<p>Electromagnetic forward modeling and inversion methods have extensive applications in geophysical exploration, and large-scale controlled-source electromagnetic method has recently drawed lots of attention. However, to obtain a rigorous and efficient forward solver for this large-scale three-dimensional problem is difficult, since it usually requires to solve for a large number of unknowns from a system of equations describing the complicate scattering behavior of electromagnetic waves that happened within inhomogeneous media. As for the development of an efficient inversion solver, because of the nonlinear, non-unique and ill-posed properties of the problem, it is also a very challenging task. </p><p>In the first part of this dissertation, a fast three-dimensional nonlinear reconstruction method is proposed for controlled-source electromagnetic method. The borehole-to-surface and airborne electromagnetic survey methods are investigated using synthetic data. In this work, it is assumed that there is only electric contrast between the inhomogeneous object and the layered background medium, for the reason that the electric contrast is much more dominant than magnetic contrast in most cases of the earth formation. Therefore, only the EFIE is needed to solve. While the forward scattering problem is solved by the stabilized bi-conjugate gradient FFT (BCGS-FFT) method to give a rigorous and efficient modeling, the Bore iterative method along with the multiplicative regularization technique is used in the inversion through frequency hopping. In the inversion, to speed up the expensive computation of the sensitivity matrix relating every receiver station to every unknown element, a fast field evaluation (FFE) technique is proposed using the symmetry property of the layered medium Green's function combined with a database strategy. The conjugate-gradient method is then applied to minimize the cost function after each iteration. Due to the benefits of using 3D FFT acceleration, the proposed FFE technique as well as the recursive matrix method combined with an interpolation technique to evaluate the LMGF, the developed inversion solver is highly efficient, and requires very low computation time and memory. Numerical experiments for both 3D forward modeling and conductivity inversion are presented to show the accuracy and efficiency of the method. </p><p>Some recent research on artificial nanoparticles have demonstrated the improved performance in geophysical imaging using magnetodielectric materials with enhanced electric and magnetic contrasts. This gives a promising perspective to the future geophysical exploration by infusing well-designed artificial magnetodielectric materials into the subsurface objects, so that a significantly improved imaging can be achieved. As a preparation for this promising application, the second part of the dissertation presents an efficient method to solve the scattering problem of magnetodielectric materials with general anisotropy that are embedded in layered media. In this work, the volume integral equation is chosen as the target equation to solve, since it solves for fields in inhomogeneous media with less number of unknowns than the finite element method. However, for complicated materials as magnetodielectric materials with general anisotropy, it is a very challenging task, because it requires to simultaneously solve the electric field integral equation (EFIE) and magnetic field integral equation (MFIE). Besides that, the numerous evaluation of the layered medium Green's function (LMGF) for the stratified background formation adds on the difficulty and complexity of the problem. To my knowledge, there is no existing fast solver for the similar problem. In this dissertation, using the mixed order stabilized biconjugate-gradient fast Fourier transform (mixed-order BCGS-FFT) method, a fast forward modeling method is developed to solve this challenging problem. Several numerical examples are performed to validate the accuracy and efficiency of the proposed method.</p><p> </p><p>Besides the above mentioned two topics, one-dimensional inversion method is presented in the third part to determine the tilted triaxial conductivity tensor in a dipping layered formation using triaxial induction measurements. The tilted triaxial conductivity tensor is described by three conductivity components and three Euler angles. Based on my knowledge, due to the highly nonlinear and ill-posed nature of the inverse problem, this study serves as the first work that investigates on the subject. There are six principal coordinate systems that can give the same conductivity tensor. Permutation is performed to eliminate the ambiguity of inversion results caused by the ambiguity of the principal coordinate system. Three new Euler angles after permutation for each layer can be found by solving a nonlinear equation. Numerical experiments are conducted on synthetic models to study the feasibility of determining triaxially anisotropic conductivity tensor from triaxial induction data. This project is accomplished during my internship in the Houston Formation Evaluation Integration Center (HFE) at Schlumberger, a world-leading oilfield service company.</p> / Dissertation

