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  • 1. Abramovitz, T
    et al.
    Berthelsen, A
    Schjoth, F
    Thybo, H
    Balling, N
    Nielsen, L
    Flueh, ER
    Hubinger, S
    Reston, T
    Pedersen, LB
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Geophysics.
    Schmidt, J
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Geophysics.
    England, RW
    Hobbs, RW
    Maguire, PKH
    MONA LISA: Deep seismic investigations of the lithosphere in the southeastern North Sea1997In: TECTONOPHYSICS, ISSN 0040-1951, Vol. 269, no 1-2, p. 1-19Article in journal (Other academic)
    Abstract [en]

    The MONA LISA collaborative project has collected 1112 km of seismic normal-incidence reflection data (recorded to 26 s) and wide-angle data from 26 onshore and 2 offshore locations along 4 profiles in the southeastern North Sea. The seismic data clearly

  • 2.
    Bastani, Mehrdad
    Uppsala University, Department of Geophysics.
    EnviroMT: A new controlled source/radio magnetotoelluric system2001Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In the frame of an EU project, a new electromagnetic prospecting instrument was designed and constructed for environmental and engineering applications. The instrument works in the frequency domain in the band 1-250 kHz, whereby the collected data can be used to study the variation of ground resistivity from the surface to a maximum depth of few hundred meters. The system is operational in two modes, the Radio Magnetotelluric (RMT) and the Controlled Source Tensor Magnetotelluric (CSTMT). The RMT method makes use of the signals generated by powerful distant radio transmitters operating in the frequency range 14-250 kHz. The CSTMT technique, with a remotely controllable double magnetic dipole source covering the band 1-100 kHz, is utilized when deeper targets are aimed at or low resistivity terrains are studied. With the aid of user-friendly database software the collected data can be visualized and modeled in1D, directly in the field.

    In November 1998, the first test survey at the Collendoorn dumpsite in the Netherlands was conducted to map the vertical boundaries and lateral extensions of the pollution plume along four RMT profiles located east of the dumpsite. The results of 1D inversion of the data correlated well with the bore-hole logs, revealing that the estimated resistivities and depths to the top of the pollution plume were realistic. The pollution plume appears to be extending more in the northern part.

    Another test was carried out north of Uppsala, at Skediga in Sweden, using both RMT and CSTMT techniques, to study the morphology of a clay-sandy aquifer. The depth of the resistive crystalline bedrock as estimated by 1D and 2D inversion of the EnviroMT data is in good agreement with bore-hole data. To the east, towards River Fyris the conductive clay lenses become thicker.

  • 3.
    Chouliaras, Gerasimos
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Geophysics.
    Seismic quiescence patterns and earthquake prediction research: Testing hypotheses for Greece and China1999Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The main aim of this investigation is to evaluate the most well known hypotheses and models in earthquake prediction research. For this reason we investigate the "localized" seismic quiescence hypothesis using the Z value griding method as well as the recently introduced Seismolap hypothesis of "extended" seismic quiescence.

    The results indicate that precursory seismic quiescence has preceded crustal main shocks in China and Greece and also that aseismic quiescence can be attributed to "silent earthquakes " such as creep events in subduction zones. The application of the Z-value method in investigating the recent seismic quiescence regime in Greece has given indications of three areas in Greece where significant quiescence is observed and which also are candidate areas for strong earthquakes according to their previous seismic history.

    Recently, the Gulf of Corinth in Central Greece has been appointed as an official multiparameter site for earthquake prediction research by the European Council and in this framework, magnetotelluric and seismological investigations revealed the presence of fluids in the Earth's crust that are mainly responsible for the observed seismic and electromagnetic anisotropy which correlates well with the local stress field as revealed from GPS measurements. In addition, the spectral fitting method was used on digital short period data from this region in order toobtain dynamic source parameters from local earthquakes and also to determine scaling relations using two different earthquake source models.

