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1.	Selway, Kate, et al. (författare) Magnetotelluric constraints on the temperature, composition, partial melt content and viscosity of the upper mantle beneath Svalbard 2020 Ingår i: Geochemistry Geophysics Geosystems. - : American Geophysical Union (AGU). - 1525-2027. ; 21:5 Tidskriftsartikel (refereegranskat)abstract Long‐period magnetotelluric (MT) data can be used to interpret upper mantle temperature, hydrogen content, and the presence of partial melt, all of which strongly influence mantle viscosity. We have collected the first long‐period MT data in Svalbard and have combined them with pre‐existing broadband MT data to produce a model of the electrical resistivity of Svalbard's upper mantle. Asthenospheric resistivities are low compared to stable continental settings but more comparable to young oceanic asthenosphere, suggesting that the physical state of Svalbard's upper mantle is controlled by its proximity to the Mid‐Atlantic Ridge. Interpretation of the MT model using a petrologically‐constrained genetic algorithm approach shows that partial melt is present in the uppermost asthenosphere beneath Svalbard. This is the first direct evidence of partial melt in Svalbard's asthenosphere from deep geophysical soundings. Viscosities calculated from the geophysical data show a low viscosity layer (~1018 Pa s) coincident with the partial melt layer, underlain by a higher viscosity layer (~1020 Pa s) extending to the transition zone. Viscosities calculated from glacial isostatic adjustment (GIA) data in Svalbard show a considerable range due mainly to uncertainties in past ice sheet models. Improved constraints on Svalbard's mantle viscosity from geophysical data may help to improve these GIA models.
2.	Abtahi, Sayyed Mohammad, et al. (författare) A new reference model for 3D inversion of airborne magnetic data in hilly terrain : A case study from northern Sweden 2018 Ingår i: Geophysics. - : SOC EXPLORATION GEOPHYSICISTS. - 0016-8033 .- 1942-2156. ; 83:1, s. B1-B12 Tidskriftsartikel (refereegranskat)abstract 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.
3.	Abtahi, Sayyed Mohammad, et al. (författare) A new reference model for 3D inversion of airborne magnetic data in hilly terrain – a case study from northern Sweden Tidskriftsartikel (refereegranskat)
4.	Abtahi, Sayyed Mohammad (författare) Airborne Gravity Gradient, Magnetic and VLF datasets : Case studies of modelling, inversion and interpretation 2016 Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract Northern Sweden is one of the largest hosts for mineral resources in Europe and always has been an interesting area for researchers from various disciplines of Earth sciences. This dissertation is a comprehensive summary of three case study papers on airborne VLF, gravity gradient and magnetic data in the area.In the first paper, tensor VLF data is extracted from an old data set which contains only the total and the vertical magnetic components. The anomalous part of the horizontal magnetic field components is computed by a Hilbert transform of the vertical magnetic field. The normal part of the horizontal magnetic field component is computed as a function of total, vertical and anomalous part of horizontal magnetic fields. The electric field is also calculated for TE mode and impedance tensor and apparent resistivity are computed. In addition tippers are calculated for two transmitters and inverted by a 3D inversion algorithm. Comparison of the estimated model and geology map of bedrock shows that lower resistivity zones are correlated with mineralizations.The second paper deals with the internal consistency of airborne gravity gradient data. The six components of the data are estimated from a common potential function. It is shown that the data is adequately consistent but at shorter land clearances the difference between the estimated data and the original data is larger. The technique is also used for computing the Bouguer anomaly from terrain corrected FTG data. Finally the data is inverted in 3D, which shows that the estimated density model in shallow depth is dominated by short wave length features.Inversion of TMI data is the topic of the third paper where a new type of reference model for 3D inversion of magnetic data is proposed by vertically extending the estimated magnetization of a 2D terrain magnetization model. The final estimated 3D result is compared with the magnetization model where no reference model is used. The comparison shows that using the reference model helps the high magnetization zones in the estimated model at shallow depths to be better correlated with measured high remanent magnetization from rock samples. The high magnetization zones are also correlated with gabbros and volcanic metasediments.
5.	Abtahi, Sayyed Mohammad, et al. (författare) Consistency investigation, vertical gravity estimation and inversion of airborne gravity gradient data – A case study from northern Sweden 2016 Ingår i: Geophysics. - : Society of Exploration Geophysicists. - 0016-8033 .- 1942-2156. ; 81:3, s. B65-B76 Tidskriftsartikel (refereegranskat)abstract 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.
6.	Abtahi, Sayyed Mohammad, et al. (författare) Extracting geoelectrical maps from vintage very-low-frequency airborne data, tipper inversion, and interpretation : A case study from northern Sweden 2016 Ingår i: Geophysics. - 0016-8033 .- 1942-2156. ; 81:5, s. B135-B147 Tidskriftsartikel (refereegranskat)abstract 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.
7.	Bastani, Mehrdad, et al. (författare) 2D joint inversion of RMT and ERT data versus individual 3D inversion of full tensor RMT data : An example from the Trecate site in Italy 2012 Ingår i: Geophysics. - 0016-8033 .- 1942-2156. ; 77:4, s. WB233-WB243 Tidskriftsartikel (refereegranskat)abstract 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.
8.	Bastani, Mehrdad, et al. (författare) CSRMT measurements in the frequency range 1-250 kHz to map a normal fault in the Volvi Basin, Greece 2011 Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 75:2, s. 180-195 Tidskriftsartikel (refereegranskat)abstract In order to gain a better understanding of the geometry of surface faults, five Controlled Source/Radio Magnetotelluric (CSRMT) profiles were measured across the Volvi basin, 45 km northeast of the city of Thessaloniki in Greece. The data were collected in two frequency ranges: a) 1–12.5 kHz using a remotely controlled double horizontal magnetic dipole transmitter (CSAMT measurements), and b) 15–250 kHz using the signal from distant radio transmitters (RMT measurements). The transition from the RMT band to the CSAMT band was smooth and continuous allowing us to combine both datasets for plane-wave modeling. The surface geology shows a predominantly 2D structure, and therefore we planned the survey into profiles perpendicular to the geological strike. We have used a 2D interpretation tool to model the data in TE, TM, TE + TM and determinant modes. Using a 4% error floor on the impedance, 2D resistivity models from inversion of the determinant data provide lower RMS data fits (4.2 and 1.2 for resistivity and phase, respectively) compared to the combined TE + TM data (4.4, 2.8, overall resistivity and phase, respectively). 2D inversion of the measured tensor data shows a sharp change in the depth to the top of resistive gneiss–schist basement that is overlain by a less resistive overburden at southern basin flanks. The change in depth to the bedrock is clearly seen in all 2D models along the measured profiles suggesting the existence of normal faults with strike directions of NE–SW to E–W. The 2D electrical resistivity models suggest that the bedrock deepens towards south-west. The resistivity models are also compared with the existing borehole information in the area and show a reasonable correlation. For example the sharp change of depth to the bedrock towards the center of the basin as seen in the resistivity models are also confirmed by the borehole data.