The Master's thesis "Geophysical survey in the area of the mediaeval fortress in Popovice" is divided into two parts. The first part is theoretical and summarizes the possibilities of using electrical resistivity tomography method (ERT) in archaeology. The second part deals with the actual geophysical prospecting in the area of the mediaeval fortress in Popovice (Benešov county). General information on the locality including geological setting, past geophysical exploration in the area, history of the object, archaeological findings and overview of construction development are addressed in this part, prior to the description of the field measurements. Methods used on the site are then specified: symmetrical resistivity profiling, dipole electromagnetic profiling, electrical resistivity tomography and ground penetrating radar. Subsequently, the process of field measuring in the area of the fortress is described and the data processing methods are discussed. The results of these measurements and their interpretation on the basis of compiled geophysical maps and sections are presented finally. Key words: geophysical survey, archaeology, fortress in Popovice (Benešov county).

The aim of the research was to provide new sea-level index points,with the intention of identifying any cross-channel similarities and comparing the results to geophysical models of relative sea-level change. The research successfully provided new sea-level index points from the Pevensey levels, East Sussex; the Canche Estuary, Pas de Calais and the Somme Estuary, Picardie, which were validated using stratigraphic pollen,diatom,foraminiferal snd AMS radiocarbon dating analysis. The results pointed to the presence of a coastal barrier thoughout the mid-Holocene at Pevensey and the Somme which complicated the pattern of coastal sedimentation observed at these sites. A clear pattern of barrier de-stabilisation can be seen to take place either side of the Channel c.3000 years cal BP. A cross-channel comparison identified three similar transgressive events either side of the English Channel, c. 5500, 3000 and 2200 cal years BP. The observed results from each site were then compared to the predicted data( Lambeck pers.comm) .in order to determine whether the observational data could be used to validate the modelled data. The comparisons showed that for the sites in south east England the modelled data tended to over-predict sea-level rise for the mid-t o late- Holocene whereas the model tended to under-predict sea-level rise for the sites in northern France. The new observational data which the research provided could be used to further validate Lambeck's (1990,1997) geophysical model. The effects of local coastal processes, such as tidal range, crustal subsidence and barrier-dynamics were used to aid the comparisons between the pattern of sediment deposition and thus the sea-level signals. These findings suggested that reconstructions should be restricted to sites at an estuary-sized scale.

Abstract not available

The self-potential method responds to the electrokinetic phenomenon of streaming potential and has been applied in hydrogeologic and engineering investigations to aid in the evaluation of subsurface hydraulic conditions. Of specific interest is the application of the method to embankment dam seepage monitoring and detection. This demands a quantitative interpretation of seepage conditions from the geophysical data. To enable the study of variably saturated flow problems of complicated geometry, a three-dimensional finite volume algorithm is developed to evaluate the self-potential distribution resulting from subsurface fluid flow. The algorithm explicitly calculates the distribution of streaming current sources and solves for the self-potential given a model of hydraulic head and prescribed distributions of the streaming current cross-coupling conductivity and electrical resistivity. A new laboratory apparatus is developed to measure the streaming potential coupling coefficient and resistivity in unconsolidated soil samples. Measuring both of these parameters on the same sample under the same conditions enables us to properly characterize the streaming current cross-coupling conductivity coefficient. I present the results of a laboratory investigation to study the influence of soil and fluid parameters on the cross-coupling coefficient, and characterize this property for representative well-graded embankment soils. The streaming potential signals associated with preferential seepage through the core of a synthetic embankment dam model are studied using the forward modelling algorithm and measured electrical properties to assess the sensitivity of the self-potential method in detecting internal erosion. Maximum self-potential anomalies are shown to be linked to large localized hydraulic gradients that develop in response to piping, prior to any detectable increase in seepage flow through the dam. A linear inversion algorithm is developed to evaluate the three-dimensional distribution of hydraulic head from self-potential data, given a known distribution of the cross-coupling coefficient and electrical resistivity. The inverse problem is solved by minimizing an objective function, which consists of a data misfit that accounts for measurement error and a model objective function that incorporates a priori information. The algorithm is suitable for saturated flow problems or where the position of the phreatic surface is known.