  • 4.
    Frenje, Lena
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Geophysics.
    Scattering of seismic waves in random velocity models: Finite difference simulation2000Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The small-scale heterogeneities within the Earth's crust and the effect these heterogeneities have on seismic waves have gained increasing interest in the last 15 years. One approach to study seismic wave scattering is through numerical simulations in random media. This thesis is mainly concerned with the random media used in scattering simulations. The properties of frequently used random media (Gaussian, exponential andself-similar) are thoroughly investigated with emphasis on the difference between continuous and discrete media. The investigation shows that the standard deviation differs between the two cases and that this should not be corrected for in the modelling. A comparison between the scattering attenuation observed in finite difference simulations and the predictions of single scattering theory implies that it is difficult to estimate the random properties of the scattering medium from seismic data alone and that it maybe difficult to separate scattering and intrinsic attenuation in real data. Two alternative models to the classical random media are presented, aimed to better simulate a fractured crust: (1) a self-similar medium with a lognormal velocity distribution instead of a Gaussian, where the low velocity tail observed in sonic logs from fractured areas is taken intoaccount, and (2) a fracture zone model where the fracture zones are modelled as low velocity zones in a constant high velocity host rock. Properties such as preferred direction and fractal distribution of the widths and lengths of the zones are implemented. Synthetic seismograms produced with finite difference modelling using fracture zone models appear to have more of the characteristics observed in real data than those from the lognormal and Gaussian random models. Scattering simulations of reflection seismicsfrom Iceland, using fracture zone models, indicate that imaging the Moho on low-fold reflection seismic data on Iceland may be difficult due to the scattering in the upper part and random noise levels at later times. Further, in numerical simulations of high resolution reflection seismics, the modelling has to be performed viscoelastically, in some cases with intrinsic Q values as low as 4 in the top 10 m.

  • 5.
    Friberg, Magnus
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Geophysics.
    Tectonics of the middle Ural2000Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis concerns the crustal structure and tectonic evolution of the Middle Urals. Theresults are based on near-vertical reflection seismic profiling, geological fieldobservations and single zircon geochronology,

    The Ural Mountains delimit. the eastern margin of Baltica. Prior to PalaeozoicUralian orogeny, this margin was bounded by an ocean. The closing of this ocean wassimultaneous with the formation of a series of intra-oceanic island-arcs during theSilurian and Devonian. In the Middle Urals, these arcs collided with Baltica during theLate Carboniferous and Early Permian. New isotope-age data show that high-grademetaigneous complexes in the hinterland are not microcontinents, as previouslythought, but rather the roots of the island-arcs. The reflection seismic data reveal that thearcs are separated from the continent and each other by major thrusts, that are E-dippingin the west and W-dipping in the east. The suture between Baltica and the accretedterranes is the Main Uralian Fault Zone (MUFZ), a complex E-dipping deformationzone that records both early suturing and later extension. Normal displacements are alsoobserved on faults in the hinterland, some normal movement probably occurring at thesame time as the collision, and some related to Triassic opening of the West SiberianBasin (WSB). The deposits of the WSB now cover large parts of the eastern MiddleUrals. The seismic data show prominent, sub-horizontal reflections from the lower crust(38 to 43 km depth) under the WSB and the Middle Urals, east of the MUFZ. The Moho is, in most places, interpreted to be at the base of this reflective band. However, directly east of the MUFZ and for 50 km eastwards, the Moho, as identified from other geophysical methods, is at c. 50 km depth, i.e. below the base of the sub-horizontal reflectivity. The features observed in the lower crust, based on their truncation of the Palaeozoic structures and by comparison with seismic data from other areas, are interpreted to have formed from crustal stretching and to be related to the opening of the WSB.

  • 6.
    Gharibi, Mehran
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Geophysics.
    Electromagnetic studies of the continental crust in Sweden2000Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Plane-wave electromagnetic methods are powerful tools for probing the earth from the surfaceto depths of a few hundred kilometers. The airborne VLF technique is widely used to map theelectrical conductivity variations of shallow geological structures. A new iterative algorithmhas been developed to allow a quantitative interpretation of VLF data by estimating the E-polarixation impedance at the surface of a 2-D geological structure from magnetic fieldmeasurements. Using theoretical and field survey data a perfect performance of algorithm inrecovering the impedance has been observed as long as a sufficiently long profile is used.In the magnetotelluric method natural plane-wave electromagnetic field fluctuations aremeasured. 1-D inversion of MT response functions provides a fast overview of the subsurfaceconductivity distribution and also facilitates 2- and 3- dimensional interpretation of the data.The problem of finding starting model for the solution of the 1-D inverse problem has beenaddressed and solved by splitting the data into several sub-bands, starting with highestfrequencies sub-band, i.e. stripping the earth from top to bottom Logarithmic reparameterization of the parameters stabilizes the inversion process and helps convergencetoward a global minimum.

