R. N. Edwards

GGG (France)

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L. K. Law

Geological Survey of Canada

Andrei Swidinsky

University of Toronto

S. J. Cheesman

University of Hull

Marion Jegen

GEOMAR Helmholtz Centre for Ocean Research Kiel

David C. Nobes

East China University of Technology

Liming Yu

Southwestern Institute of Physics

Jianwen Yang

University of Windsor

Peter Wolfgram

Fugro (United States)

Eleanor C. Willoughby

Ronin Institute

G. D. Garland

University of Toronto

Cooperative Institutions

University of Toronto

Geological Survey of Canada

Colorado School of Mines

Geophysical Laboratory

University of Waterloo

Center for Scientific Research and Higher Education at Ensenada

Scripps Institution of Oceanography

Ministry of Energy, Northern Development and Mines

GGG (France)

Natural Resources Canada

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The transient electromagnetic response of a resistive sheet: an extension to three dimensions

Geophysical Journal International (2010)

Andrei Swidinsky R. N. Edwards

Controlled-source electromagnetic prospecting systems which produce vertical current flow in the Earth are sensitive to horizontal, electrically resistive structures such as hydrocarbon deposits and fresh water lenses. The electromagnetic response of such 3-D targets can sometimes be approximated by the fields produced by an arrangement of thin resistive sheets buried in a conductive host medium. We have previously shown, through simplified 2-D modelling, that the resistive sheet problem is more subtle than the conductive case commonly used in mineral exploration. Not only does the resistive sheet require the continuity of the normal current density as opposed to the continuity of the tangential electric field, interaction terms within the coupling matrix must also be calculated in full rather than approximated by point dipole sources representing each element. The purpose of this paper is to extend these results to the more realistic and practical situation of a 3-D resistive sheet buried in a multilayered earth. The algorithm is validated against 1-D calculations for a very large sheet and 3-D finite difference modelling for a smaller sheet. A current deflection number characterizes the effect of a resistive sheet's transverse impedance and governs its response in the same way that the current channelling number does for a conductive target. The sheet can be represented to first order by a single point dipole located at its centre; at most receiver locations, arrival times are nearly identical to those found from the full solution although amplitudes show some deviation near the target. As an example of the use of the algorithm, the marine controlled-source electromagnetic response of a simple anticlinal hydrocarbon reservoir underlying near surface resistors such as shallow gas or gas hydrate deposits is modelled. Results show that such near surface anomalies, even those with low deflection numbers, significantly affect the response of the deeper target and must be included in modelling and interpretation.

Resistive touchscreen

10.1111/j.1365-246x.2010.04649.x

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Citations (13)

2-D Modeling of Sea Floor Magnetotelluric Data in the Presence of Bathymetry

AGU Fall Meeting Abstracts (2001)

Katrin Schwalenberg R. N. Edwards

Magnetotellurics

Seabed

Source

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2. The Magnetometric Resistivity Method

Society of Exploration Geophysicists eBooks (1991)

R. N. Edwards Misac N. Nabighian

INTRODUCTION The traditional resistivity method maps the electrical properties of the earth by measuring differences in potential at the earth's surface caused by galvanic current flow between two current electrodes. The magnetometric resistivity (MMR) method differs from the traditional method in that the potential electrodes are replaced by a highly sensitive coil or magnetometer and one or more components of the magnetic field are recorded.

Subsoil

Magnetotellurics

Electrical Resistivity Tomography

10.1190/1.9781560802686.ch2

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Citations (53)

Book review

Marine Geodesy (1981)

T. S. Murty R. N. Edwards

NOAA Atlas 3—The Central North Atlantic Ocean Basin and Continental Margins: Geology, Geophysics, Geochemistry, and Resources, including the Trans‐Atlantic Geotraverse (TAG), by Peter A. Rona (U.S. Department of Commerce, National Oceanic and Atmospheric Administration, Environmental Research Laboratories, 1980), 99 pp. $17.00. Master Tables for Electromagnetic Depth Sounding Interpretation by R. K. Verma (N.Y.: Plenum, 1979).