IGeoS is an integrated open-source software framework for geophysical data processing under development at the UofS seismology group. Unlike other systems, this processing monitor supports structured multicomponent seismic data streams, multidimensional data traces, and employs a unique backpropagation execution logic. This results in an unusual flexibility of processing, allowing the system to handle nearly any geophysical data.<p> In this project, a modern and feature-rich Graphical User Interface (GUI) was developed for the system, allowing editing and submission of processing flows and interaction with running jobs. Multiple jobs can be executed in a distributed multi-processor networks and controlled from the same GUI. Jobs, in their turn, can also be parallelized to take advantage of parallel processing environments such as local area networks and Beowulf clusters.<p> A 3D/2D interactive display server was created and integrated with the IGeoS geophysical data processing framework. With introduction of this major component, the IGeoS system becomes conceptually complete and potentially bridges the gap between the traditional processing and interpretation software.<p> Finally, in a specialized application, network acquisition and relay components were written allowing IGeoS to be used for real-time applications. The completion of this functionality makes the processing and display capabilities of IGeoS available to multiple streams of seismic data from potentially remote sites. Seismic data can be acquired, transferred to the central server, processed, archived, and events picked and placed in database completely automatically.

IGeoS is an integrated open-source software framework for geophysical data processing under development at the UofS seismology group. Unlike other systems, this processing monitor supports structured multicomponent seismic data streams, multidimensional data traces, and employs a unique backpropagation execution logic. This results in an unusual flexibility of processing, allowing the system to handle nearly any geophysical data.<p> In this project, a modern and feature-rich Graphical User Interface (GUI) was developed for the system, allowing editing and submission of processing flows and interaction with running jobs. Multiple jobs can be executed in a distributed multi-processor networks and controlled from the same GUI. Jobs, in their turn, can also be parallelized to take advantage of parallel processing environments such as local area networks and Beowulf clusters.<p> A 3D/2D interactive display server was created and integrated with the IGeoS geophysical data processing framework. With introduction of this major component, the IGeoS system becomes conceptually complete and potentially bridges the gap between the traditional processing and interpretation software.<p> Finally, in a specialized application, network acquisition and relay components were written allowing IGeoS to be used for real-time applications. The completion of this functionality makes the processing and display capabilities of IGeoS available to multiple streams of seismic data from potentially remote sites. Seismic data can be acquired, transferred to the central server, processed, archived, and events picked and placed in database completely automatically.

In the northern Gulf of Mexico, seafloor hydrocarbon fluid and gas seepage is an ubiquitous process on the continental margin. Although seafloor seepage and seep-related features (mud volcanoes, carbonate formation) have been studied for many years, little is known about their mechanisms of formation and the relationship of sub-surface structure to current seep activity. In this study, we examined three seafloor seeps in the Garden Banks and Mississippi Canyon areas using exploration and reprocessed 3D multi-channel seismic (MCS) data augmented with side-scan sonar (Garden Banks site) to characterize hydrocarbon seep activity and develop an understanding of the processes that led to their formation. Side-scan sonar data provided high resolution coverage of the seafloor while the exploration seismic data were used to image near and deep sub-surface features. Additionally, the 3D amplitude extraction maps were useful in delineating amplitude anomalies often associated with seep related activity. The reprocessed 3D seismic data were used to map in greater detail near seafloor features and amplitude anomalies. Using remote sensing geophysical data, we were effectively able to map sub-surface features such as salt topography, seep-related faults and geophysical indicators of hydrocarbons and correlate them with seafloor amplitude anomalies and fault traces in order to characterize seep activity level. The southern mud volcano in the Garden Banks site is characterized as an established high flux seep vent owing to signs of active seepage and sediment flows as well as the build-up of hard grounds. The northern mud volcano in the area, with greater hard ground build-up and fewer signs of active seepage represents an established low flux seep vent. In the Mississippi Canyon area, the data suggest that the seep mound can be characterized as a mature high flux vent due to the extensive build-up of hard ground, evidence of gas hydrates and signs of active seepage and sediment flows. The mechanisms of formation are similar between the two study sites. Upwelling salt appears to have fractured the sub-surface leading to the formation of fault induced depressions. Mapping of geophysical indicators of hydrocarbons implies that hydrocarbon migration is occurring along bedding planes to the fault systems underlying the depressions. Here they appear to migrate vertically to the seafloor creating the topographic features and seafloor amplitude anomalies that characterize the seeps

