Browsing by Subject "geophysics"
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Item A finite element approach to the 3D CSEM modeling problem and applications to the study of the effect of target interaction andtopography(Texas A&M University, 2005-11-01) Stalnaker, Jack LeeThe solution of the secondary coupled-vector potential formulation of Maxwell??s equations governing the controlled-source electromagnetic (CSEM) response of an arbitrary, threedimensionalconductivitymodelmust be calculatednumerically.The finite elementmethod is attractive, because it allows the model to be discretized into an unstructured mesh, permitting the specification of realistic irregular conductor geometries, and permitting the mesh to be refined locally, where finer resolution is needed. The calculated results for a series ofsimple test problems, ranging from one-dimensionalscalar differentialequations to three-dimensional coupled vector equations match the known analytic solutions well, with error values several orders of magnitude smaller than the calculated values. The electromagnetic fields of a fully three-dimensional CSEM model, recovered from the potentials using the moving least squares interpolation numerical differentiation algorithm, compares well with published numerical modeling results, particularly when local refinement is applied. Multiple buried conductors in a conductive host interact via mutual induction and current flow through the host due to the dissipation of charge accumulated on the conductor boundary. The effect of this interaction varies with host conductivity, transmitter frequency, and conductor geometry, orientation, and conductivity. For three test models containingtwo highly conductive plate-like targets, oriented in various geometries (parallel, perpendicular, and horizontal), mutual coupling ranges as high as twenty times the total magnetic field. The effect of varying host conductivity is significant, especially at high frequencies. Numerical modeling also shows that the vorticity of the currents density induced in a vertically oriented plate-like conductor rotates from vertical at high frequencies, to horizontal at low frequencies, a phenomenon confirmed by comparison with time domain field data collected in Brazos County, Texas. Furthermore, the effect of the presence of a simple horst on the CSEM response of a homogeneous conductive earth is significant, even when the height of the horst is only a fraction of the skin depth of the model. When the transmitter is placedon topofthe horst, the currents inducedtherein account for nearly all of the total magnetic field of the model, indicating that topography, like mutual coupling must be accounted for when interpreting CSEM data.Item A near-surface geophysical investigation of the effects of measured and repeated removal of overlying soil on instrument response(Texas A&M University, 2005-11-01) Long, Zachary RyanA geophysical survey presents many challenges. A scientist must be able to not only understand the theory and nature of the geophysics being applied but must also be able to identify features of interest in a dataset. It is also of extreme importance to be able to determine where, in the subsurface, the features identified in the data occur. This research is designed in an attempt to identify the locations of subsurface heterogeneities that affect geophysical instrument response. An experiment was conducted in which topography, magnetics, ground-penetrating radar (GPR), and electromagnetic induction (EM) data were collected over a defined survey line. An excavator with a modified flat-bladed bucket was used to remove, or skim, a 5 to 10 cm thick layer of material from the survey line. Upon removal of the material, datasets from the above mentioned instruments were again collected along the same survey line. This process was repeated for 10 skims, resulting in a total of 11 sets of data for each instrument. Having collected data with various instruments in the same location as material was progressively removed allowed for an empirical study with the goal of noting how the response of each instrument changed with respect to the removal of material. By observing how the anomalies changed in the data from one skim to the next, a better understanding of the location of the causative heterogeneities could be had. Data for each instrument was compared to the equivalent data collected from each subsequent skim to determine how similar or different the data appeared as the depth of the trench increased. The experiment also sought to determine if the topographic variations, or roughness, along the survey line had any impact of the geophysical signals. The data collected from each instrument were compared to the topographic roughness of the survey line for the corresponding skim.Item Fault Characterization using Induced Electric Currents Presumed to Undergo Anomalous Diffusion in Mason, Texas(2014-08-01) Murphy, Blaine CullenA tensorial (9-component) controlled source electromagnetic survey over an oblique slip fault in Mason County, Texas was acquired and the data were analyzed