    Interpretation of MT data, conducted in the central-south Scandinavian Caledonides along a175 km long profile with fifty-two stations, revealed a wide resistivity distribution of geological structures range from less than 0.1 Ωm to more than 100000 Ωm. The results exhibit a successful application of the MT method by resolving a highly conducting graphiterich layer outcropping in the Caledonian front and gently dipping westwards (ca. 2o) along the MT profile. The Jämtland supergroup shows more conductive structures of about 1000 Ωm in compared with the rest of the Caledonian rocks to the west. A few conductive anomalousregions have been detected in the upper and middle crust, two of them highly conductive with resistivities less than 0.1 Ωm.

    An algorithm has been developed to remove magnetic field disturbances caused by DCcurrents in power line cables from the airborne total-field magnetic data. The power line issplit into several short straight lines and its total magnetic field is estimated by the vector sumof magnetic fields due to individual straight short lines. Assuming that Earth's magnetic fieldis much stronger than magnetic field caused by the current in the power cable, a correction isapplied by projecting the estimated magnetic field on to the main field direction andsubtracted from the total-field magnetic data. This technique has been applied to an airbornemagnetic data set, strongly distorted by DC power line magnetic fields, resulting in asuccessful removal of the disturbance field from the airborne magnetic data.

  • 7.
    Lund, Björn
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Geophysics.
    Crustal stress studies using microearthquakes and boreholes2000Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The state of stress in the crust is essential for the understanding of geodynamic processes such as the transmission of plate driving forces through the lithosphere and the triggering of earthquakes. This thesis describes the development and application of new methods for the estimation of crustal stresses. The first study revisits the stress state of the Swedish upper crust using data from the deep boreholes in Siljan. Theanalysis of wellbore image and other logging data enables us to constrain orientations and magnitudes of in situ stress. We find a strike-slip faulting regime at depth with the maximum horizontal stress in a WNW-ESE direction and SH - SH ~ 60 MPa at 5 km depth. The remaining three studies in this thesis are concerned with stress estimation from microearthquakes. We utilize earthquake data from the Icelandic SIL network in southwest Iceland. The first microearthquake study develops a stress tensor inversion scheme for focal mechanisms, including new methods for the selection of fault planefrom the nodal planes as well as a novel technique to account for the errors in the focal mechanisms. The second study introduces a new method to assess the similarity of focal mechanisms for earthquakes with closely located hypocenters. The method is also promising as a monitoring instrument of earthquake repeating patterns. Finally, these new methods are applied to one year of seismicity before the November 13, 1998, ML = 5.0 Ölfus earthquake. During the year we find significant temporal variations in microearthquake repeating patterns, and a large anomalyending at the main event. The estimated background state of stress is well constrained, stress inversions over time show a rather stable, subhorizontal direction of σ3 with more variations in σ1 and σ2.

  • 8.
    Muiuane, Elónio
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Geophysics.
    Hydrogeophysics of Tropical Africa: Recent advances and perspectives1999Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In the present work an attempt is made to put into perspective the hydrogeophysical investigations for rural water supply carried out over the last two decades in tropical Africa, paying special attention to groundwater targets in the weathered rocks in and above the crystalline basement. The applicability of geophysical techniques in groundwater investigations in tropical Africa, particularly the DC resistivity and electromagnetic techniques is discussed in the light of recent developments in instrumentation, data collection and interpretation procedures.

