Geophysics

Permanent URI for this collectionhttps://hdl.handle.net/1807/71876

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YVESET and CHANNEL: A Subroutine Package for Stable Transformation of Sparse Frequency Domain Electromagnetic Data to the Time Domain
(1981-01) Holladay, J. Scott
It is often necessary to transform frequency domain (FD) response data for electromagnetic (EM) prospecting instruments near a system of conductors into the time domain (TD), in order to estimate the response of a TD EM system to the same system of conductors. Straightforward approaches utilizing interpolation and Fourier transform techniques are expensive and sensitive to data error. Lamontagne (1975) introduced a method in which the FD data is fitted by stabilized least squares techniques with a finite set of "basis functions" for which the Fourier transforms (FT's) are known. The TD response of the system can then be synthesized by performing a sum over the FT's of the basis functions using the same weights determined for the FD fit. Lamontagne's method has been implemented in a subroutine package including two alternative subroutines called YVESFT and CHANNEL which calculate the TD response of a system to a variety of primary field waveforms. CHANNEL calculates point-sampled impulse and step responses, and windowed UTEM, INPUT, PEM and SIROTEM responses in negligible computer time using analytical rules, and can also calculate the system's response to an arbitrary periodic waveform using a more time-consuming numerical approach. YVESFT is a smaller, somewhat simpler program which can calculate point-sampled impulse and step responses, and windowed step and UTEM responses using analytical formulas. Both packages are written in VAX-11 FORTRAN, and are fully documented both internally and externally. The routines have been extensively tested against numerical and analogue models and found to be fast, accurate and stable.
Wideband Time-Domain EM Project 1976-8: Reports 1-5
(1980-01) UTEM
A numerical modelling method has been used to compute the response of several common EM systems to a conductor in the form of a rectangular thin plate. Host rock and overburden conductivity is assumed to be negligible, so only simple eddy current induction occurs in the conductor. The model data provided by this study show the depth and conductivity range over which a large plate (dimensions 300x600 m) can be detected by the EM system. A variety of attitudes of the conductor have been considered.
Electromagnetics for Geophysical Exploration: Part II
(1984) West, G. F.; Macnae, J. C.
From Part I introduction - Electromagnetic exploration methods suffer seriously from the fact that many users have difficulty in visualizing the physics which is taking place in the ground to produce the survey results. In this respect, EM is unlike the gravity and magnetic methods, where most users can sketch for themselves the potential field configurations around various sources of attraction, or seismic methods where simple Snell' s law ray theory offers a reasonable first approximation. Furthermore, we have not yet reached the stage where EM induction processes can be routinely simulated by computed models, so a certain amount of trained intuition is definitely required by an interpreter. The object of these notes is to assist you in visualizing EM induction processes in a qualitative manner. The quantitative mathematical theory of EM fields can be developed in a parallel manner 1 but examining the mathematical niceties too carefully can distract one from the essential physics. Also, several very convenient mathematical analogies with wave theory can obscure the fact that EM induction at very low frequencies has more in common with diffusive potential field processes such as thermal diffusion and with electric circuit theory than with wave processes as epitomized by optical ray theory.
Electromagnetics for Geophysical Exploration: Part I
(1984-03) West, G. F.; MacNae, J. C.
Electromagnetic exploration methods suffer seriously from the fact that many users have difficulty in visualizing the physics which is taking place in the ground to produce the survey results. In this respect, EM is unlike the gravity and magnetic methods, where most users can sketch for themselves the potential field configurations around various sources of attraction, or seismic methods where simple Snell's law ray theory offers a reasonable first approximation. Furthermore, we have not yet reached the stage where EM induction processes can be routinely simulated by computed models, so a certain amount of trained intuition is definitely required by an interpreter. The object of these notes is to assist you in visualizing EM induction processes in a qualitative manner. The quantitative mathematical theory of EM fields can be developed in a parallel manner, but examining the mathematical niceties too carefully can distract one from the essential physics. Also, several very convenient mathematical analogies with wave theory can obscure the fact that EM induction at very low frequencies has more in common with diffusive potential field processes such as thermal diffusion and with electric circuit theory than with wave processes as epitomized by optical ray theory.
An Atlas of Primary Fields Due to fixed Transmitter Loop EM Sources + Erratum
(1980-05) MacNae, James C.
It has long been known in exploration geophysics that any fixed transmitter EM system has "blind zones" for thin, plate-like conductors. These occur whenever the primary field is nearly parallel to the plane of the conductor, with the result that very little current is induced. The body may then escape detection, particularly if noise or other conductors are present. Recent work in borehole EM has revived interest in this topic. As well as this interest in the shape of primary field, knowledge of its amplitude is also very important in order to asses the relative strength of secondary fields cause by induction, and in the determination of optimum loop sizes ...
The Thomas Township Test Target: An Example of EM Interpretation Using Simple Models
(1981-10) MacNae, J. C.; Walker, P.
