VictorJuiz/thesis huggingface dataset:
The Effect of Current Channelling in Square Plates with Applications to MMR
Keywords: Geophysics

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
The quantitative Interpretation of Airborne Electromagnetic Data
Keywords: Geophysics

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.
Development and Application of a Short-Baseline Electromagnetic Exploration Technique for the Ocean Floor
Keywords: Geophysics

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.
On the Development of Downhole Magnetometric Resistivity
Keywords: Geophysics

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.
Electromagnetic induction in an inhomogeneous Conductive Thin Sheet
Keywords: Geophysics

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.
A theoretical Investigation of Airguns and Airgun Arrays
Keywords: Geophysics

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.
The Horizontal Loop EM Response of a Thin Vertical Conductor in a Conducting Half-Space
Keywords: Geophysics

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.
The Magnetometric Off-Shore Electrical Sounding (MOSES) Method and Its Application in a Survey of Upper Jervis Inlet, British Columbia
Keywords: Geophysics

Magnetometric off-shore electrical sounding, acronym MOSES, is an adaptable sea floor electrical exploration method. It circumvents the shortcomings of traditional exploration methods, such as dipole arrays and magnetotelluric soundings, which are adversely affected by the high conductivity of sea water. In this thesis, I describe the development of the MOSES theory, the equipment and methodology, and the processing and interpretation of data from a successful test survey. The method employs a vertical bipolar wire source. Current is injected through a sea floor source electrode and returns to a sea surface sink electrode. The current penetrating the sea bed generates a sea floor azimuthal magnetic field which is proportional to the contra.st between sea and crustal resistivities. Transverse isotropy does not affect the inherent axial symmetry of the method. I derive the magnetic field equations in wavenumber domain for a. transversely isotropic layered earth. The solutions for a halfspace with and without a thin intermediate layer show that intermediate conductive and resistive layers can be resolved. Current channelling is affected more by the host medium anisotropy than by the layer anisotropy. I derive apparent resistivity and anisotropy formulae which aid interpretation. The sea floor receiver is a remote encapsulated microprocessor controlled magnetometer which is deployed at the start of a survey and recovered at the end. My algorithm triggers the preset periodic data acquisition and stacking cycles, and the data retrieval. The transmitter is driven by a ship-board 2 kilowatt square-wave current transvertor which is synchronized with the receiver clock. I show that the source-receiver geometry must be known for proper data interpretation. Survey lines were placed in Princess Royal and Queen's Reaches of Jervis Inlet. I process the stacked station data to yield only the transmitted signal harmonics, and interpret the data using one- and two-dimensional and angled wire source models. The sediment resistivities are about 2 ohm-metres; the apparent porosities agree with core samples. The Princess Royal Reach sediment thickness and basement resistivity are not resolved. The Queen's Reach sediments a.re about 600 metres thick. The basement is very resistive; thus if basement hydrothermal flow is causing observed high heat flow in upper Jervis Inlet, it is localized.
The Development and Evaluation of a Crosshole Seismic System for Crystalline Rock Environments
Keywords: Geophysics

Crosshole seismic techniques appear to be useful in the evaluation of the mechanical properties of inhomogeneous crystalline rock masses, particularly when near surface weathered zones prohibit the use of high resolution surface seismic methods. A project has been undertaken to develop a crosshole seismic system specifically designed to meet the criteria of the Canadian Nuclear Fuel waste Management Program. Because the rock masses to be studied are at depth, the system was required to operate in environments with high hydrostatic pressures of up to 15 mPa, and was required to be non-destructive, because of the high cost of the boreholes. The system that has been built utilizes piezoelectric ceramics in a wall-clamped accelerometer geometry as both receiving and transmitting transducers. Although the peak power of the transmitter is limited by the design criteria, the highly controllable nature of the piezoelectric ceramic allows the use of a continuous transmitted signal. The impulse response of the rock mass is then estimated using cross-correlation techniques. In this manner significant noise reduction is obtained and the detection of low level signals is enhanced. The received signals are digitized and processed in the field using a microcomputer system. Tests have been conducted which demonstrate that crosshole seismology is capable of evaluating the mechanical properties of rock masses. P-wave transmissions on paths of up to 300 m and S-wave transmissions on paths of up to so m nave been recorded, and show that both velocity and amplitude information in the data reflect geological and geophysical effects observed using other methods. The system has also been used to study tube wave phenomena, which can provide information on the shear modulus of the rock in the absence of direct shear arrivals. Although problems exist in accurately determining the position of the probes and in calibrating the amplitude response of the system, valuable information can be collected and interpreted with the present system. Future developments in instrument design and in methods of tomographic reconstruction hold great promise in several fields of application, including mining engineering and rock mass assessment.
The calculation of electromagnetic fields in a horizontally layered earth for an arbitrary source
Keywords: Geophysics

