VictorJuiz/thesis huggingface dataset:
A Novel Method of Computing the EM Response of a Conductive Plate in a Conductive Medium
Keywords: Geophysics

This thesis describes the development of a novel, simple, and robust integral equation technique to compute the electromagnetic response of a three dimensional plate-like conductor embedded in a conductive host. Thin sheet or plate-like conductors are mathematical models of many base metal mineral deposits, including mineralized veins, shears, and dipping strat. Solutions for the thin sheet models are usually formulated using the integral equation technique with the electric field as the variable. The appeal in this approach is its simplicity in derivation and the ease in interpretation of solutions. However, many of the approximate computer algorithms derived directly from the exact theory have suffered to some extent from the defect that the solutions fail to generate strong vortex and tend to become unstable when the host conductivity becomes too small. The determination of the scattered electric field within the plate requires the solution of an integral equation in which inductive and channeling terms couple together. The magnitudes of the terms vary with the selection of parameters. They can differ by many orders of magnitude particularly for very resistive hosts. The accuracy with which the terms can be derived numerically depends on the selection of appropriate basis functions. A small set of functions which can describe accurately both the current flows is difficult to find. There are several powerful techniques to circumvent the problem, but these methods usually result in complicated and costly software. Our approach to resolve the problem is to deform the thin plate so as to create an equivalent network model for which the channeling and vortex currents can be described equally well by one simple set of unknowns. The new model uses a set of basis functions with a spatial representation bearing a strong resemblance to a lattice composed of many thin conductive strips, called elements, arranged to look like a bottle-box with the top and base removed. It has finite length, width, and thickness, and consists of m x n basic square loops. In order to check our formulation, the EM responses of the new model in a conductive whole space are compared with equivalent responses of a thin disk derived by West and Edwards (1985) for a wide range of induction and channeling response parameters. Other tests of the lattice plate have shown that the solutions are accurate, stable, and robust, and the algorithm is efficient. The lattice plate is used in forward modeling to study the responses of plate-like conductors in the earth and in the sea floor to various Controlled Source Electromagnetic sounding systems.
A Plausible Mechanism for Generating Negative Coincident-Loop Transient Electromagnetic Responses
Keywords: Geophysics

Coincident-loop transient electromagnetic measurements are normally positive at all delay times, but occasionally they become negative at late delay times. The most likely cause of these sign reversals are polarizable conductivity structures (i.e. structures whose conductivity varies in the sense $\partial\sigma$/$\partial$s $>$ 0, where s is the Laplace transform variable representing the frequency). However, structures such as half-spaces, buried conductors or buried conductive layers are not capable of explaining the negatives unless the polarizabilities are implausibly large. The negatives can be explained with realistic polarizabilities if the structure is such that the following three 'favourable coupling conditions' are satisfied at the positions where negatives occur: (1) the transmitter couples well to the body, (2) at late delay times the electromagnetically induced current couples poorly to the receiver, and (3) the polarization couples well to the receiver. Polarizable structures which satisfy the favourable coupling conditions are: a buried conductor in a very resistive host, a thin conductive overburden, two interacting conductors, an overburden which comes to an edge and a thin dipping dyke. The field examples of negative transients presented in this thesis are all associated with these structures and the polarizabilities required to model the negatives are small, so the favourable coupling conditions provide a plausible general explanation of the negative transients.
A Robust Integral Equation Solution for EM Scattering by a Plate in Conductive Media
Keywords: Geophysics

A new integral solution formulated with the Galerkin method is derived and applied to model the electromagnetic response of a thin conducting plate in a stratified medium. The solution is expressed as two coupled integral equations, one for the scalar potential of the equivalent current density and the other for the corresponding magnetic field. The solution adopts the form of a single equation plus a constraint on the current density in two limiting cases. Where the host medium is resistive, the scalar potential equation forces the current to be divergence free. For static field excitation, the magnetic field equation forces the current to be curl free. This property of the solution is responsible for the robustness of the method for a wide range of model parameters. The Galerkin method requires multiple integration which can be extremely time-consuming when the required Green function is not analytical. To overcome this problem, new integration techniques are introduced in which the Green function is represented as a product of geometrical and electrical factors. The geometrical factors are time-consuming to calculate but can be pre-calculated and used in many model evaluations. The electrical factor can be computed relatively easily, but cannot be saved. This separation permits the response of the plate in a layered environment to be calculated extremely efficiently. Parametric scaling arguments are used to determine when it is necessary to compute the EM response with a solution accounting for both galvanic and inductive effects. Estimators are developed and tested which allow one to predict if the complete solution to be replaced by simpler solutions such as those which treat the inductive and galvanic components separately. These estimators may be used to predict the mode of current excitation itself, or the mode of current excitation which dominates the sensed fields.
The Effects of Inhomogeneities on Magnetotellurics
Keywords: Geophysics