The Thomas conductor, located near to Timmins, Ontario, is a short, wide, graphitic zone located under about 90m of poorly conducting, glacially deposited overburden. Due both to its ease of access and the large quantity of data that has already been collected over it, it has the potential to become a standard geophysical test site. Joint interpretation of moving and fixed transmitter EM data, together with previous limited drilling have accurately defined the geometrical and electrical structure of the conductive zone. The interpretation study showed how simple models such as a thin plate or sphere were able to accurately fit the observed secondary EM fields, but that a good fit did not necessarily imply that a good interpretation had been made. In particular, for fixed transmitter systems, interpreted plate conductors dipping perpendicular to the primary field may have significant thickness.
The Electromagnetic Response of the Different Conductive Phenomena of the Athabasca Basin Area of Saskatchewan to the Slingram Coil Configuration
(1982-03) Betz, John E.
This study throws some light onto the types of conductors in the Athabasca Basin area of Saskatchewan, which respond to air and ground electromagnetic surveying systems. By relating laboratory physical scaled modelling results to field results, for a ground EM system using the slingram or horizontal loop coil configuration, it becomes apparent there are: a) moderately to steeply-dipping, moderately-conductive tabular basement zones, and b) very broad, weakly-conductive basements units – in the overall picture. By further relating the Input air EM results to the interpretation, based on the 'slingram' ground EM results, supported at times by drill hole information, some interpretive aids can be obtained for the Input air EM system, which to date have been rather incomplete for an environment such as the Athabasca Basin. The incompleteness of the Input interpretation has often led to improper procedures. In conclusion, the results of this study will improve the quality of the interpretation for both the Input air EM system, and ground EM systems using the slingram coil configuration.
User Manual for Programs PLATE and SPHERE
(1980-01) Dyck, A. V.; Bloore, M.; Vallee, M. A.
A FORTRAN-computer program is provided on the DEC VAX 11/780 system to complete the· response of any common EM prospecting system to a conductor in the form of a rectangular thin plate. Host rock and overburden conductivity are assumed to be negligible, so only simple eddy current induction occurs in the conductor. The program can be run either interactively or in batch mode. Results are provided as listings in data files to be printed or as graphics for output on the Versatec plotter.
The Cavendish Test Site: A UTEM Survey Plus a Compilation of Other Ground Geophysical Data
(1980) MacNae, James C.
This report presents data from three recent University of Toronto surveys at the Cavendish geophysical test site, namely UTEM, horizontal loop (HLEM) and ground magnetic surveys. Interpretation of the UTEM survey has clearly defined the presence of a short strike-length pod of high conductance within the regional zone A conductor, present on line C. The pod is interpreted to have a conductance in excess of 400 Siemens, compared to about 10 Siemens for the regional zone A conductor extending across the grid. HLEM data interpretation strongly confirm this conclusion. Zone B is interpreted to have the top of its main conductivity at some depth. This conclusion, if true would indicate that the crosscutting westerly dip for zone B postulated by Williams et al. (on the basis of one drill hole that did not intersect sulphides) may no longer be necessary to explain the drill data. The detailed magnetic field data collected, when contoured, show a very different picture to that of earlier data. On the basis of UTEM late-time limit interpretation it was possible here to determine the nature of the source of the near surface static magnetic anomalies i.e. whether induced or remnant magnetization is predominant. The second part of the report presents a compilation of ground geophysical data at Cavendish obtained from a variety of sources. This data set is by no means complete, and due to the constraints of the original surveys, no valid inter-system comparisons can be made. A reference list of other, easily available, data on the Cavendish test site is also included. In an appendix the vector diffusion process of EM induction is illustrated in the UTEM E field case for the Cavendish test site.
MMR2DFD: A Programme for Magnetometric Resistivity 2D Modelling by the Finite Difference Method
(1983-07) Pai, D. M.; Edwards, R. N.
A computer programme for two dimensional magnetometric resistivity modelling is described. The electric potential is calculated by the finite difference method1 and from the resulting current distribution the magnetic field response is obtained by an integration through the two dimensional conductivity plane. The current sources and receiver points can be specified anywhere (not restricted to the two dimensional conductivity plane). Several sources can be specified at once. Solutions (single-source) for all specified sources are obtained simultaneously. Computed results are compared to analytical solutions for three conductivity models: the uniform conductivity model, the contact model and the multi-layer model. Instructions for running the programme are provided.
EM Response of an Arbitrary Source on a Layered Earth, A New Computational Approach
(1972-11) Lajoie, J.; Alfonso Roche, J.; West, G. F.
The Fast Fourier transform is used to compute the EM Induction response in a layered earth by an arbitrary grounded or ungrounded source (e.g. dipole, Turam looo, grounded wire). The induction problem is expressed in the horizontal wavenumber domain, the 2-D Fourier transform of the x-y plane, where a source term and an earth transfer function may be considered separately. This new procedure results in a significant reduction in computation time and flexible straightforward programming. Several examples are described to demonstrate the advantages and limitations of the method.
EM Response of a Rectangular Thin Plate
(1971-02) Lamontagne, Y.; West, G. F.