9.	Blake, S., et al. (författare) A multi-disciplinary investigation of Irish warm springs and their potential for geothermal energy provision 2015 Ingår i: Proceedings World Geothermal Congress 2015. ; , s. 11p.- Konferensbidrag (refereegranskat)
10.	Buntin, Laura M. (författare) Efficient Electromagnetic Induction Modelling : Adaptive mesh optimisation, advanced boundary methods and iterative solution techniques 2023 Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract Forward modelling of electromagnetic induction data simulates the electric and magnetic fields within a computational domain for a given distribution of electromagnetic material properties and a given source of the electromagnetic field. The quantities of interest are the fields at receiver locations at the Earth's surface. Reliable results require high accuracy solutions at the receivers. First and foremost, numerical computations need to be accurate, but ideally they are also resource efficient, i.e., as fast and cheap as possible. Run time and memory demand mainly depend on the size of the numerical problem to be solved. This thesis addresses specific steps within the forward modelling procedure of electromagnetic induction data in order to improve the solution accuracy of forward modelling as well as to reduce computational resources. The solution accuracy is strongly influenced by the spatial discretisation of the computational domain, which directly correlates with the numerical problem size. To optimise the solution accuracy while keeping the numerical problem size as small as possible, a goal-oriented adaptive mesh refinement scheme for three-dimensional controlled-source electromagnetic models is developed. In addition, this thesis investigates the influence of different types of boundary methods on the solution accuracy at the receivers. To replace inhomogeneous boundary conditions in magnetotelluric total-field modelling by perfectly-matched layers (PML), a domain decomposition approach (the total and scattered field decomposition) is adapted for Earth models. By reducing boundary effects, the approach yields superior solution accuracy for specific types of models. The fastest and most memory-efficient way to solve large numerical problems are iterative solution methods. Iterative solvers, however, work poorly for numerical systems arising from domains bounded by PML. In this thesis a preconditioned iterative solution framework that efficiently solves PML-bounded magnetotelluric models is proposed.
11.	Buntin, Laura M., et al. (författare) Improved accuracy of plane-wave electromagnetic modelling by application of the total and scattered field decomposition and perfectly matched layers 2023 Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 235:2, s. 1201-1217 Tidskriftsartikel (refereegranskat)abstract In 2-D magnetotelluric modelling, the standard application of Dirichlet boundary conditions (BC) may severely diminish the solution accuracy, because the unknown scattered part of the electromagnetic field is erroneously reflected at the domain boundary. Therefore, we adapt the total and scattered field decomposition (TSFD) to geophysical modelling, enabling the application of fully absorbing boundary methods, here perfectly matched layers (PML), to the scattered field. Our novel TSFD divides the modelling domain into two regions. In the total-field region containing the area of interest, the solution is computed for the total field. In the scattered-field region containing the boundaries, the solution is obtained for the scattered field, which is fully absorbed by PML at the boundaries. The plane-wave source is excited at the TSFD interface between both regions. Thus, boundary reflections are eradicated leading to superior solution accuracy, and boundaries can be placed closer to the receivers, shrinking the computational problem. Especially for challenging models with strong lateral changes, the solution accuracy of the TSFD is superior to that of the standard Dirichlet approach. Owing to the linearity of Maxwell's equations, the inaccuracy introduced to the electric and magnetic fields by using Dirichlet BC can be expected to partly cancel out in the magnetotelluric transfer functions, for example the impedance tensor. In this work, we quantify this cancellation effect. The inaccuracy is less than typical measurement errors in the vast majority of apparent resistivity and phase data, even, when the primary fields are strongly inaccurate.
12.	Buntin, Laura M., et al. (författare) Using preconditioned iterative methods in electromagnetic geophysical modelling including perfectly-matched layers Annan publikation (övrigt vetenskapligt/konstnärligt)
13.	Chen, Chaojian, et al. (författare) Exact solutions of the vertical gravitational anomaly for a polyhedral prism with vertical polynomial density contrast of arbitrary orders 2018 Ingår i: Geophysical Journal International. - : Oxford University Press. - 0956-540X .- 1365-246X. ; 214:3, s. 2115-2132 Tidskriftsartikel (refereegranskat)abstract We present general closed-form solutions for the vertical gravitational anomaly caused by a polyhedral prism with mass density contrast varying with depth. Our equations are the first ones to implement a polynomial vertical mass density contrast of arbitrary order. Singularities in the gravity field which arise when the observation site is close to or in the anomalous polyhedral prism are removed in our analytic expressions. Therefore, the observation site can be located outside, on the faces of or inside the anomalous mass bodies. A simple prismatic body of anomalous density is adopted to test the accuracy of our newly developed closed-form solution. Cases of constant, linear, quadratic, cubic and quartic polynomial orders of mass density contrast are tested. For cases of constant, linear, quadratic and cubic polynomial orders, the relative errors between our results and other published exact solutions are less than 10(-11)%. For the case of quartic polynomial order, relative errors less than 10(-10)% are obtained between our solutions and those computed by a high-order Gaussian quadrature rule (512 x 512 x 512 = 134 217 728 quadrature points), where our new analytic solution needs significantly less computational time (0.0009 versus 31.106 s). These numerical experiments not only verified the accuracy of our new formula but also demonstrated their potential in computing exact gravity anomalies for complicated mass density distributions in the Earth.
14.	Cherevatova, M., et al. (författare) Crustal structure beneath southern Norway imaged by magnetotellurics 2014 Ingår i: Tectonophysics. - : Elsevier BV. - 0040-1951 .- 1879-3266. ; 628, s. 55-70 Tidskriftsartikel (refereegranskat)abstract We use data from two magnetotelluric profiles, ToSca10 and ToSca'09, over the Scandinavian Mountains to study the crustal structure in southern Norway. The profiles cross the major tectonic structures of the Caledonian orogen as well as the western margin of the Precambrian Baltica. Dimensionality and strike analyses indicate generally 3-D behavior of the data. However, the majority of the used data distinguishes a preferable strike direction, which is supported by the geology of the region. Hence, we employ 2-D inversion and choose to invert the determinant of the impedance tensor to mitigate 3-D effects in the data on our 2-D models. Magnetotelluric data from both profiles are inverted using a damped least squares solution based on a singular value decomposition. We improved the solution by defining the inverse model covariance matrix through gradient or Laplacian smoothing operators. The two-dimensional inversion models of the ToSca'09 and ToSca'10 field data from southern Norway derived from the damped least squares scheme with the Laplacian inverse model covariance matrix are presented. Resistive rocks, extending to the surface, image the autochthonous Southwest Scandinavian Domain and the allochthonous Western Gneiss Region. Near-surface conductors, which are located between the resistive Caledonian nappes and Precambrian basement, delineate highly conductive shallow-sea sediments, so called alum shales. They exhibit a decollement along which the Caledonian nappes were overthrust. A deeper, upper to midcrustal conducting layer in the Southwest Scandinavian Domain may depict the remnants of closed ocean basins formed during the accretions and collisions of various Sveconorwegian terranes. In ToSca'10, the Caledonian nappes, the conducting alum shales and the deeper conductor are terminated in the west by the Faltungsgraben shear complex which represents a crustal scale boundary between the Western Gneiss Region in the west and the Southwest Scandinavian Domain in the east.
15.	Donohue, Shane, et al. (författare) The Use of Geophysics for Sensitive Clay Investigations 2014 Ingår i: Landslides in Sensitive Clays. - Dordrecht : Springer Netherlands. ; , s. 159-178 Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract Marine clay deposits in coastal, post-submarine areas of Scandinavia and North America may be subjected to quick clay landslides and hence significant efforts are being taken to map their occurrence and extent. Recently, considerable efforts by a number of researchers have been made to investigate areas of sensitive clay using a range of geophysical techniques. Although the majority of this work has focussed on measurements of electrical resistivity, other electromagnetic and seismic geophysical techniques have also received attention in the literature. The purpose of this paper is to review recent research concerning the effectiveness of a number of geophysical techniques for investigating sensitive clays. In addition to discussing a number of case studies, this review will also consider recent work showing the correlation of geophysical measurements, and in particular electrical resistivity, with a range of relevant engineering properties.
