VictorJuiz/thesis huggingface dataset:
Tomographic Inversion of Crosshole Seismic Data

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.

Keywords: Geophysics
Advisor: Chapman, Chris
Bailey, Richard
A Three-Component Drillhole EM Receiver Probe

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.

Keywords: Geophysics
Advisor: West, G. F.
Electromagnetic Induction Studies of the Meager Creek Geothermal Area, British Columbia

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.

Keywords: Geophysics
Advisor: Edwards, R. N.
The Interpretation of Inductive Transient Magnetic Sounding Data

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.

Keywords: Geophysics
Advisor: West, G. F.
The Effect of Current Channelling in Square Plates with Applications to MMR

A knowledge of the types of anomalies produced by current channelling is necessary for the successful interpretation of a variety of geophysical exploration techniques, including transient electromagnetic (TEM), magnetotelluric (MT) and magnetometric resistivity (MMR) surveys. In order to investigate current channelling the author has developed a numerical modelling program. This computer program, named MULT, uses the integral equation technique to compute the anomalies created by current channelling in one or more thin conductive plates. The program is written in VAX Fortran. The position, size, dip angle, and conductance of each plate are specified in the input to the program along with the host resistivity and the position of the source(s). In addition, each plate may bend a number of times in the down-dip direction, the maximum number of bends being determined by the discretization. Output from the program consists of the current flow impressed in each plate section, along with the magnetic field (and if required the electric field) as measured on a receiving line. The receiving line may lie on the surface, or inside a borehole within the host medium. The program has application not only to the present work, but also to additional work such as resistivity modelling or induced polarization (IP). The behaviour of anomaly shape and amplitude is considered as a function of plate size, depth5 conductivity and dip angle. The ability to determine these parameters from a given profile is investigated using generalized linear inversion. The anomalies of multiple plate models are shown to be derivable from the anomaly of a single plate through the use of current channelling numbers and relative effect. This may substantially reduce the amount of computation necessary in a forward modelling procedure. The conditions necessary to resolve two closely spaced plates are delineated. The degree of resolution is a strong function of the anomaly halfwidth. In a demonstration of the wide range of applicability of the program MULT, a slightly modified version is used in a demonstration of the resistivity anomalies which might be expected to be produced by a conductive square plate-like object buried beneath the sea.floor using the MOSES technique. Although the emphasis of this thesis is on MMR modeling, the type of treatment done may, with some modifications, be applied to MT or TEM modelling

Keywords: Geophysics
Advisor: Edwards, Nigel
The quantitative Interpretation of Airborne Electromagnetic Data

Airborne electromagnetic (AEM) surveys have been used since the 1950's as a rapid means of economically searching large, geologically potential areas to detect conductive zones of mineralization. As exploration continues, new target conductors are becoming increasingly more difficult to detect, while the cost of ground follow-up work is escalating. Therefore, it is clear that time is well spent on interpreting AEM data as carefully as possible. With modem multi-channel, multi-coil AEM systems it is now possible to apply a reasonable quantitative interpretation of the data. In this thesis, present AEM interpretation techniques a.re refined. To do this, use was made of numerical modelling methods for determining the response of thin rectangular plates. The effect of geometrical parameters for targets embedded in a resistive host are examined and interpretation aids provided for a number of different models. Thereafter, an approximate method for interpreting conductors embedded in a resistive host beneath a conductive overburden layer is developed whereby the overburden is approximated as a low-pass frequency filter. By applying this method to interpretation of helicopter electromagnetic (HEM) data, the anomaly is divided by the complex filter coefficient, and free-space phasor diagrams can then be used. Finally, the effect of a conductive host medium is examined and a suite of model results provided.

Keywords: Geophysics
Advisor: West, G. F.
Development and Application of a Short-Baseline Electromagnetic Exploration Technique for the Ocean Floor

Hydrothermal springs on mid-oceanic ridge crests and on seamounts create polymetallic sulphide deposits that are thought to be models for many ancient ore-deposits now found on land. For the first time, possible subsurface extensions of these sulphide deposits and the properties of the upper few tens of meters seafloor basalt have been investigated with a geophysical method. An electromagnetic technique has been developed to determine the electrical resistivity structure beneath the seafloor. A self-contained generator at the seafloor feeds a commutated electric current to two electrodes at either end of a vertical, long, insulated wire. The receiver is a remote induction coil magnetometer. Both instruments are self-contained and microprocessor-controlled, recording transmitted and received waveforms in random access memory. They are deployed from the ship to the seafloor where a deep submersible adjusts their separation to prearranged values at prearranged times. Both instruments are subsequently recovered and interrogated. The data are measurements of two horizontal components of the magnetic field as a function of transmitter-receiver horizontal separation and transmitter frequency. The acronym MINI-MOSES was chosen for the method because - similar to the MOSES method previously developed but over smaller spatial dimensions - its geometry is carefully arranged to remove many of the adverse effects of the relatively conductive sea water. In particular, accurate estimates of sea floor resistivity are possible because the data are proportional to the transmitted current from the source into the crustal material. A sea test of the method in 2.2 km deep water was conducted near an active hydrothermal sulphide zone on the Northern Juan de Fuca seafloor ridge. The survey results are consistent with 20 Ωm electrical resistivity or 12% porosity for the upper few tens of meters of basalt. Any sub-bottom metallic sulphide deposit at the site was found to be smaller than economic deposits on land. The technique should now be applied to a deposit located in an inactive hydrothermal area, or off the ridge-axis, or on a seamount. Detailed geological information from such a survey could provide a better understanding of many ancient deposits found on land.