in order to characterize the structure of the fault and observe the diffusion of the induced currents in a fractured medium. The fault juxtaposes a Cambrian sandstone against Precambrian granite and both are highly resistive media that are not generally conducive to electromagnetic surveys. The loop-loop method used in this project induces an eddy current system in the subsurface that moves down and out like a smoke ring. The eddy currents generate a transient magnetic field that is recorded by the receiver loop, which is stepped out from the transmitter loop at various offsets. The 9-component data set includes responses with the 3 orthogonal source orientations and 3 similar receiver orientations at each offset, and data were recorded over the fault zone along two oppositely-directed profiles. Induced currents were presumed to diffusive anomalously in the presence of a geologically rough medium, in this case the fractured subsurface of the fault zone. Classic diffusion corresponding to a continuously differentiable medium would result in a decay of the received signal according to a t^(-5/2) power law. Deviation of the power law decay from a -5/2 slope indicates anomalous diffusion of the subsurface eddy current system. A faster decay in the transient response at long offsets was observed, and it suggests highly resistive fractures in the fault zone at depth. A shallower decay at near offset, and for the shallower-probing vertical co-planar orientation, suggests more conductive fractures in the near surface A higher resolution 3-dimensional survey is required to rigorously test whether induced currents indeed undergo anomalous diffusion in a rough geological medium.Item Static Load Balancing using Non-Uniform Mesh Partitioning based on Ray Density Prediction for the Parallel Wavefront Construction Method(2014-08-01) Alyabes, Abdullah FahadThe Wavefront Construction (WFC )method, which was developed based on ray theory, is one of the most efficient tools in seismic modeling. The main idea of this method is to propagate a wavefront represented by rays in a computational mesh that is interpolated whenever an accuracy criterion is violated. Recently, a parallel WFC was developed using the Standard Template Adaptive Parallel Library. However, due to wavefront density adaptivity, the parallel implementation exhibits inefficient performance owing to load imbalances between multiple processors.This paper applies a static load balancing approach based on a method for predicting future loads for a synthetic salt dome model, in order to improve the performance.The approach utilizes a preliminary conventional ray simulation to estimate the cost (future load) of each cell in the WFC's initial wavefront mesh.Then it applies a non-uniform mesh decomposition that results in a more efficient parallel WFC. Our implementation shows better and stable scalability in most WFC simulations. Overall, this paper contributes to understanding the behavior of wavefront mesh adaptability and predicting earth model complexities, and it serves as a guide for achieving the ultimate goal, a fully load-balanced parallel WFC.Item The Effects of Fluid Flow On Shear Localization and Frictional Strength From Dynamic Models Of Fault Gouge During Earthquakes(2013-12-02) Bianco, RonaldThis thesis explores the effects of fluid flow on shear localization and frictional strength of fault gouge through the use of a coupled 2-phase (pore fluid-grain) Finite Difference-Discrete Element Numerical model. The model simulates slip at earthquake velocities (~1m/s) in a fluid saturated gouge-filled fault. We find three types of shear behavior: (I) distributed shear, (II) random internal localization, and (III) boundary localization. Each shear type is dependent on the applied shear velocity, V, effective confining stress, N, and internal permeability, k. Through quantitative analysis of the positions and magnitude of localized shear bands, we show under which conditions the presence of and transitions between these shear types will occur. During shear, fluid pressure deviations, delta P, are generated by dilation and compaction cycles. The fluid effects on the system are more pronounced in simulations with higher V and lower N and k. Relative to the dry experiments, fluid saturated systems have an increased localization toward the boundaries of the gouge layer (type III), and no occurrence of distributed (type I) shear. Systems with lower N and k show liquefaction events. Liquefaction events originate from increases in fluid pressure, delta P, around force chains between grains. Once delta P, the high pressures weaken the frictional forces between grains and destroy force chains. Shear then occurs at essentially zero friction until a new grain configuration recreates force chains. A reduction in mean friction is seen for systems with large liquefaction events (without inclusion of thermal pressurization), which could introduce a new mechanism in low friction faults. We also find that systems undergoing different types of shear will all trend toward type (III) shear following a liquefaction event.