The magnetotelluric (MT) method is a low-frequency, electromagnetic (EM), geophysical method which is widely used for investigating the conductivity structure of the Earth's crust and upper mantle. Although MT often seeks to determine the gross conductivity structure and the variation of conductivity with depth, it suffers from the ubiquitous presence of EM scatterers of all scales. The thesis advances the understanding of the effects of multiple and small-scale inhomogeneities on the MT impedance tensor. One of the most pressing difficulties at present in MT are distortions of the EM fields by small-scale, 3-D inhomogeneities which are often found near the surface of the Earth. The thesis investigates the effects of these inhomogeneities on the observed impedance tensor. These studies show that for a great many cases the impedance tensor conforms to a particular factorization, each part of which is related either to the local inhomogeneities or to the large scale structure. Each factor is associated with a different physical effect. This factorization is used to explain the necessary requirements for a useful parametrization of the data contained within the impedance tensor. The development of such a complete, useful decomposition is begun with a method for dealing with the most significant effect of small-scale surface bodies; namely galvanic or frequency-independent distortion of the horizontal electric fields. In addition, analytic and statistical solutions are provided to a number of relevant low-frequency EM scattering problems which model the response of inhomogeneities. An analytic solution for two semi-infinite slabs over a basement is developed in the thesis. This extends existing solutions to this problem so as to include all frequencies and arbitrary basement conductivities. For the particular case of an insulating or perfectly-conducting basement, the above method is extended to provide the EM response for a model of a quasi-anisotropic layer. This model was used to study the benefits of extensive spatial sampling of the electric field in complex media and the presence of fictitious conducting layers when data due to large lateral inhomogeneities are interpreted one-dimensionally. The study showed, for this particular model, that the use of large electrode spacing could be beneficial if used with care. The anisotropy model was also employed to investigate the question of what bulk properties the MT method samples. A statistical technique is used for investigating the response of very fine-bedding in MT demonstrating that such structures can have a relatively significant effect of up to ten degrees on the impedance phase. These results are corroborated by a deterministic model.
A Generalized Approach to the Interpretation Of Controlled Source Electromagnetic Data Collected in Sedimentary Basins
Keywords: Geophysics

Electrical and electromagnetic (EM) methods are widely used to explore layered geological structures such as sedimentary basins. Current methods are well developed but have rather low resolution and are easily disturbed if the earth is not well layered. To improve the resolution and accuracy of sounding methods requires that a very large set of field measurements be taken over a wide range of spatial scales and frequencies. In practice, such measurements are usually made about a spatially extended EM source. However, the mathematical complexities of modeling extended source surveys tends to obscure the simple physics behind the method which, in turn, makes interpretation difficult. I have devised a new method to aid the interpretation of survey data acquired about an extended source on a quasi-stratified terrain. The method overcomes the deleterious effects of a complicated source/receiver geometry and makes it possible to account for moderate lateral structure in the earth. It is shown that EM fields can always be factored into "geometrical" operators and "electrical" kernels. The electrical kernels represent the geometrical expansion of the EM field in the earth and are independent of the survey geometry. Conversely, the geometrical operator is independent of the earth model and represents the source/receiver configuration. Field data are transformed to estimate the "average" electric kernels of the horizontal layering. The layered earth data is then interpreted using standard inversion techniques to obtain the best fitting one dimensional model. The EM fields scattered from lateral heterogeneity are defined as the residual data which cannot be interpreted in terms of layered earth models. I develop some simple linear methods of interpreting the scattered fields in terms of anomalous lateral structure in the 1D earth. To test the interpretation concepts, a field study was carried out in the sedimentary sequence of southern Ontario using a grounded bipole source. The survey site is mainly layered, but there is also strong evidence of scattering in the data. The data were successfully separated into layered earth electric kernels and the EM fields scattered from the lateral structure, and subsequently interpreted. The results of the interpretation are consistent with the known geology indicating the utility of the methods developed in this thesis.
The Generalized Electrosounding Method for Sedimentary Basin Exploration
Keywords: Geophysics