In order to explore sedimentary environments with electromagnetics techniques, a greater understanding of the benefits and problems with the available methods must be obtained. In this thesis, an efficient method of calculating the response of a horizontally layered earth to an electric or magnetic dipole source has been developed. The computational effort involved in calculating response of a large grounded or ungrounded source was significantly reduced by using the concept of lagged convolution, and a novel integration technique. The integration technique permits the evaluation of the fields due to any arbitrary source on the surface of a layered earth. The combination of the two techniques will hopefully reverse the present thinking that it is much too expensive to do routine interpretation of large source responses using generalized inversion schemes. Since many rocks exhibit anisotropy and induced polarization, both effects are described in terms of the fields of a grounded electric bipole on the surface of a layered earth. Furthermore, the concept of considering a grounded source as two individual sources is used to explain the field responses to half-space and layered earth models. In general, the galvanic part of the source is sensitive to anisotropy and IP, while the inductive part of the source is not. The major advantage of a bipole source is that the fields due to subtle changes in conductivity can be discriminated. This makes a grounded bipole source suitable for EM exploration of sedimentary environments where various conductivity contrasts may exist between different layers in a stratigraphic section.
On the electromagnetic response of non-uniform overburden layers: scale model experiments
Keywords: Geophysics

The first problem the interpreter of electromagnetic data in mineral exploration is required to solve is to differentiate between anomalies caused by metallic conductors in the bedrock and those caused by any other feature. Many subsurface conductors other than ore deposits are conductive enough to cause electromagnetic anomalies. One of the main sources of such "geological noise" is the overburden layer or inhomogeneities in it. In this thesis the electromagnetic response of these inhomogeneities are investigated by means of scale model experiments with the horizontal loop technique of geophysical prospecting, Several characteristics of the response that will help to distinguish the electromagnetic response of a non-uniform overburden layer, from that caused by a bedrock-mineralized conductor are reported. To recognize these characteristics, survey data at several frequencies or coil separations are required.
Interpretation of Airborne EM (Electromagnetic) Measurements Based on Thin Sheet Models
Keywords: Geophysics

An airborne electromagnetic (AEM) prospecting system is used as a very rapid means of economically searching large geologically potential areas for sulphide ore bodies within the top few hundred feet of the earth's surface. This is possible because massive sulphide ores are usually much more electrically conductive than most host rocks. The system usually consists of a transmitting coil and a receiving coil flown by an aircraft. The transmitting coil generates a time-varying magnetic field in the low audio-frequency range (100-4000 hz). Any small perturbations of a component of the field are then recorded continuously as the aircraft flies along flight lines. The record over a conducting body is called an anomaly profile. The magnitude of an anomaly depends on conductivity3 size and shape of the conducting body causing it3 and the flight direction and height of the AEM system. Interpretation is based on a qualitative or quantitative comparison of the observed response (anomaly) with the responses that would be observed by the system over simple idealized conductors. Many geological conductors are somewhat sheet-like in form and dip into the earth at an appreciable angle. The extent of the sheet is often large compared to the region sensed by the system. Both for airborne and ground EM measurements the single most important model has been the (thin) half-plane. For the multitude of AEM systems actually in use even this simple model has not been fully explored. The only available data are the peak amplitudes of the vertical half-plane for 'small-scale' systems (inaccurate) and the Lockwood quadrature system, and some profiles over perfectly conducting half-planes for a variety of systems plus a few random model profiles for a few systems. The thesis contains systematic studies with several AEM systems over vertical and dipping half-planes, vertical and flat ribbons, strike limited sheets, vertical half-plane under a thin flat overburden and two parallel half-planes. Studies have been restricted to thin conductors because of modelling convenience, and to keep the number of parameters under control. While thick bodies are interesting, the simpler thin case must obviously be done first. Field and model data have been compared in a number of cases and it is found that most of the field data is explicable without further elaboration of the models. However, certain discrepancies are found even in these selected cases which indicate that additional features of the conductors may have to be taken into account (and cast suspicion on the quality of altimetry). In the cases examined, it did not seem that the conductivity of host rocks was having much influence. The study indicates that it is often possible to discriminate flat-lying conductors from dipping conductors with the present systems. Other effects such as finite extent, influence of overburden, and fringe conductors are difficult to identify from a single flight with a current AEM system.