Keywords: Geophysics
Advisor: Edwards, R. Nigel
On the Development of Downhole Magnetometric Resistivity

The Magnetometric Resistivity (MMR) method is an electrical method used in exploration for conductive, mineralized bodies. The MMR technique measures low level magnetic fields caused by currents injected into the ground. The cross-hole variant, introduced in 1980 by Newmont Exploration Ltd. in conjunction with the University of Toronto, requires at least two boreholes in the vicinity of a target. In the first, two fixed current electrodes are located, one above the other. They are effectively joined by a single cable carrying the low frequency transmitter current. In another borehole, a sensitive coil measures, as a function of depth, the component of the magnetic field of the transmitted current parallel to the hole. The practical distance scale of Cross-Hole MMR is of the order of one kilometer. This thesis describes the instrumentation of the down-hole probe. The sensor consists of a 101 centimeter long ferrite core of relative permittivity 204. Twenty-four thousand turns of magnet wire of radius 0.95 cm to 1.27 cm are wrapped in ten equal sections about the core. The coil is connected in series with capacitors of total capacitance 8.63 micro-farads to resonate at the operating frequency of 7.5 hertz. Five tuned, active filter stages connected in cascade, amplify the received signal before transmission up hole in Kevlar reinforced, shielded cable. The circuit diagrams of the probe assembly as well as their Bode diagrams are given. The diameter of the finished probe is 3.2 cm and can fit into EX (3.6 cm diameter) sized holes. Nonvertical boreholes cause the probe to sense the primary magnetic fields produced by the current wire and current sources. This effect is removed in a layer over a half-space earth model. Dip and tropari (azimuthal) measurements of the boreholes are used to determine the location and orientation of the probe, current wire, and current sources. The mathematical development of Edwards et al (1978) of a thick outcropping dyke is rotated to produce the expressions for current sources in a layer over a half-space earth. Primary magnetic fields of the current wire sensed by the coil a.re removed through the use of the Biot-Savart law. Data collected from field tests of the down-hole probe is presented. Lateral conductivity changes are observed in the data. The data is corrected for the effects of overburden and nonvertical boreholes. A comparison of the Cross-Hole MMR results with Crone Pulse EM data collected previously in the same boreholes a.re shown.

Keywords: Geophysics
Advisor: Edwards, R. N.
Electromagnetic induction in an inhomogeneous Conductive Thin Sheet

In electrically conductive geological environments electromagnetic prospecting systems experience difficulty in sensing bodies buried below conductive overburden. Not only may coupling exist between the conductor and the overburden, but in many cases an anomalous response may be due entirely to lateral variations in the conductivity of the overburden. This thesis presents a method of calculating the electromagnetic response of a thin sheet with laterally varying conductivity-thickness product. The thin sheet is suspended in free space. This model will approximate the effect of an overburden layer lying on an unweathered resistive crystalline rock. Two solution methods were used. The first was a frequency domain method in which a matrix equation was solved to find the horizontal wavenumber components of the induced vertical magnetic field. The method is best suited to calculating the response of the sheet when excited by frequency domain airborne E.M. systems. The second method calculates the time domain step-response of the sheet by time stepping the component of the magnetic field which cuts normally through the sheet. To calculate the time derivative of the normal (vertical) field, the horizontal components of the magnetic field must be known. The conversion of the vertical components of the magnetic horizontal components of the magnetic field is done in the wavenumber domain. To transform the field to wavenumber domain the fast Fourier transform is used. The time domain method works well when the sheet is excited by a large loop placed a small distance above the sheet. The conductance of the sheet must vary smoothly. Instabilities result at late times, but these have been overcome by forcing the solution towards the solution for a uniform thin sheet.

Keywords: Geophysics
Advisor: Edwards, R. N.
A theoretical Investigation of Airguns and Airgun Arrays

Airguns and airgun arrays have received a great deal of use in the exploration of marine sedimentary environments for possible oil and gas reserves in the last few years. In an effort to obtain a greater understanding of the basic physics behind the airgun, this thesis investigates the thermodynamic processes that can occur within the airgun. Consequently, a new model has been developed which takes account of the processes of evaporation and condensation of water as the stabilizing influences on the air bubble released by the airgun. The signal from a single airgun is far from being the ideal seismic impulse, so airgun arrays are designed to correct for some of the deficiencies of a single airgun. The physical size of an array is limited by mechanical constraints and other constraints1 and interaction of airgun signals occurs. A novel Lagrangian dynamics approach to solving this interaction problem is introduced here. A general formulation of the interactive system is given along with a first-order solution. The major advantage of this method is that the complexity of the solution can be increased to incorporate higher order approximations.

Keywords: Geophysics
Advisor: Bailey, R. C.
The Horizontal Loop EM Response of a Thin Vertical Conductor in a Conducting Half-Space

The frequency domain electromagnetic response of a thin vertical tabular plate in a conductive two-layer earth can be obtained by solving the electric field integral equation using the method of point collocation with global test functions. When it is assumed that the components of the scatter current perpendicular to the plate edges vanish at the plate edges, and when test functions are chosen which satisfy this condition, EM responses can usually be obtained for plates ranging from the resistive to the inductive limits. For highly resistive host media, numerical failure occurs when the plate conductivity is high enough that magnetic effects predominate. The solution is valid over a sufficiently wide range of parameters as to permit the computation of horizontal loop (Slingram) profiles over a plate in a layered earth. Suites of horizontal loop Argand diagrams are presented which illustrate the peak (in-phase and quadrature) anomaly values for a plate imbedded in a conductive earth. The suites show anomaly enhancement and phase rotation due to the conductive host and can be used to obtain, from field surveys, estimates of conductor depth and conductivity-thickness which are independent of the prospecting frequency.

Keywords: Geophysics
Advisor: Hall, D. H.