16.	Green, Alan G., et al. (författare) An ancient mega-lake and mega-delta in southern Africa: hydrogeophysical investigations across the Okavango Delta 2014 Ingår i: Near-surface geophysics and geohazards. ; , s. 9-13 Konferensbidrag (refereegranskat)
17.	Hunkeler, Priska A., et al. (författare) A glimpse beneath Antarctic sea ice : Platelet layer volume from multifrequency electromagnetic induction sounding 2016 Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 43:1, s. 222-231 Tidskriftsartikel (refereegranskat)abstract In Antarctica, ice crystals emerge from ice-shelf cavities and accumulate in unconsolidated layers beneath nearby sea ice. Such sub-ice platelet layers form a unique habitat, and serve as an indicator for the state of an ice shelf. However, the lack of a suitable methodology impedes an efficient quantification of this phenomenon on scales beyond point measurements. In this study, we inverted multi-frequency electromagnetic (EM) induction soundings of > 100 km length, obtained on fast ice with an underlying platelet layer in the eastern Weddell Sea. EM-derived platelet-layer thickness and conductivity are consistent with other field observations. Our results further suggest that platelet-layer volume is higher than previously thought in this region, and that platelet-layer ice-volume fraction is proportional to its thickness. We conclude that multi-frequency EM is a suitable tool to determine platelet-layer volume, with the potential to obtain crucial knowledge of associated processes in otherwise inaccessible ice-shelf cavities.
18.	Hunkeler, Priska A., et al. (författare) Improved 1D inversions for sea ice thickness and conductivity from electromagnetic induction data : Inclusion of nonlinearities caused by passive bucking 2016 Ingår i: Geophysics. - 0016-8033 .- 1942-2156. ; 81:1, s. WA45-WA58 Tidskriftsartikel (refereegranskat)abstract 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.
19.	Juhlin, Christopher, et al. (författare) Seismic imaging in the eastern Scandinavian Caledonides: Siting the 2.5 km deep COSC-2 borehole, central Sweden 2016 Ingår i: Solid Earth. - : Copernicus GmbH. - 1869-9510 .- 1869-9529. ; 7:3, s. 769-787 Tidskriftsartikel (refereegranskat)abstract The Collisional Orogeny in the Scandinavian Caledonides (COSC) project, a contribution to the International Continental Scientific Drilling Program (ICDP), aims to provide a deeper understanding of mountain belt dynamics. Scientific investigations include a range of topics, from subduction-related tectonics to the present-day hydrological cycle. COSC investigations and drilling activities are focused in central Scandinavia where rocks from the mid to lower crust of the orogen are exposed near the Swedish-Norwegian border. Here, rock units of particular interest occur in the Seve Nappe Complex (SNC) of the so-called Middle Allochthon and include granulite facies migmatites (locally with evidence of ultra-high pressures) and amphibolite facies gneisses and mafic rocks. This complex overlies greenschist facies metasedimentary rocks of the dolerite-intruded Särv Nappes and underlying, lower grade Jämtlandian Nappes (Lower Allochthon). Reflection seismic profiles have been an important component in the activities to image the sub-surface structure in the area. Sub-horizontal reflections in the upper 1-2 km are underlain and interlayered with strong west- to northwest-dipping reflections, suggesting significant east-vergent thrusting. Two 2.5 km deep fully cored boreholes are a major component of the project which will improve our understanding of the sub-surface structure and tectonic history of the area. Borehole COSC-1 (IGSN: http://hdl.handle.net/10273/ICDP5054EEW1001), drilled in the summer of 2014, targeted the subduction-related Seve Nappe Complex and the contact with the underlying allochthon. The COSC-2 borehole will be located further east and investigate the lower grade, mainly Cambro-Silurian rocks of the Lower Allochthon, the Jämtlandian décollement and penetrate into the crystalline basement rocks to identify the source of some of the northwest-dipping reflections. A series of high resolution seismic profiles have been acquired along a composite c. 55 km long profile to help locate the COSC drill holes. We present here the results from this COSC-related composite seismic profile (CSP), including new interpretations based on previously unpublished data acquired between 2011 and 2014. These seismic data, along with shallow drill holes in the Caledonian thrust front and previously acquired seismic, magnetotelluric, and magnetic data, are used to identify two potential drill sites for the COSC-2 borehole.
20.	Kalscheuer, Thomas, et al. (författare) A hybrid regularization scheme for the inversion of magnetotelluric data from natural and controlled sources to layer and distortion parameters 2012 Ingår i: Geophysics. - 0016-8033 .- 1942-2156. ; 77:4, s. E301-E315 Tidskriftsartikel (refereegranskat)abstract 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.
21.	Kalscheuer, Thomas, 1975-, et al. (författare) A hybrid scheme for the inversion of CSTRMT data for layer and distortion parameters 2010 Konferensbidrag (refereegranskat)
22.	Kalscheuer, Thomas, et al. (författare) A non-linear truncated SVD variance and resolution analysis of 2D magnetotelluric models 2006 Konferensbidrag (populärvet., debatt m.m.)abstract The resolution and variance properties of 2D models of electrical resistivity derived from magnetotelluric measurements are analysed with a truncated singular value decomposition (TSVD) scheme on a local subspace partly taking into account the non-linearity of the inverse problem.The TSVD resolution and variance analysis is performed on a local subspace pertaining to one single cell of interest at a time. The trade-off between model variance and model resolution is used to determine a level of truncation by fixing a variance threshold.Non-linear semi-axes describe the non-linear confidence surface in the directions of the model eigenvectors and replace the inverse singular values in the computation of model variances. The model variance of the cell considered is estimated from the sum of squares of the non-linear semi-axes up to the given variance threshold. This - in turn - gives the truncation level of the TSVD and the row of the model resolution matrix belonging to the considered cell can be computed from the model eigenvectors of the TSVD.The validity of our non-linear model variance and resolution estimates is tested with a most-squares inversion which gives an independent and improved estimate of model variability.A field example from the Skediga area (Sweden) shows that the electrical resistivity distribution of sand and gravel formations which are only laterally bounded by conductive clay lenses is relatively well resolved whereas there is little resolution for the transition between the sand and gravel layer and the basement under a clay cover.
23.	Kalscheuer, Thomas, et al. (författare) A non-linear truncated SVD variance and resolution analysis of two-dimensional magnetotelluric models 2007 Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 169:2, s. 435-447 Tidskriftsartikel (refereegranskat)abstract A novel approach to assess variance and resolution properties of 2-D models of electrical resistivity derived from magnetotelluric measurements is presented. Based on a truncated singular value decomposition (TSVD) scheme on a local subspace, it partly takes the non-linearity of the inverse problem into account. The TSVD resolution and variance analysis is performed on a single cell at a time. A variance threshold is selected and the resulting model resolution is determined. As an improvement over existing schemes, non-linear semi-axes are introduced to describe the non-linear confidence surface in the directions of the model eigenvectors and they replace the inverse singular values entering into the standard expression of model variances. The model variance of the cell considered is estimated from the sum of squares of the non-linear semi-axes up to the given variance threshold. This, in turn, gives the truncation level of the TSVD and the row of the model resolution matrix belonging to the considered cell can be computed from the model eigenvectors of the TSVD. The information contained in the resolution matrix is condensed to easily comprehensible measures like the centre of resolution and horizontal and vertical resolution lengths.The validity of our non-linear model variance and resolution estimates is tested with a most-squares technique which gives an improved estimate of model variability.A synthetic model with a conductive block in a homogenous half-space is analysed. TSVD analyses for model cells on the upper edge of the block and outside the block illustrate how the truncation process works. Typically, the linear and non-linear semi-axes are almost equal up to a certain singular value number, after which the non-linear semi-axes increase much less than the linear semi-axes. This important result indicates that the resolution of 2-D magnetotelluric models is significantly better than previously suggested by linear schemes for the computation of model variance and resolution properties. A field example from the Skediga area (Sweden) shows that the electrical resistivity distribution of sand and gravel formations which are only laterally bounded by conductive clay lenses is relatively well resolved whereas there is little resolution for the transition between the sand and gravel layer and the basement under a clay cover.