Galvanic resistivity and electromagnetic induction sounding are presently the standard techniques for mapping the electrical structure of sedimentary basins. Taken in isolation, these methods have reached the limits of their interpretational accuracy, which can be only marginally improved by refinement of instrumental accuracy or field procedure. The Generalized Electrosounding (GES) method synthesizes the galvanic resistivity and electromagnetic techniques of sounding/profiling to utilize the wealth of information present in high-precision, broadband (0.05 Hz to 5 kHz) electric and magnetic field measurements made in the vicinity of a grounded controlled source. Analysis of these measurements should permit resolution of thin layer properties and apparent aeolotropy, which are not normally resolved by conventional electroprospecting datasets. A prototype measurement system was designed and constructed to investigate the practical acquisition of GES data. Analysis of data obtained with this system during field tests indicates that the instrumentation problem is now thoroughly understood. The principal obstacle to the use of GES for mapping geological structure and formation aeolotropy is "geological noise," a consequence of spatial undersampling of the measured fields. The most promising strategy for control of geological noise requires spatial "filtering" through the use of sensing dipoles which are large with respect to the receiver station spacing. Another feature which distinguishes G ES from conventional electroprospecting methods is the degree of possible redundancy in the data. This possible redundancy, which is a result of measuring both electric and magnetic field components over a wide range of frequencies and spatial offsets, is particularly useful in identifying non-stratified structure which might otherwise be misinterpreted as variations in stratigraphy.
Tomographic Inversion of Crosshole Seismic Data
Keywords: Geophysics

A crosshole seismic experiment deploys sources and receivers at many depth positions in parallel boreholes. The resulting ray paths form a crisscrossing pattern through the intervening rock. Many tomographic interpretations of the traveltimes have approximated the ray paths with straight lines, ignoring ray path curvature. While the approximation is valid in weakly inhomogeneous media, many regions of interest have large velocity variations which cause significant ray curvature. The interpretation problem is that the ray paths are required in order to accurately determine the velocity structure but these ray paths depend on the unknown velocity structure. This thesis presents a method of interpreting crosshole data which accurately accounts for the curved ray paths in an inhomogeneous medium. The nonlinear inverse problem of estimating the velocity structure from traveltime data is solved iteratively. The traveltimes and partial derivatives for the current model are calculated during raytracing. The difference between the data and current model traveltimes are then used to determine perturbations to the velocity model. The partial derivatives relating the velocity perturbations to the traveltime differences are recalculated at each iteration, as they depend upon the changing ray paths. Difficulties in interpreting crosshole seismic data arise because of limited angular coverage. The impossibility of imaging features parallel to the boreholes is investigated, as are the resolution limitations inherent in other source-receiver borehole geometries. A method of estimating the spatial resolution possible with large datasets is also presented. Results using synthetic data demonstrate the accuracy of the inversion method, which is then successfully applied to two sets of real crosshole data. Regions of low seismic velocity in the final models correlate well with known zones of fracturing along the boreholes. The good correspondence of synthetic seismograms and observed seismograms also confirms the validity of the velocity models obtained with this method. The same methods are extended to the inversion of amplitude data for the determination of attenuation structure, accurately taking into account the previously determined velocity structure. They are applied to a field dataset and the resulting attenuation structure is shown to agree with other geophysical and geological data.
A Three-Component Drillhole EM Receiver Probe
Keywords: Geophysics