Plenum space

10.1080/15210608109379425

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Inversion and SVD analysis of frequency-domain EM data

Expanded abstracts with biographies/SEG technical program expanded abstracts (1985)

Stephen Reford Norman R. Paterson R. N. Edwards

PreviousNext No AccessSEG Technical Program Expanded Abstracts 1985Inversion and SVD analysis of frequency‐domain EM dataAuthors: S. W. RefordN. R. PatersonR. N. EdwardsS. W. RefordPaterson, Grant and Watson Ltd., N. R. PatersonPaterson, Grant and Watson Ltd., and R. N. EdwardsUniv. of Toronto, Canadahttps://doi.org/10.1190/1.1892694 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail FiguresReferencesRelatedDetails SEG Technical Program Expanded Abstracts 1985ISSN (print):1052-3812 ISSN (online):1949-4645Copyright: 1985 Pages: 643 publication data© 1985 Copyright © 1985 Society of Exploration GeophysicistsPublisher:Society of Exploration Geophysicists HistoryPublished: 07 Mar 2005 CITATION INFORMATION S. W. Reford, N. R. Paterson, and R. N. Edwards, (1985), "Inversion and SVD analysis of frequency‐domain EM data," SEG Technical Program Expanded Abstracts : 251-254. https://doi.org/10.1190/1.1892694 Plain-Language Summary PDF DownloadLoading ...

10.1190/1.1892694

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Approximate calculations of the transient electromagnetic response from buried conductors in a conductive half‐space

Geophysics (1984)

JN McNeill R. N. Edwards Gad Levy

The transient electromagnetic (TEM) response from a conductive plate buried in a conductive half‐space and energized by a large‐loop transmitter is investigated in a heuristic manner. The vortex and galvanic components are each calculated directly in the time domain using an approximate procedure which ignores the electromagnetic coupling present in the complete solution. In modeling the vortex and galvanic current flows, the plate is replaced with a single‐turn wire loop of appropriate parameters and a distribution of current dipoles, respectively. The results of calculations of the transient magnetic field at the surface of the earth are presented for a few selected cases of practical interest. The relative importance of the vortex and galvanic components varies with the half‐space resistivity. The vortex component dominates if the half‐space is resistive, in which case free‐space algorithms suffice for numerical modeling. Furthermore the measured responses give much useful information about the target, and large depths of exploration should be achieved. As the half‐space resistivity decreases, a significant half‐space response is observed, caused by currents induced in the half‐space itself. This response can be very large. Spatial variations in it caused by relatively small changes in resistivity, i.e., geologic noise, obscure the response from deep targets making them difficult to detect. The effect of the half‐space is also to delay, distort, and reduce the vortex component in comparison with the free‐space response. The behavior of the galvanic component is determined by the haft‐space current flow. The presence of this component explains the large enhancement of overall target response seen at early times over relatively resistive ground and the departure from an exponential decay seen over more conductive ground, again with respect to responses predicted by free‐space modeling. In more conductive ground the galvanic component completely dominates the vortex component, resulting in the loss of useful diagnostic information. Although target location and depth can still be determined, target shape and orientation are poorly defined. Because of galvanic current saturation good conductors are difficult to distinguish from poor ones.

Transient (computer programming)

Half-space

Resistive touchscreen

10.1190/1.1441737

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Citations (42)

On crosswell diffusive time-domain electromagnetic tomography

Geophysical Journal International (1997)

Liming Yu R. N. Edwards

We investigate the reconstruction of a conductive target using crosswell time-domain electromagnetic tomography in the diffusive limit. The work is a natural extension of our ongoing research in the modification of time-domain methods for the rugged marine mid-ocean-ridge environment, an environment characterized by extreme topography and pronounced variations in crustal conductivity on all scales. We have proved both in theory and in practice that ‘traveltime’, the time taken for an electromagnetic signal to be identified at a receiver following a change of current in the transmitter, is an excellent, robust estimator of average conductivity on a path between transmitter and receiver. A simple estimate of the traveltime for a parallel electric dipole-dipole system is the time at which the derivative of the electric field with respect to logarithmic time at the receiver reaches its maximum. We have derived the fundamental relationship between the traveltime and the conductivity of the medium for a uniform whole-space. We have applied the concept of the traveltime inversion to the related crosswell problem and demonstrated reconstructions of finite targets based on tomographic analyses. Results show that the crosswell time-domain electromagnetic tomography can supply useful information, such as the location and shape of a conductive target.

10.1111/j.1365-246x.1997.tb05660.x

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Cross‐hole magnetometric resistivity (MMR)

Geophysics (1984)

Misac N. Nabighian Gary L. Oppliger R. N. Edwards B. B. H. Lo S. J. Cheesman

The last decade has seen a growing acceptance of the magnetometric resistivity (MMR) method as a viable exploration technique in various geologic environments. Until recently, MMR exploration was carried out with both current electrodes and recording magnetometer located on the surface of the Earth. Significant improvements in anomaly amplitude can be achieved by lowering the recording magnetometer inside a drill hole. In contrast, the lowering of current electrodes beneath the surface does not always improve surface MMR responses. The advantages of locating the magnetic detector in a drill hole are illustrated numerically, anomaly calculations being carried out with a novel yet simple integral equation technique for a plate‐like body. The practicality of the cross‐hole MMR technique is demonstrated with a successful case history. Massive sulfide mineralization is mapped at a depth exceeding 500 m.

