borehole. This was especially important given that the 3D surface seismic data in the area have a low fold at the injection site. Mapping of the sandy layers in the target formation (Stuttgart Formation) at around 630 m, the approximate CO2 injection depth, was another objective of the research. A comparison with repeat moving source profile surveys, during and after the injection, will be done in the future. A processing sequence consisting in hodogram analysis, wavefield separation and prestack migration was applied to the moving source profile data. A median filter was used to separate the downgoing and upgoing wave modes. The data were processed to generate depth migrated images in the vicinity of the borehole that could be compared with the 3D surface seismic data. Both the modelling studies that were carried out and the migrated images, indicate that the sandy layers within the Stuttgart Formation can potentially be imaged in the moving source profile data whereas reflections from these layers are not as clearly observed in the 3D surface seismic data.

  • 8.
    Yordkayhun, Sawasdee
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Juhlin, Christopher
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Norden, Ben
    3D seismic reflection surveying at the CO2SINK project site, Ketzin, Germany: A study for extracting shallow subsurface information2009Ingår i: Near Surface Geophysics, ISSN 1569-4445, E-ISSN 1873-0604, ISSN 1569-4445, Vol. 7, nr 2, s. 75-91Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Three-dimensional (3D) reflection seismic data were recorded as part of a pilot scale carbon dioxide (CO2) geological storage project (CO2 SINK) with the aim of mapping the structural geometry of the site and providing a 3D baseline prior to CO2 injection. Standard processing  originally focused on the storage target, a saline aquifer at 500-700 m  depth and successful imaged coherent reflections from 150 ms to 900 ms in the seismic volume. However, the relatively sparse distribution of sources and receivers, the frequency content of the data and artefacts   of the processing resulted in the uppermost 150 ms being poorly   resolved. This depth range contains caprock, shallow faults and an aquifer system. Thus, characterizing the shallow subsurface is important in terms of site delineation of potential leakage paths and monitoring after CO2 injection. In order to study the potential of   mapping the uppermost reflectors and shallow structures associated with  major fault zones, a comprehensive reprocessing effort on a subset of the 3D data was performed. The challenge in imaging shallow reflections is dependent upon the separation of ground roll and refracted energy   from the reflected energy, as well as compensation for time shifts due   to statics. Among the processing sequences, refraction static corrections, careful muting and filtering, velocity analysis and 3D   time migration were key steps for enhancing the resolution and   coherency of shallow seismic reflections. This study images a  previously unmapped horizon, close to the Quaternary-Tertiary boundary, at about 95-120 ms (similar to 65-90 m depth). Correlation of lateral variations in reflectivity along this boundary, lateral velocity   variations in the tomographic image and the seismic signature in   modelling studies suggest an aquifer/aquitard complex and variable   lithology with associated localized silty or clayey sediments,   overlying the Tertiary Rupelton clay unit. In the previous processing   it was not clear if the deeper faults imaged on the 3D seismic survey   extended to shallower levels than the base Tertiary. Thus, a   comprehensive fault detection technique, multi-attributes and neural networks analysis, was employed in this study to allow a more reliable  fault geometry to be interpreted. Tracking of faults in the seismic   image and comparisons with a tomography study indicate that some deeper faults may penetrate into the overlying Tertiary unit. These findings are important for understanding potentially risky areas and can be used as a database for future monitoring programmes at the site.

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