24.	Kalscheuer, Thomas, et al. (författare) Electromagnetic evidence for an ancient avalanche caldera rim on the south flank of Mount Merapi, Indonesia 2007 Ingår i: Journal of Volcanology and Geothermal Research. - : Elsevier BV. - 0377-0273 .- 1872-6097. ; 162:1-2, s. 81-97 Tidskriftsartikel (refereegranskat)abstract Long-Offset Transient Electromagnetic (LOTEM) data and VIBROTEM data from the south flank of Mount Merapi on Java island, Indonesia, are interpreted with one-dimensional (1D) inversions as well as two-dimensional (2D) forward modelling. One-dimensional joint inversions of several components of the electromagnetic field with Occam's method reduce the number of equivalent models, which were derived from inversions of single components and fit the data to a similar misfit. The 1D results, together with results from other geophysical measurements, serve as the basic model for further 2D forward modelling. The final model depicts a layering that follows the topography of the strato-volcano. In the depth range of 500 m to 1000 m, the resistivity of the layers decreases rapidly downwards into a good conductor with resistivities below 10 Ohmm. The deepest layer has a resistivity of 0.4 Ohmm which is quantitatively explained with a combination of saline fluids and hydrothermally altered minerals. Furthermore, the final model supports a hypothesis from the interpretation of central-loop TEM (Transient Electromagnetic) data that there is a fault structure below the southern flank, approximately 7.3 km south of the summit. To the north of the fault, the top of the good conductor is lowered from a depth of 500 m to 1000 m. We propose that the fault structure coincides with an ancient avalanche caldera rim.
25.	Kalscheuer, Thomas, 1975- (författare) Improvement and Assessment of Two-Dimensional Resistivity Models Derived from Radiomagnetotelluric and Direct-Current Resistivity Data 2008 Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract Two-dimensional (2-D) models of electrical resistivity are improved by jointly inverting radiomagnetotelluric (RMT) and direct-current resistivity (DCR) data or by allowing for displacement currents in the inversion of RMT data collected on highly resistive bedrock. Uniqueness and stability of the 2-D models are assessed with a model variance and resolution analysis that allows for the non-linearity of the inverse problem.Model variance and resolution are estimated with a truncated singular value decomposition (TSVD) of the sensitivity matrix. In the computation of model errors, inverse singular values are replaced by non-linear semi-axes and the number of included eigenvectors is increased until a given error threshold is reached. Non-linear error estimates are verified with most-squares inversions. For the obtained truncation levels, model resolution matrices are computed. For RMT data, non-linear error appraisals are smaller than linearized ones. Hence, the consideration of the non-linearity in RMT data leads to reduced model errors or enhanced model resolution.The dielectric effect on RMT data is investigated with a new 2-D forward and inverse code that allows for displacement currents. As compared to the quasi-static approximation, apparent resistivities and phases of the impedance tensor elements are found to be significantly smaller and the vertical magnetic transfer function exhibits more distinct sign reversals. More reliable models of electrical resistivity are obtained from areas with highly resistive bedrock, if displacement currents are allowed for. In contrast, inversions with a quasi-static scheme introduce artefactual structures with extremely low or high resistivities.A smoothness-constrained 2-D joint inversion of RMT and DCR data is presented. The non-linear model variance and resolution analysis is applied to single and joint inverse models. For DCR data, the errors estimated by most-squares inversions are consistently larger than those estimated by the non-linear semi-axes, indicating that DCR models are poorly resolved. Certain areas of the joint inverse models are better resolved than in the single inverse models.
26.	Kalscheuer, Thomas, et al. (författare) Joint inversions of three types of electromagnetic data explicitly constrained by seismic observations : results from the central Okavango Delta, Botswana 2015 Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 202:3, s. 1429-1452 Tidskriftsartikel (refereegranskat)abstract The Okavango Delta of northern Botswana is one of the world’s largest inland deltas or megafans. To obtain information on the character of sediments and basement depths, audio-magnetotelluric (AMT), controlled-source audiomagnetotelluric (CSAMT) and central-loop transient electromagnetic (TEM) data were collected on the largest island within the delta. The data were inverted individually and jointly for 1-D models of electric resistivity. Distortion effects in the AMT and CSAMT data were accounted for by including galvanic distortion tensors as free parameters in the inversions. By employing Marquardt–Levenberg inversion, we found that a 3-layer model comprising a resistive layer overlying sequentially a conductive layer and a deeper resistive layer was sufficient to explain all of the electromagnetic data. However, the top of the basal resistive layer from electromagnetic-only inversions was much shallower than the well-determined basement depth observed in high-quality seismic reflection images and seismic refraction velocity tomograms. To resolve this discrepancy, we jointly inverted the electromagnetic data for 4-layer models by including seismic depths to an interface between sedimentary units and to basement as explicit a priori constraints. We have also estimated the interconnected porosities, clay contents and pore-fluid resistivities of the sedimentary units from their electrical resistivities and seismic P-wave velocities using appropriate petrophysical models. In the interpretation of our preferred model, a shallow ∼40 m thick freshwater sandy aquifer with 85–100 Ohmm resistivity, 10–32 per cent interconnected porosity and <13 per cent clay content overlies a 105–115 m thick conductive sequence of clay and intercalated salt-water-saturated sands with 15–20 Ohmm total resistivity, 1−27 per cent interconnected porosity and 15–60 per cent clay content. A third ∼60 m thick sandy layer with 40–50 Ohmm resistivity, 10–33 per cent interconnected porosity and <15 per cent clay content is underlain by the basement with 3200–4000 Ohmm total resistivity. According to an interpretation of helicopter TEM data that cover the entire Okavango Delta and borehole logs, the second and third layers may represent lacustrine sediments from Paleo Lake Makgadikgadi and a moderately resistive freshwater aquifer comprising sediments of the recently proposed Paleo Okavango Megafan, respectively.
27.	Kalscheuer, Thomas, et al. (författare) Non-linear model error and resolution properties from two-dimensional single and joint inversions of direct current resistivity and radiomagnetotelluric data 2010 Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 182:3, s. 1174-1188 Tidskriftsartikel (refereegranskat)abstract P>For the first time, a comparative analysis of the resolution and variance properties of 2-D models of electrical resistivity derived from single and joint inversions of dc resistivity (DCR) and radiomagnetotelluric (RMT) measurements is presented. DCR and RMT data are inverted with a smoothness-constrained 2-D scheme. Model resolution, model variance and data resolution analyses are performed both with a classical linearized scheme that employs the smoothness-constrained generalized inverse and a non-linear truncated singular value decomposition (TSVD). In the latter method, the model regularization used in the inversion is avoided and non-linear semi-axes give an approximate description of the non-linear confidence surface in the directions of the model eigenvectors. Hence, this method analyses the constraints that can be provided by the data. Model error estimates are checked against improved and independent estimates of model variability from most-squares inversions. For single and joint inverse models of synthetic data sets, the smoothness-constrained scheme suggests relatively small model errors (typically up to 30 to 40 per cent) and resolving kernels that are spread over several cells in the vicinity of the investigated cell. Linearized smoothness-constrained errors are in good agreement with the corresponding most-squares errors. The variability of the RMT model as estimated from non-linear semi-axes is confirmed by TSVD-based most-squares inversions for most model cells within the depth range of investigation. In contrast to this, most-squares errors of the DCR model are consistently larger than errors estimated from non-linear semi-axes except for the smallest truncation levels. The model analyses confirm previous studies that DCR data can constrain resistive and conductive structures equally well while RMT data provide superior constraints for conductive structures. The joint inversion can improve error and resolution of structures which are within the depth ranges of exploration of both methods. In such parts of the model which are outside the depth range of exploration for one method, error and resolution of the joint inverse model are close to those of the best single inversion result subject to an appropriate weighting of the different data sets.