By putting the fundamental equations in terms fo the stream potential of the surface current density, it is possible to express the rectangular thin plate problem in a single equation subject to simple boundary conditions. A finite difference approximation of this equation reduces the problem to that of solving a large set of linear algebric equations. The solution of these equations by a modified Gauss-Seidel iterative method yields the stream potential and thus permits visualization of the eddy currents circulating inside the plate conductors. The secondary field calculated from the stream potential compares well will scale model measurements provided that the intervals used in the finite differences are small enough. Using a further approximation, inductively thick conductors can also be simulated if they are not also geometrically thick.
Quantitive Interpretation of Input AEM Measurements
(1973-12) Palacky, G. J.; West, G. F.
Recent improvements of the INPUT airborne electromagnetic system have made possible a more quantitative approach to interpretation. The necessary interpretational aids can be obtained in two ways: either by correlating the system and ground EM measurements, or by devising computational or analog quantitative models. Both approaches have been explored. In the former, the system decay rate can be correlated with the apparent conductivity‐thickness (σt) estimated by ground surveys. In the latter, four quantitative models were investigated, vertical half‐plane, vertical ribbon, dipping half‐plane, and homogeneous half‐space. Nomograms have been constructed which make it possible to determine σt, conductor depth, and dip for sheet‐like conductors, and conductivity for a homogeneous half‐space. Field examples show that this procedure can be used satisfactorily in the routine interpretation of records obtained by this system.
Interpretation of EM Sounding Data for a Stratified Earth by Means of Non-Linear Regression
(1981-01) Holladay, J. Scott; West, G. F.; Physics
Direct inversion methods for electromagnetic soundings have not been used widely for routine interpretation of field data, even though they provide more useful information to the interpreter than do indirect methods, because existing inversion routines require large amounts of computer time and are relatively difficult to use. In an attempt to make direct inverse methods accessible for routine use, a study was made of the various available methods and a new computer program was written. This program solves for conductivity on a layering specified by the user, and uses the Fast Hankel transform with Anderson’s (1979) filter coefficients, a quintic spline approximation, and improvements in the Marquardt-Fletcher nonlinear regression algorithm to reduce the computer time required for convergence. The routine was tested with numerically generated sounding data which includes some random noise, as well as with several sets of field data. The results indicate that the routine solves easily for simple structures with moderate (≤ 10:1) conductivity contrasts, while larger contrasts in the conductivity structure require some care in setting up the starting model. Resistive structures are not resolved well, but the depth to conductive features in the stratigraphy can be determined accurately. Results from the inversion of five sets of field data are given: they were found to compare well with geophysical information from well logs or with interpretations made by other means.
A Short Baseline Transient Electromagnetic Method for Use on the Sea Floor
(1989-07) Cheesman, Stephen J.; Edwards, R. Nigel; Physics
A towed electromagnetic system capable of mapping the electrical conductivity of the sea floor over a large area has many possible applications, including mapping Quaternary geology, and understanding the physical properties of midocean ridge hydrothermal systems in association with massive sulphide deposits. However, the electrical conductivity of seawater is usually much greater than that of the sea floor, rendering the majority of electromagnetic systems presently employed on land unsuitable for marine use. A theoretical study of the transient step-on responses of some common controlled source, electromagnetic systems to adjoining conductive half-spaces shows that for two systems, the horizontal, in-line, electric dipole-dipole (ERER) and horizontal, coaxial, magnetic dipole-dipole (HRHR), the position in time of the initial portion of the transient indicates the conductivity of the sea floor, while at distinctly later time, a second characteristic of the transient is a measure of the larger seawater conductivity. The diagnostic separation between the two parts of the transient response does not occur for many other systems, including several commonly used for exploration on land. The concept of apparent conductivity is defined for the transient system in terms of the time of the first arrival of the transient signal. This apparent conductivity is used to produce characteristic curves of the HRHR system response for different layered earth models. A prototype HRHR system operating on a 100 m scale has been designed and constructed. A successful test of the system in shallow water was conducted in the coastal waters of Vancouver Island. The survey yielded 37 conductivity measurements along three lines. The instruments were towed by a ship along the sea floor. The tow cable carried both current to the transmitter coil and the received signal back to the ship for processing. Both the shape and amplitude of the received signal are indicative of the conductivity of the bottom sediments. Inversion of the data suggest that a varying thickness of 1.2 S·m-1mud overlies rock or sediment with a conductivity of about .1 S·m-1 . Improvements in equipment design might allow the production in real time of a continuous map of the conductivity structure of the sea floor.
A Novel Method of Computing the EM Response of a Conductive Plate in a Conductive Medium
(1989) Kwan, Karl C. H.; Edwards, R. N.; Physics
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
(1988-01) Smith, Richard S.; Physics
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
(1988-04) Walker, Peter W.; West, G. F.; Physics
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
(1988-04) Groom, Ross W.; Bailey, Richard; Physics
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
(1987-11) Boerner, David E.; West, G. F.; Physics
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.

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