Anomaly (physics)

Drill hole

10.1190/1.1441758

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Citations (49)

Numerical computation of hydrothermal fluid circulation in fractured Earth structures

Geophysical Journal International (1998)

Jianwen Yang Konstantin Latychev R. N. Edwards

Hydrothermal fluid circulation through porous Earth materials is an important physical phenomenon occurring in both submarine and continental environments. Irregularly interconnected discrete fractures are pervasive in nearly all Earth materials, providing preferential paths for fluid flow and controlling the circulating fluid patterns. Most mathematical algorithms addressing hydrothermal convection problems treat rocks as πecewise continuous media. The representation of local, large changes in permeability requires a high level of discretization for accurate results and a corresponding large number of unknowns. The alternative is to incorporate fractures discretely through special adaptation of the numerical code. We adopt this approach to solve the coupled, time-dependent heat and fluid transport differential equations using the finite element method. The final algorithm is validated against both an analytical solution and νmerical solutions from a complementary but less general finite difference scheme. Case studies of some simplified fractured models indicate that fractures can induce and maintain hydrothermal fluid circulation in media which would otherwise be passive. Fracture location can control both convection pattern and vigour in a closed system. Discrete fractures can also significantly change an established convection pattern. Multiply fractured porous media are comparable with the homogeneously aniso-tropic media in the numerical solutions if the effective average horizontal and vertical permeabilities are kept the same.

Poromechanics

10.1046/j.1365-246x.1998.00669.x

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Citations (43)

Transient marine electromagnetics: the 2.5-D forward problem

Geophysical Journal International (1993)

Mark E. Everett R. N. Edwards

Marine controlled-source electromagnetic experiments are designed to measure the electrical conductivity of the sea-floor. The apparatus consists of a transmitter, typically an electric current dipole, and a series of remote receivers. Variations in the current through the dipole cause correlated variations in the electric and magnetic fields at the receivers. The signals contain information about the electrical conductivity of the crustal rocks. Electrical conductivity is related to such critical physical parameters as porosity, temperature, composition, fluid content and texture. Many interesting sea-floor structures, such as the mid-ocean ridge or the continental margin, may be approximated by a 2-D model. There is a defined local horizontal strike direction and the conductivity along strike is approximately constant. We investigate the response of an arbitrary 2-D structure to an artificial, compact source deployed on or near the sea-floor, a case commonly described as having 2.5 dimensions. Our aim is to improve the design of sea-going experiments and provide a tool for the interpretation of data. We transform the governing Maxwell equations into the Laplace and along-strike spatial Fourier domains. Two coupled linear-differential equations result whose dependent variables are the along-strike components of the electric and magnetic fields. The equations are solved by the finite element method. The accuracy of the numerical solution is dramatically improved by exploiting the known rate of convergence towards the exact solution with systematic doubling of node density. Responses in the space-time domain are recovered by a combination of inverse Laplace and Fourier transforms. We selected the Gaver-Stehfest algorithm to compute the inverse Laplace transform because it requires the evaluation of iespnses at vniy a small number of real values of the Laplace variable s, eliminating the need for any complex arithmetic. The output from the software we present here are fields on the sea-floor that result from a sudden increase in current through an electric-dipole transmitter, transient step responses. Computed transient responses are checked for accuracy against the analytic solution for a double half-space model and equivalent numerical solutioiis for an appropriate test structure. Two practical applications of the aigorithm are demonstrated. First, it has been suggested that the traveltimes of signals between a transmitter and a receiver array towed along the sea-floor may be rapidly inverted for variations in sea-floor conductivity, a type of tomography. We verify that the traveltime method works effectively for vertical structures. Second, the response of a fast-spreading mid-ocean ridge segment is modelled in detail. The diffusion of signals through the structure and their distortion by the conductive axial magma chamber and the near-surface zones of hydrothermal fluid circulation are presented as a sequence of snapshots. The geometry and physical properties of the magma chamber and the subsurface hydrothermal circulation, key components of all proposed geological models of ocean crust formation, may be constrained. The perturbations of the signal on the sea-floor are measurable and diagnostic of the presence of these conductors.

Seabed

10.1111/j.1365-246x.1993.tb04651.x

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Citations (65)