28.	Kalscheuer, Thomas, 1975-, et al. (författare) Non-linear smoothness-constrained model error and resolution estimates 2010 Konferensbidrag (refereegranskat)
29.	Kalscheuer, Thomas, 1975-, et al. (författare) Radiomagnetotelluric Two-Dimensional Forward and Inverse Modelling Accounting for Displacement Currents 2008 Konferensbidrag (refereegranskat)
30.	Kalscheuer, Thomas, et al. (författare) Radiomagnetotelluric two-dimensional forward and inverse modelling accounting for displacement currents 2008 Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 175:2, s. 486-514 Tidskriftsartikel (refereegranskat)abstract Electromagnetic surface measurements with the radiomagnetotelluric (RMT) method in the frequency range between 10 and 300 kHz are typically interpreted in the quasi-static approximation, that is, assuming displacement currents are negligible. In this paper, the dielectric effect of displacement currents on RMT responses over resistive subsurface models is studied with a two-dimensional (2-D) forward and inverse scheme, that can operate both in the quasi-static approximation and including displacement currents.Forward computations of simple models exemplify how responses that allow for displacements currents deviate from responses computed in the quasi-static approximation. The differences become most obvious for highly resistive subsurface models of about 3000 Ohm*m and more and at high frequencies. For such cases, the apparent resistivities and phases of the transverse magnetic (TM) and transverse electric (TE) modes are significantly smaller than in the quasi-static approximation. Along profiles traversing 2-D subsurface models, sign reversals in the real part of the vertical magnetic transfer function (VMT) are often more pronounced than in the quasi-static approximation. On both sides of such sign reversals, the responses computed including displacement currents are larger than typical measurement errors.The 2-D inversion of synthetic data computed including displacement currents demonstrates that serious misinterpretations in the form of artefacts in inverse models can be made if displacement currents are neglected during the inversion. Hence, the inclusion of the dielectric effect is a crucial improvement over existing quasi-static 2-D inverse schemes. Synthetic data from a 2-D model with constant dielectric permittivity and a conductive block buried in a highly resistive layer which in turn is underlain by a conductive layer are inverted. In the quasi-static inverse model, the depth to the conductive structures is overestimated, artefactual resistors appear on both sides of the conductive block, and a spurious conductive layer is imaged at the surface.High-frequency RMT field data from Ävrö, Sweden, are re-interpreted using the newly developed 2-D inversion scheme which includes displacement currents. In contrast to previous quasi-static modelling, the new inverse models have electrical resistivity values comparable to a normal-resistivity borehole log and boundaries between resistive and conductive structures which correlate with the positions of seismic reflectors.
31.	Kalscheuer, Thomas, 1975-, et al. (författare) Two-dimensional audiomagnetotelluric and magnetotelluric modelling of ore deposits : Improvements in model constraints by inclusion of borehole measurements 2018 Ingår i: Surveys in geophysics. - : Springer Science and Business Media LLC. - 0169-3298 .- 1573-0956. ; 39:3, s. 467-507 Tidskriftsartikel (refereegranskat)abstract A combination of magnetotelluric (MT) measurements on the surface and in boreholes (without metal casing) can be expected to enhance resolution and reduce the ambiguity in models of electrical resistivity derived from MT surface measurements alone. In order to quantify potential improvement in inversion models and to aid design of electromagnetic (EM) borehole sensors, we considered two synthetic 2D models containing ore bodies down to 3000 m depth (the first with two dipping conductors in resistive crystalline host rock and the second with three mineralisation zones in a sedimentary succession exhibiting only moderate resistivity contrasts). We computed 2D inversion models from the forward responses based on combinations of surface impedance measurements and borehole measurements such as (1) skin-effect transfer functions relating horizontal magnetic fields at depth to those on the surface, (2) vertical magnetic transfer functions relating vertical magnetic fields at depth to horizontal magnetic fields on the surface and (3) vertical electric transfer functions relating vertical electric fields at depth to horizontal magnetic fields on the surface. Whereas skin-effect transfer functions are sensitive to the resistivity of the background medium and 2D anomalies, the vertical magnetic and electric field transfer functions have the disadvantage that they are comparatively insensitive to the resistivity of the layered background medium. This insensitivity introduces convergence problems in the inversion of data from structures with strong 2D resistivity contrasts. Hence, we adjusted the inversion approach to a three-step procedure, where (1) an initial inversion model is computed from surface impedance measurements, (2) this inversion model from surface impedances is used as the initial model for a joint inversion of surface impedances and skin-effect transfer functions and (3) the joint inversion model derived from the surface impedances and skin-effect transfer functions is used as the initial model for the inversion of the surface impedances, skin-effect transfer functions and vertical magnetic and electric transfer functions. For both synthetic examples, the inversion models resulting from surface and borehole measurements have higher similarity to the true models than models computed exclusively from surface measurements. However, the most prominent improvements were obtained for the first example, in which a deep small-sized ore body is more easily distinguished from a shallow main ore body penetrated by a borehole and the extent of the shadow zone (a conductive artefact) underneath the main conductor is strongly reduced. Formal model error and resolution analysis demonstrated that predominantly the skin-effect transfer functions improve model resolution at depth below the sensors and at distance of similar to 300-1000 m laterally off a borehole, whereas the vertical electric and magnetic transfer functions improve resolution along the borehole and in its immediate vicinity. Furthermore, we studied the signal levels at depth and provided specifications of borehole magnetic and electric field sensors to be developed in a future project. Our results suggest that three-component SQUID and fluxgate magnetometers should be developed to facilitate borehole MT measurements at signal frequencies above and below 1 Hz, respectively.
32.	Kalscheuer, Thomas, 1975-, et al. (författare) Two-Dimensional Single and Joint Inversion of Direct Current Resistivity and Radiomagnetotelluric Data: Comparison of Non-linear Model Variance and Resolution Properties 2008 Konferensbidrag (refereegranskat)
33.	Meier, P., et al. (författare) Hydrogeophysical investigations in the western and north-central Okavango Delta (Botswana) based on helicopter and ground-based transient electromagnetic data and electrical resistance tomography 2014 Ingår i: Geophysics. - : Society of Exploration Geophysicists. - 0016-8033 .- 1942-2156. ; 79:5, s. B201-B211 Tidskriftsartikel (refereegranskat)abstract The Okavango Delta is a huge alluvial megafan in northwestern Botswana. Despite numerous geologic, geochemical, geophysical, and hydrologic investigations over the past half-century, the sedimentary units underlying the delta are largely unknown. To address this issue, helicopter transient electromagnetic data (HTEM) have been collected across the entire delta and coincident ground-based electrical resistance tomographic (ERT) and transient electromagnetic (TEM) data have been acquired at two locations, one along the delta’s western margin and one in its north-central region. Inversions of the HTEM data have yielded three-layer resistivity models in which a relatively homogeneous conductive layer is sandwiched between two resistive layers. The three-layer HTEM model is reproduced in models obtained from independently and jointly inverting the ground-based data. The conductive layer’s low resistivities and depths to its upper and lower boundaries are practically equal in the HTEM and ground-based models. Resistivities of the upper resistive layer are similar in the various models, with the ground-based estimates being somewhat higher than those of the HTEM model at one site and somewhat lower at the other site. For the basal resistive layer, it can only be concluded that its resistivity must be substantially higher than that of the overlying conductive layer. An interpretation of the HTEM and ground-based resistivity models in the delta’s north-central region, appropriately constrained by the surface geology, high-resolution seismic refraction-reflection models, and borehole logs suggests the following structure: basement overlain at progressively shallower depths by freshwater-saturated sand and gravel that represent the remnants of a Paleo Okavango Megafan, saline-water-saturated sand, and lacustrine clay originally deposited in Paleo Lake Makgadikgadi, and freshwater-saturated megafan and fluvial sediments of the current Okavango Delta.