Most downhole EM receiver probes currently in use measure only the component of the magnetic field vector parallel to the borehole. This is mainly due to the difficulties of fitting coils of sufficient sensitivity and bandwidth sideways within a probe. While the so-called axial component data is very useful, it does not allow direct determination of the direction to the conductor from the drillhole, an important parameter. An EM probe which measures the transverse components of the magnetic field as well as the axial component would, however, give an indication of the direction to the conductor, provided that the orientation of the probe can be determined. The object of this thesis project was to develop such a probe. The difficulties inherent in the design of a transverse sensor were overcome by using an assemblage of ferrite slabs as a magnetic flux collector. The collector serves to funnel the magnetic flux from a wide area through the coil at its center, greatly increasing the coil's effective area. The sensors developed in this research are only 30 cm long, yet have an effective area of 63 m². The axial sensor is also somewhat unusual in that it uses larger blocks of ferrite at its ends to act as flux collectors. These greatly increase the sensitivity over what a single narrow rod the same length would have. The axial sensor is about 21 cm long but has an effective area of 250 m² and a bandwidth around 10 kHz. All three sensors plus batteries and electronics were fitted into a probe 41 mm in diameter and about 2 m long. The probe was tested at the Gertrude West test site near Sudbury Ont. After processing of the data to correct for the rotation of the probe as it was lowered down the drillhole, profiles showing the vertical, the east and the north components of the field were produced. These showed that a conductive body dipping towards the north passed just south of the drillholes tested, an interpretation in agreement with the geology as known from previous drilling. Whereas the data from a single component probe would only give an idea of the distance, size, shape etc. of the conductor, that of the three-component probe was able to give a good estimate of the direction to the conductor as well.
Electromagnetic Induction Studies of the Meager Creek Geothermal Area, British Columbia
Keywords: Geophysics

The magnetotelluric (MT) method has been among the most useful techniques for the mapping of geothermal zones. Its quantitative interpretation in terms of three-dimensional structures, however, has been hindered by the complexity of these environments and by the vast amounts of computer resources used to model such structures with the present numerical techniques. This thesis presents, for the first time, an interpretation of actual MT observations in terms of multiple three-dimensional confined conductors by means of a rapid low-frequency technique. Meager Creek is one of the most promising geothermal reservoirs in Canada. It is located at the base of a Quaternary volcano in southern British Columbia. This zone is characterized by impressive topographic reliefs, by an irregular and heterogeneous overburden, and by several conductive zones with different geometries. The electrical structure of the area of study is estimated through careful one- and two-dimensional interpretations of the MT and controlled source electromagnetic (CSEM) sounding data and with the use of previous studies in the region. The MT survey consists of seven sites crossing the thermal anomaly, measuring the three magnetic and two electric field natural variations. Responses at forty frequencies from 380 to .00055 Hz are obtained. The CSEM study consists of 22 sounding sites measuring the vertical magnetic field at nine frequencies from 3 to 1000 Hz where a finite loop is used as energizing source. A rapid integral equation algorithm is developed to approximate the MT galvanic response by ignoring the low-frequency inductive coupling present in the complete solution. The model consists of a set of arbitrary dipping plates embedded in a stratified earth. The method is based on the representation of current channeling by a distribution of surface current dipoles on the plane of the plate. Several different tests with existing published solid body results obtained from the complete solution are performed, showing a good agreement and a saving in computer execution time of at least one order of magnitude.
The Interpretation of Inductive Transient Magnetic Sounding Data
Keywords: Geophysics

Within the last two decades time domain electromagnetic (EM) methods have found wide application in earth soundings as applied to sedimentary basin exploration, geothermal and groundwater exploration, and engineering. The inversion of EM sounding data by nonlinear regression methods which minimize the residual between the observed field amplitudes and the response of a model is an expensive process, particularly when an EM response is observed at many locations of the receiver. In these cases the data sets are typically very large and the inversion often requires the solution of large, unstable linear systems through many iterations. This thesis demonstrates that the inversion of transient magnetic field data using an inductive source may be considerably simplified if the step response, or a reasonable approximation to it, has been observed. In these cases the misfit between the observations and the synthetic response may be described in terms of a well-defined time delay ⌂lt. Unlike the amplitude residuals which are highly dependent on the source-receiver geometry, the delay is very nearly independent of this geometry. This property allows one to reduce the measured response at any number of positions, from any number of sources, or of any directional components, into a single universal delay function ⌂t(t). This thesis develops an iterative nonlinear inversion method which seeks to minimize the delay with respect to the parameters of a model. This method iii permits very rapid construction of a model with a response that fits the data within the errors. The significance of structure in the model is analyzed by applying the method of Backus and Gilbert (1970). Both the construction and evaluation methods are illustrated on synthetic, noisy UTEM data.