    Although there has been an increase in the use of 2D resistivity surveying systems, standard VES is still the main tool of geophysical surveys for groundwater in Africa, where it is often used to characterise the lithology of the weathered layer. In the present work, two automatic 1D inversion schemes based on iterative least squares procedures with singular value decomposition are presented. In order to reinforce the convergence of the inversion schemes towards a global minimum, the standard least squares procedure is combined with other measures of robustness and search strategies in the parameter space. Tests with synthetic and field data prove their usefulness for fast interpretation of DC resistivity data. Compared with plane wave RMT data DC resistivity data generally seem to have less resolving power for the models tested.

  • 9.
    Persson, Leif
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Geophysics.
    Analysis of regional seismic data by means of higher-order spectra2000Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The use of higher-order statistical and spectral estimators as complementary analysis for conventional regional seismic data processing is studied. The issues addressed are; the properties of the complex bispectrum of regional seismic phases, the experimental detection performance of higher-order statistical detectors, discrimination between local earthquakes and explosions, statistical tests and bicoherence, biphase and spatial bispectrum analysis of regional P-, S, and Lg-phases. The complex bispectral measures show a more antisymmetric waveform for mine explosions. Third- and fourth-order measures can add information about seismic phases for the first regional P- and S-phase. The fourth-order cumulant for the first P-phase outperforms the second- and third-order cumulants. A master-event correlation methodis used for discrimination between explosions and earthquakes. A library of seismic events with an a priori knowledge of the source type is employed. The method compares unknown events with the library through second-, third- and fourth-order cross-correlations. The discrimination by the third- and fourth-order master-event correlation outperforms by a factor of two the second-order correlation. Statistical tests based on bispectra can also provide information about seismic noise and seismic phases important for both conventional analysis and detection. Ambient seismic noise recorded with an ocean bottom seismometer is characterized as stationary, symmetric and linear. Prior to spectral estimation, bispectral preconditioning of the data is recommended. The first P-phase is mainly characterized as non-symmetric (non-Gaussian), about one third of recorded second P-phases, and S-phases and one quarter of the Lg-phase are non-Gaussian. This means that conventional analysis of regional phases, will gain by using higher-order statistics and spectra. All of the non-Gaussian phases are characterized as linear. Thus, linear models for regional, even non-Gaussian, phases can be used. The bicoherence and the biphase were estimated for twelve events. First P-phases exhibit considerable values useful for a frequency-wavenumber analysis, which represents additional information about the arriving wave field in the complex frequency-wavenumber bispectrum.

  • 10.
    Quintero, Ronnie
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Geophysics.
    Seismotectonics and lithospheric structure of Costa Rica1999Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The interaction of different tectonic features on a short-distance scale of 100's of km produces the complexity of the Costa Rica region. The intraplate Caribbean earthquake activity of shallow-depth nature is mainly concentrated along an E-W axis in the central part of the country. All types of faults are presented in this region, although a strike-slip fault system, with NB-SW direction is proposed. Intraplate Caribbean esrthquakes in northwestem Panama are shallow and dominated by thrust fault motion. The seismicity along the Pacific coast is mainly associated with the subduction of the Cocos under theCaribbean plate, with predominant thrust fault mechanisms. The interaction of the Cocos and Nazca plates in southeastem Costa Rica, along the Panama fracture fault system produces events with strike-slip mechanism.

    A proposed 1D P-velocity model for the region is presented. Relocation of earthquakes of this area made through this ID-velocity model provides a reliable geometry of the Wadati-Benioff zone, with a slab dipping at 45° to a maximum depth of approximately 250 km.

    Station corrections and P- and S-velocity distribution correlate well with the major upper crust geological features of the area. The P- and S-velocity distribution for depth below 30 km shows a lateral velocity variation, which is associated with the subductionslab.

  • 11.
    Schmidt, Johannes
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Geophysics.
    Deep seismic studies in the western part of the Baltic shield2000Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The Caledonides form an ancient orogenic belt at the western margin of the Balticshield and in Scotland. They are the result of a three-part continent collision thattook place c. 430-390 million years ago. Later extension events have heavily over-printed, and in some areas completely removed, a mountain chain that probablyhad dimensions similar to the Himalayas today.

    In order to understand the tectonic processes involved in the collision and tomap what is still visible today, deep seismic data were acquired along profile lines that cross the border zone between the undeformed Baltic Shield and the accreted continents.