34.	Podgorski, Joel E., et al. (författare) Integrated interpretation of helicopter and ground-based geophysical data recorded within the Okavango Delta, Botswana 2015 Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 114, s. 52-67 Tidskriftsartikel (refereegranskat)abstract Integration of information from the following sources has been used to produce a much better constrained and more complete four-unit geological/hydrological model of the Okavango Delta than previously available: (i) a 3D resistivity model determined from helicopter time-domain electromagnetic (HTEM) data recorded across most of the delta, (ii) 2D models and images derived from ground-based electrical resistance tomographic, transient electromagnetic, and high resolution seismic reflection/refraction tomographic data acquired at four selected sites in western and north-central regions of the delta, and (iii) geological details extracted from boreholes in northeastern and southeastern parts of the delta. The upper heterogeneous unit is the modern delta, which comprises extensive dry and freshwater-saturated sand and lesser amounts of clay and salt. It is characterized by moderate to high electrical resistivities and very low to low P-wave velocities. Except for images of several buried abandoned river channels, it is non-reflective. The laterally extensive underlying unit of low resistivities, low P-wave velocity, and subhorizontal reflectors very likely contains saline-water-saturated sands and clays deposited in the huge Paleo Lake Makgadikgadi (PLM), which once covered a 90,000 km(2) area that encompassed the delta, Lake Ngami, the Mababe Depression, and the Makgadikgadi Basin. Examples of PLM sediments are intersected in many boreholes. Low permeability clay within the PLM unit seems to be a barrier to the downward flow of the saline water. Below the PLM unit, freshwater-saturated sand of the Paleo Okavango Megafan (POM) unit is distinguished by moderate to high resistivities, low P-wave velocity, and numerous subhorizontal reflectors. The POM unit is interpreted to be the remnants of a megafan based on the arcuate nature of its front and the semi-conical shape of its upper surface in the HTEM resistivity model. Moderate to high resistivity subhorizontal layers are consistent with this interpretation. The deepest unit is the basement with very high resistivity, high P-wave velocity, and low or complex reflectivity. The interface between the POM unit and basement is a prominent seismic reflector. (C) 2015 Elsevier B.V. All rights reserved.
35.	Podgorski, Joel E., et al. (författare) Paleo-megalake and paleo-megafan in southern Africa 2013 Ingår i: Geology. ; 41:11, s. 1155-1158 Tidskriftsartikel (refereegranskat)
36.	Podgorski, Joel E., et al. (författare) Processing and inversion of commercial helicopter time-domain electromagnetic data for environmental assessments and geologic and hydrologic mapping 2013 Ingår i: Geophysics. - 0016-8033 .- 1942-2156. ; 78:4, s. E149-E159 Tidskriftsartikel (refereegranskat)abstract Helicopter time-domain electromagnetic (HTEM) surveying has historically been used for mineral exploration, but over the past decade it has started to be used in environmental assessments and geologic and hydrologic mapping. Such surveying is a cost-effective means of rapidly acquiring densely spaced data over large regions. At the same time, the quality of HTEM data can suffer from various inaccuracies. We developed an effective strategy for processing and inverting a commercial HTEM data set affected by uncertainties and systematic errors. The delivered data included early time gates contaminated by transmitter currents, noise in late time gates, and amplitude shifts between adjacent flights that appeared as artificial lineations in maps of the data and horizontal slices extracted from inversion models. Multiple processing steps were required to address these issues. Contaminated early time gates and noisy late time gates were semiautomatically identified and eliminated on a record-by-record basis. Timing errors between the transmitter and receiver electronics and inaccuracies in absolute amplitudes were corrected after calibrating selected HTEM data against data simulated from accurate ground-based TEM measurements. After editing and calibration, application of a quasi-3D spatially constrained inversion scheme significantly reduced the artificial lineations. Residual lineations were effectively eliminated after incorporating the transmitter and receiver altitudes and line-to-line amplitude factors in the inversion process. The final inverted model was very different from that generated from the original data provided by the contractor. For example, the average resistivity of the thick surface layer decreased from similar to 1800 to similar to 30 Omega m, the depths to the layer boundaries were reduced by 15%-23%, and the artificial lineations were practically eliminated. Our processing and inversion strategy is entirely general, such that with minor system-specific modifications it could be applied to any HTEM data set, including those recorded many years ago.
37.	Reiser, Fabienne, et al. (författare) Constraining helicopter electromagnetic models of the Okavango Delta with seismic-refraction and seismic-reflection data 2014 Ingår i: Geophysics. - 0016-8033 .- 1942-2156. ; 79:3, s. B123-B134 Tidskriftsartikel (refereegranskat)abstract Electrical resistivity models derived from exceptionally high-quality helicopter transient electromagnetic data recorded across the Okavango Delta in Botswana, one of the world's great inland deltas or megafans, include three principal layers: (1) an upper heterogeneous layer of dry and water-saturated sand, (2) an intermediate electrically conductive layer that likely comprises saline-water-saturated sand and clay, and (3) a lower fan-shaped electrically resistive layer of freshwater-saturated sand/gravel and/or crystalline basement. If part of the lower layer comprises a freshwater aquifer, it would be evidence for a recently proposed Paleo Okavango Megafan and a major new source of freshwater. In an attempt to constrain the interpretation of the lower layer, we acquired two high-resolution seismic refraction and reflection data sets at each of two investigation sites: one near the center of the delta and one along its western edge. The interface between unconsolidated sediments and basement near the center of the delta is well defined by an similar to 1800 to similar to 4500 m/s increase in P-wave velocities, a change in seismic reflection facies, and a strong continuous reflection. This interface is about 45 m deeper than the top of the lower resistive layer, thus providing support for the Paleo Okavango Megafan hypothesis. Subhorizontal seismic reflectors are additional evidence for a sedimentary origin of the upper part of the lower resistive layer. In contrast to the observations at the delta's center, the interface between unconsolidated sediments and basement along its western edge, which is also defined by a similar to 1800 to similar to 4500 m/s increase in P-wave velocities and a continuous reflection, coincides with the top of the resistive layer.
38.	Ren, Zhengyong, et al. (författare) A finite-element-based domain-decomposition approach for plane wave 3D electromagnetic modeling 2014 Ingår i: Geophysics. - : Society of Exploration Geophysicists. - 0016-8033 .- 1942-2156. ; 79:6, s. E255-E268 Tidskriftsartikel (refereegranskat)abstract We developed a novel parallel domain-decomposition approach for 3D large-scale electromagnetic induction modeling in the earth. We used the edge-based finite-element method and unstructured meshes. Unstructured meshes were divided into sets of nonoverlapping subdomains. We used the curl-curl electric field equation to carry out the analysis. In each subdomain, the electric field was discretized by first-order vector shape functions along the edges of tetrahedral elements. The tangential components of the magnetic field on the interfaces of the subdomains were defined as a set of Lagrange multipliers. The unknown Lagrange multipliers were solved from an interface problem defined on the interfaces of the subdomains. With the availability of the Lagrange multipliers, the electric field values in each subdomain were solved independently. Three synthetic examples were evaluated to verify our code. Excellent agreement with previously published solutions was obtained. Synthetic examples revealed that our domain decomposition technique is scalable with respect to the number of subdomains and robust with regard to frequency and the heterogeneous distribution of material parameters, i.e., electric conductivity, electric permittivity, and magnetic permeability.