<p>A great number of field measurements with different methods and instruments were conducted in attempts to develop a method for an optimal combination of various geochemical and geophysical methods in archaeological prospecting. The research presented in this thesis focuses on a study of how different anthropogenic changes in the ground can be detected by geochemical and geophysical mapping and how the results can be presented.</p><p>A six-year pilot project, Svealand in Vendel and Viking periods (SIV), was the beginning of this work in which EM- measurements, gradiometer and GPR surveys together with phosphate mapping succeeded in detecting settlements, mainly from the Iron Age. During the project, a new field kit for soil phosphate analysis was also developed and patented.</p><p>Another major project was the examination of the Kings’ Mounds in Old Uppsala where a multi method survey including two different slingrams, three different types of GPR equipments and two different gradiometers was used for detecting structures inside the mounds. The Thing Mound was interpreted not to be a burial mound. The layers indicated by the GPR survey are most probably natural glaciofluvial layers. At the Eastern Mound the measurements detected an excavation tunnel from 1846-47, some known old brick furnaces from the time of building of the church, the original esker surface and the central cairn. The survey also detected two linear structures from the mounds base to the top and a depression under the mound in the old esker. The linear structures were interpreted as possible boulder rows and the depression as a possible older burial preceding the construction of the mound.</p><p>Other surveys with slingram and GPR were used to detect subsurface features at historical gardens and historical glass works. A number of surveys to detect older church remnants were also carried out. In a project at Arethousa, Greek Macedonia, slingram measurements pointed out possible subsurface stonewalls and a possible hearth, which were confirmed later by excavations. At Old Uppsala, Sweden a GPR survey was successfully used for detecting the older cathedral under the present church.</p><p>The overall results led to a recommended concept of combined methods for archaeological prospecting, especially in northern Europe. Slingram measurements of both electrical and magnetic components combined with phosphate mapping and GPR surveys proved to be useful methods for such prospecting.</p>

This thesis describes the results of new analysis of gravity studies over West Antarctica. Set on the Siple Coast, an airborne geophysical survey was flown between 1994 and 1997 that covered the trunk of Bindschadler Ice Stream and the up-stream areas, including Whitmore Accommodation Zone and Byrd Subglacial Basin. The new gravity reduction methodology removed vertical and horizontal accelerations, the Eötvös effect, and the theoretical gravity; unlike previous analyses, this reduction did not level individual lines, preserving the high frequency data and avoiding introduction of new errors. This reduction provided the free-air gravity disturbance over the area, which was then leveled and registered by the more regional extensive GOCE satellite gravity. The processing and reduction of the data improved the high frequency signal over previous work on the data, giving better definition of small scale, short wavelength features, which works well with satellite gravity data that emphasizes the large scale, long wavelength features. The leveled free-air gravity was then processed with a FORTRAN 90 program that calculates the Bouguer disturbance based on the free-air gravity and the topography. The topography was gathered concurrently with the gravity with ice penetrating radar during the airborne survey. The Bouguer disturbances provide a crustal model of the area. The final Bouguer disturbance was also corrected for the ice above sea level, which was calculated with a simple Bouguer slab correction. Finally, a power spectrum analysis was run on a profile in the Bouguer gravity disturbance in order to complete a spectral analysis. The spectral analysis provides crustal density boundaries for a density anomaly near the surface, a mid-crustal anomaly, and the Moho boundary. The improved the high frequency content of the data allows spectral analysis down to 4 km. The differing crustal thickness from spectral analysis also shows the character and extent of the West Antarctic Rift System, the northern flank of which extends out from Marie Byrd Land and into the survey area. Bindschadler Ice Stream is located on the WARS rift floor and MacAyeal Ice Stream sits on the rift flank. / text