    The seismic method is one of the most important geophysical tools used toimage the structure of the deep crust and lithosphere. The combined use of wide angle seismics and reflection seismics provides reliable information on the velocity distribution and the smaller scale reflectivity pattern in the crystalline crust.This dissertation focusses on the wide-angle seismic data which are used to create two-dimensional velocity models along profiles in the southeastern North Sea and in the central Scandinavian Caledonides. The North Sea models are used to depth migrate the reflection seismic data. Thanks to the profile geometry, new insights contribute to the knowledge on the collision process. In the Scandinavian Caledonides, features in the upper part of the velocity model correlate well with other geophysical results and reveal the crustal thickness, which could not be resolved by other methods. This work contributes substantial new insights to the understanding of the processes involved in the Caledonian orogeny.

  • 12.
    Sirat, Manhal
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Geophysics.
    Structural and neural network analyses of fracture systems at the Äspö Hard Rock Laboratory, SE Sweden1999Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The > 10,000 fractures documented in the 450m deep Äspö Hard Rock Laboratory (HRL) provide a unique opportunity to study brittle deformation of a Swedish bedrock mass. The fracture population consists of six major sets, one subhorizontal and five sub-vertical. A classical structural analysis explored the interrelations between geometry and frequency of both dry and wet fractures with respect to depth and in-situ stresses. Three main findings are:

    In-situ stresses govern frequency distributions of dilated, hence water-bearingfractures. About 68.5% of subhorizontal fractures are dilated in the thrustregime above a depth of ca. 230m while 53% of sub-vertical fractures aredilated in the underlying wrench regime.

    Fractures curve both horizontally and vertically, a finding confirmed by theapplication of artificial neural networks that included Back-Propagation andSelf-Organizing (Kohonen) networks.

    The asymmetry of the total fracture population and tilts of the sub-Cambrianpeneplain demonstrates that multiple reactivations of fractures have tilted theÄspö rock mass 6° to the west.

    The potential space problem raised by this tilt is negated by systematic curvature of steep fractures, some of which sole out to gently dipping fracture zones. Fractures probably developed their curvature when they formed deep in crystalline crust in Precambrian times but have since reactivated at shallow depths. These findings add significantly to the conceptual model of Äspö and should be taken into account in future studies regarding the isolation of Sweden's high-grade radioactive waste in crystalline bedrock.

  • 13.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Geophysics.
    Seismic tomography: Inversion for P- and S-wave velocities1998Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Seismology is undoubtedly the most powerful method for determining the structure of the Earth below the deepest borehole. This thesis describes the, use of seismic signals from both man-made and earthquake sources to model seismic velocities within a volume of the Earth. Based on data from explosions, a two-dimensional model for the P-wave velocities from the Baltic Shield in southern Sweden, across Denmark and into the North GermanBasin is presented. The observed crustal structures show clear signs of extension, estimated to be at least 20% along the profile. However, the emphasis of my work has been on modeling seismic velocities in three dimensions using local earthquake travel times. Two studies have been conducted, one using data from the Long Valley caldera, California. Originally formed in a catastrophic eruption 0.76 My ago, the caldera is today characterized by intense seismic activity. To better understand this volcanic system we have imaged the three-dimensional P- and S-wave velocity structure beneath the caldera. Low seismic velocities are observed below the resurgent dome down to depths of 9 km, which we believe are caused by hydrothermal circulation of fluids rather than by a large magma chamber. The observed lower than normal Vp/Vs ratios (<1.7) are associated with fractured rock containing compressible and low density fluids. Partially molten or hotmaterial will reduce the S- more than the P-wave velocities, thus producing elevated Vp/Vs ratios. The second study exploits the earthquake data catalog from southwest Iceland. Within the study region are several volcanic centers, active and extinct. The seismically most active one is the Hengill volcanic complex, within 30 km of Reykjavík. The large fissure system is characterized by lower than normal velocities and, similar to the Long Valley caldera, low Vp/Vs ratios. We conclude that supercritical magmaticfluids, such as CO2 and H20, circulate in the fissure system and cause the reduced velocities. These findings are important for understanding the nature of the volcanism and volcanic hazard in both study regions.

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