39.	Ren, Zhengyong, et al. (författare) A goal-oriented adaptive finite-element approach for plane wave 3-D electromagnetic modelling 2013 Ingår i: Geophysical Journal International. ; 194:2, s. 700-718 Tidskriftsartikel (refereegranskat)
40.	Ren, Zhengyong, et al. (författare) A hybrid boundary element-finite element approach to modeling plane wave 3D electromagnetic induction responses in the Earth 2014 Ingår i: Journal of Computational Physics. - : Elsevier BV. - 0021-9991 .- 1090-2716. ; 258, s. 705-717 Tidskriftsartikel (refereegranskat)
41.	Ren, Zhengyong, et al. (författare) Accurate and robust evaluation of singularboundary element integrals for computing 3D radiomagnetotelluric fields 2010 Konferensbidrag (refereegranskat)
42.	Ren, Zhengyong, et al. (författare) Boundary element solutions for broad-band 3-D geo-electromagnetic problems accelerated by an adaptive multilevel fast multipole method 2013 Ingår i: Geophysical Journal International. ; 192:2, s. 473-499 Tidskriftsartikel (refereegranskat)
43.	Ren, Zhengyong, et al. (författare) Fast 3-D large-scale gravity and magnetic modeling using unstructured grids and an adaptive multilevel fast multipole method 2017 Ingår i: Journal of Geophysical Research - Solid Earth. - : American Geophysical Union (AGU). - 2169-9313 .- 2169-9356. ; 122:1, s. 79-109 Tidskriftsartikel (refereegranskat)abstract A novel fast and accurate algorithm is developed for large-scale 3-D gravity and magnetic modeling problems. An unstructured grid discretization is used to approximate sources with arbitrary mass and magnetization distributions. A novel adaptive multilevel fast multipole (AMFM) method is developed to reduce the modeling time. An observation octree is constructed on a set of arbitrarily distributed observation sites, while a source octree is constructed on a source tetrahedral grid. A novel characteristic is the independence between the observation octree and the source octree, which simplifies the implementation of different survey configurations such as airborne and ground surveys. Two synthetic models, a cubic model and a half-space model with mountain-valley topography, are tested. As compared to analytical solutions of gravity and magnetic signals, excellent agreements of the solutions verify the accuracy of our AMFM algorithm. Finally, our AMFM method is used to calculate the terrain effect on an airborne gravity data set for a realistic topography model represented by a triangular surface retrieved from a digital elevation model. Using 16 threads, more than 5800 billion interactions between 1,002,001 observation points and 5,839,830 tetrahedral elements are computed in 453.6s. A traditional first-order Gaussian quadrature approach requires 3.77days. Hence, our new AMFM algorithm not only can quickly compute the gravity and magnetic signals for complicated problems but also can substantially accelerate the solution of 3-D inversion problems.
44.	Ren, Zhengyong, et al. (författare) Gravity Anomalies of Arbitrary 3D Polyhedral Bodies with Horizontal and Vertical Mass Contrasts 2017 Ingår i: Surveys in geophysics. - : SPRINGER. - 0169-3298 .- 1573-0956. ; 38:2, s. 479-502 Forskningsöversikt (refereegranskat)abstract During the last 15 years, more attention has been paid to derive analytic formulae for the gravitational potential and field of polyhedral mass bodies with complicated polynomial density contrasts, because such formulae can be more suitable to approximate the true mass density variations of the earth (e.g., sedimentary basins and bedrock topography) than methods that use finer volume discretization and constant density contrasts. In this study, we derive analytic formulae for gravity anomalies of arbitrary polyhedral bodies with complicated polynomial density contrasts in 3D space. The anomalous mass density is allowed to vary in both horizontal and vertical directions in a polynomial form of , where m, n, t are nonnegative integers and a, b, c are coefficients of mass density. First, the singular volume integrals of the gravity anomalies are transformed to regular or weakly singular surface integrals over each polygon of the polyhedral body. Then, in terms of the derived singularity-free analytic formulae of these surface integrals, singularity-free analytic formulae for gravity anomalies of arbitrary polyhedral bodies with horizontal and vertical polynomial density contrasts are obtained. For an arbitrary polyhedron, we successfully derived analytic formulae of the gravity potential and the gravity field in the case of , , , and an analytic formula of the gravity potential in the case of . For a rectangular prism, we derive an analytic formula of the gravity potential for , and and closed forms of the gravity field are presented for , and . Besides generalizing previously published closed-form solutions for cases of constant and linear mass density contrasts to higher polynomial order, to our best knowledge, this is the first time that closed-form solutions are presented for the gravitational potential of a general polyhedral body with quadratic density contrast in all spatial directions and for the vertical gravitational field of a prismatic body with quartic density contrast along the vertical direction. To verify our new analytic formulae, a prismatic model with depth-dependent polynomial density contrast and a polyhedral body in the form of a triangular prism with constant contrast are tested. Excellent agreements between results of published analytic formulae and our results are achieved. Our new analytic formulae are useful tools to compute gravity anomalies of complicated mass density contrasts in the earth, when the observation sites are close to the surface or within mass bodies.
45.	Ren, Zhengyong, et al. (författare) Gravity Gradient Tensor of Arbitrary 3D Polyhedral Bodies with up to Third-Order Polynomial Horizontal and Vertical Mass Contrasts 2018 Ingår i: Surveys in geophysics. - : Springer. - 0169-3298 .- 1573-0956. ; 39:5, s. 901-935 Forskningsöversikt (refereegranskat)abstract During the last 20 years, geophysicists have developed great interest in using gravity gradient tensor signals to study bodies of anomalous density in the Earth. Deriving exact solutions of the gravity gradient tensor signals has become a dominating task in exploration geophysics or geodetic fields. In this study, we developed a compact and simple framework to derive exact solutions of gravity gradient tensor measurements for polyhedral bodies, in which the density contrast is represented by a general polynomial function. The polynomial mass contrast can continuously vary in both horizontal and vertical directions. In our framework, the original three-dimensional volume integral of gravity gradient tensor signals is transformed into a set of one-dimensional line integrals along edges of the polyhedral body by sequentially invoking the volume and surface gradient (divergence) theorems. In terms of an orthogonal local coordinate system defined on these edges, exact solutions are derived for these line integrals. We successfully derived a set of unified exact solutions of gravity gradient tensors for constant, linear, quadratic and cubic polynomial orders. The exact solutions for constant and linear cases cover all previously published vertex-type exact solutions of the gravity gradient tensor for a polygonal body, though the associated algorithms may differ in numerical stability. In addition, to our best knowledge, it is the first time that exact solutions of gravity gradient tensor signals are derived for a polyhedral body with a polynomial mass contrast of order higher than one (that is quadratic and cubic orders). Three synthetic models (a prismatic body with depth-dependent density contrasts, an irregular polyhedron with linear density contrast and a tetrahedral body with horizontally and vertically varying density contrasts) are used to verify the correctness and the efficiency of our newly developed closed-form solutions. Excellent agreements are obtained between our solutions and other published exact solutions. In addition, stability tests are performed to demonstrate that our exact solutions can safely be used to detect shallow subsurface targets.
46.	Ren, Zhengyong, et al. (författare) Recursive Analytical Formulae of Gravitational Fields and Gradient Tensors for Polyhedral Bodies with Polynomial Density Contrasts of Arbitrary Non-negative Integer Orders 2020 Ingår i: Surveys in geophysics. - : SPRINGER. - 0169-3298 .- 1573-0956. ; 41:4, s. 695-722 Tidskriftsartikel (refereegranskat)abstract Exact computation of the gravitational field and gravitational gradient tensor for a general mass body is a core routine to model the density structure of the Earth. In this study, we report on the existence of closed-form solutions of the gravitational potential, gravitational field and gravitational gradient tensor for a general polyhedral mass body with a polynomial density function of arbitrary non-negative integer orders that can simultaneously vary in both horizontal and vertical directions. Our closed-form solutions of the gravitational potential and the gravitational field are singularity-free, which implies that the observation sites can have arbitrary geometric relationships with polyhedral mass source bodies. However, weak logarithmic singularities exist on the edges of polyhedra for the gravitational gradient tensor. A simple prismatic mass body with polynomial density contrast varying in the vertical direction and a complicated dodecahedral mass body with quartic-order density contrasts were tested to verify the accuracy of the newly derived closed-form solutions. For the gravitational potential, gravitational fields and gradient tensors, our closed-form solutions are in excellent agreement with previously published analytical solutions and Gaussian numerical quadrature solutions.
47.	Ren, Zhengyong, et al. (författare) Uncertainty and Resolution Analysis of 2D and 3D Inversion Models Computed from Geophysical Electromagnetic Data 2020 Ingår i: Surveys in geophysics. - : SPRINGER. - 0169-3298 .- 1573-0956. ; 41:1, s. 47-112 Tidskriftsartikel (refereegranskat)abstract A meaningful solution to an inversion problem should be composed of the preferred inversion model and its uncertainty and resolution estimates. The model uncertainty estimate describes an equivalent model domain in which each model generates responses which fit the observed data to within a threshold value. The model resolution matrix measures to what extent the unknown true solution maps into the preferred solution. However, most current geophysical electromagnetic (also gravity, magnetic and seismic) inversion studies only offer the preferred inversion model and ignore model uncertainty and resolution estimates, which makes the reliability of the preferred inversion model questionable. This may be caused by the fact that the computation and analysis of an inversion model depend on multiple factors, such as the misfit or objective function, the accuracy of the forward solvers, data coverage and noise, values of trade-off parameters, the initial model, the reference model and the model constraints. Depending on the particular method selected, large computational costs ensue. In this review, we first try to cover linearised model analysis tools such as the sensitivity matrix, the model resolution matrix and the model covariance matrix also providing a partially nonlinear description of the equivalent model domain based on pseudo-hyperellipsoids. Linearised model analysis tools can offer quantitative measures. In particular, the model resolution and covariance matrices measure how far the preferred inversion model is from the true model and how uncertainty in the measurements maps into model uncertainty. We also cover nonlinear model analysis tools including changes to the preferred inversion model (nonlinear sensitivity tests), modifications of the data set (using bootstrap re-sampling and generalised cross-validation), modifications of data uncertainty, variations of model constraints (including changes to the trade-off parameter, reference model and matrix regularisation operator), the edgehog method, most-squares inversion and global searching algorithms. These nonlinear model analysis tools try to explore larger parts of the model domain than linearised model analysis and, hence, may assemble a more comprehensive equivalent model domain. Then, to overcome the bottleneck of computational cost in model analysis, we present several practical algorithms to accelerate the computation. Here, we emphasise linearised model analysis, as efficient computation of nonlinear model uncertainty and resolution estimates is mainly determined by fast forward and inversion solvers. In the last part of our review, we present applications of model analysis to models computed from individual and joint inversions of electromagnetic data; we also describe optimal survey design and inversion grid design as important applications of model analysis. The currently available model uncertainty and resolution analyses are mainly for 1D and 2D problems due to the limitations in computational cost. With significant enhancements of computing power, 3D model analyses are expected to be increasingly used and to help analyse and establish confidence in 3D inversion models.
48.	Rosas Carbajal, Marina, et al. (författare) Focused time-lapse inversion of radio and audio magnetotelluric data 2012 Ingår i: Journal of Applied Geophysics. ; 84, s. 29-38 Tidskriftsartikel (refereegranskat)
49.	Rosas-Carbajal, M., et al. (författare) Probabilistic 3-D time-lapse inversion of magnetotelluric data: application to an enhanced geothermal system 2015 Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 203:3, s. 1946-1960 Tidskriftsartikel (refereegranskat)abstract Surface-based monitoring of mass transfer caused by injections and extractions in deep boreholes is crucial to maximize oil, gas and geothermal production. Inductive electromagnetic methods, such as magnetotellurics, are appealing for these applications due to their large penetration depths and sensitivity to changes in fluid conductivity and fracture connectivity. In this work, we propose a 3-D Markov chain Monte Carlo inversion of time-lapse magnetotelluric data to image mass transfer following a saline fluid injection. The inversion estimates the posterior probability density function of the resulting plume, and thereby quantifies model uncertainty. To decrease computation times, we base the parametrization on a reduced Legendre moment decomposition of the plume. A synthetic test shows that our methodology is effective when the electrical resistivity structure prior to the injection is well known. The centre of mass and spread of the plume are well retrieved. We then apply our inversion strategy to an injection experiment in an enhanced geothermal system at Paralana, South Australia, and compare it to a 3-D deterministic time-lapse inversion. The latter retrieves resistivity changes that are more shallow than the actual injection interval, whereas the probabilistic inversion retrieves plumes that are located at the correct depths and oriented in a preferential northâ€“south direction. To explain the time-lapse data, the inversion requires unrealistically large resistivity changes with respect to the base model. We suggest that this is partly explained by unaccounted subsurface heterogeneities in the base model from which time-lapse changes are inferred.
50.	Rosas-Carbajal, Marina, et al. (författare) Two-dimensional probabilistic inversion of plane-wave electromagnetic data : methodology, model constraints and joint inversion with electrical resistivity data 2014 Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 1365-246X .- 0956-540X. ; 196:3, s. 1508-1524 Tidskriftsartikel (refereegranskat)abstract Probabilistic inversion methods based on Markov chain Monte Carlo (MCMC) simulation are well suited to quantify parameter and model uncertainty of nonlinear inverse problems. Yet, application of such methods to CPU-intensive forward models can be a daunting task, particularly if the parameter space is high dimensional. Here, we present a 2-D pixel-based MCMC inversion of plane-wave electromagnetic (EM) data. Using synthetic data, we investigate how model parameter uncertainty depends on model structure constraints using different norms of the likelihood function and the model constraints, and study the added benefits of joint inversion of EM and electrical resistivity tomography (ERT) data. Our results demonstrate that model structure constraints are necessary to stabilize the MCMC inversion results of a highly discretized model. These constraints decrease model parameter uncertainty and facilitate model interpretation. A drawback is that these constraints may lead to posterior distributions that do not fully include the true underlying model, because some of its features exhibit a low sensitivity to the EM data, and hence are difficult to resolve. This problem can be partly mitigated if the plane-wave EM data is augmented with ERT observations. The hierarchical Bayesian inverse formulation introduced and used herein is able to successfully recover the probabilistic properties of the measurement data errors and a model regularization weight. Application of the proposed inversion methodology to field data from an aquifer demonstrates that the posterior mean model realization is very similar to that derived from a deterministic inversion with similar model constraints.

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