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  • 1
    Electronic Resource
    Electronic Resource
    Electromagnetic mapping of electrical conductivity beneath the Columbia basalts (1996)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 44 (1996), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: Sedimentary rocks beneath the Columbia River Basalt Group are recognized as having potential for oil and gas production, but the overlying layered basalts effectively mask seismic reflections from the underlying sediments. Four electromagnetic (EM) methods have been applied on profiles crossing Boylston Ridge, a typical east–west trending anticline of the Yakima Fold Belt, in an attempt to map the resistivity interface between the basalts and the sediments and to map variations in structure and resistivity within the sediments. The EM surveys detected strong variations in resistivity within the basalts, and in particular the continuous magnetotelluric array profiling (EMAP) revealed resistivity lows beneath the surface anticlines. These low resistivity zones probably coincide with fracturing in the core of the anticlines and they appear to correlate well with similar zones of low seismic velocity observed on a nearby seismic profile.The controlled-source EM surveys (in-loop transient, long-offset transient, and variable-offset frequency-domain) were designed in anticipation of relatively uniform high resistivity basalts, and were found to have been seriously distorted by the intrabasalt conductors discovered in the field. In particular, the resistivity sections derived from 1D inversions were found to be inconsistent and misleading. The EMAP survey provided the most information about the subsurface resistivity distribution, and was certainly the most cost-effective. However, both controlled-source and EMAP surveys call for accurate 2D or 3D inversion to accommodate the geological objectives of this project.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1996.tb00186.x
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  • 2
    Electronic Resource
    Electronic Resource
    EFFECTS OF WELL CASING ON POTENTIAL FIELD MEASUREMENTS USING DOWNHOLE CURRENT SOURCES (1990)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 38 (1990), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: A mathematical formulation for the electric potential from point current-sources coaxial with a metal casing has been obtained. The excitation caused by the axial point-sources will produce currents in the pipe. By assuming that the pipe can be divided into many cylindrical ring segments with constant axially-directed current, the solution of the fields inside and outside the pipe can be formulated in an integral form. The integral equation applied to the segmented pipe yields a set of simultaneous linear equations which are solved for the currents in the pipe; these are then used to calculate the potentials anywhere outside the pipe in the medium.This solution has been used to study the distribution of the potentials in a half-space for a single current-source at and beyond the bottom of a finite length of casing. For a casing 0.1 m in radius and 0.006 m in wall thickness with a conductivity of 106 S/m, in a half-space of 10-2 S/m, it was found that only in a region very near the pipe does the pipe exert substantial influence on the fields of a point-source 100 casing diameters beyond the end of the pipe. It appears that cross-hole resistivity surveys can be implemented without corrections for the casing if the source is located at least 50–100 casing diameters beyond the end of the casing. Hole-to-surface surveys are much more affected by the pipe. For a pipe-source separation of 100 casing diameters, the surface measurements must not be closer than a half pipe length for a 5% or less field distortion.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1990.tb01868.x
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  • 3
    Electronic Resource
    Electronic Resource
    COMPARISON OF FIVE LEAST-SQUARES INVERSION TECHNIQUES IN RESISTIVITY SOUNDING (1982)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 30 (1982), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: A brief history of the development of the inverse problem in resistivity sounding is presented with the development of the equations governing the least-squares inverse. Five algorithms for finding the minimum of the least-square problem are described and their speed of convergence is compared on data from two planar earth models. Of the five algorithms studied, the ridge-regression algorithm required the fewest numbers of forward problem evaluations to reach a desired minimum.Solution space statistics, including (1) parameter-standard errors, (2) parameter correlation coefficients, (3) model parameter eigenvectors, and (4) data eigenvectors are discussed. The type of weighting applied to the data affects these statistical parameters. Weighting the data by taking log10 of the observed and calculated values is comparable to weighting by the inverse of a constant data error. The most reliable parameter standard errors are obtained by weighting by the inverse of observed data errors. All other solution statistics, such as dataparameter eigenvector pairs, have more physical significance when inverse data error weighting is used.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1982.tb01334.x
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  • 4
    Electronic Resource
    Electronic Resource
    RESISTIVITY MODELLING FOR ARBITRARILY SHAPED TWO-DIMENSIONAL STRUCTURES (1979)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 27 (1979), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: A numerical technique is developed to solve the three-dimensional potential distribution about a point source of current located in or on the surface of a half-space containing arbitrary two-dimensional conductivity distribution. Finite difference equations are obtained for Poisson's equations by using point- as well as area-discretization of the subsurface. Potential distributions at all points in the set defining the half-space are simultaneously obtained for multiple point sources of current injection. The solution is obtained with direct explicit matrix inversion techniques. An empirical mixed boundary condition is used at the “infinitely distant” edges of the lower half-space. Accurate solutions using area-discretization method are obtained with significantly less attendant computational costs than with the relaxation, finite-element, or network solution techniques for models of comparable dimensions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1979.tb00961.x
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  • 5
    Electronic Resource
    Electronic Resource
    QUANTITATIVE INTERPRETATION OF TRANSIENT ELECTROMAGNETIC FIELDS OVER A LAYERED HALF SPACE (1969)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 17 (1969), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: The transient fields from a finite horizontal loop excited by a half sine wave current pulse have been computed numerically for a particular source receiver configuration at a height of 100 meters above a layered ground. The amplitude of the vertical component of the magnetic field has been chosen for the interpretation. Curves of apparent conductivity vs. time, plotted during the off-time of the signal, show that layering is easily resolved, that resonance effects are present and that polarization effects are detectable for certain types of polarization.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1969.tb02073.x
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  • 6
    Electronic Resource
    Electronic Resource
    ELECTROMAGNETIC RESPONSE OF TWO-DIMENSIONAL INHOMOGENEITIES IN A DISSIPATIVE HALF-SPACE FOR TURAM INTERPRETATION (1973)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 21 (1973), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: An integral equation technique based on the integral formulation of the electric field of a harmonically excited line-source in the presence of a two-dimensional scattering in-homogeneity is outlined. In this treatment the inhomogeneity is assumed to have an arbitrary cross-sectional shape and to be located in a layered, dissipative half-space. The scattering responses in terms of the horizontal and the vertical magnetic field components observed on the ground surface are studied for a variety of geologic models involving overburden layers, vertical or inclined veins and multiple inhomogeneities. The horizontal component of the total magnetic field appears to be the most diagnostic response parameter. A thin vein shaped target with a cross sectional area of 0.05 δ2× 0.50 δ2 (δ2 being the skin-depth in the lower half-space) located 0.50 δ2 away from the line-source and under a highly conductive overburden layer (σ1/σ2= 300) of thickness 0.02 δ2 is easily detected with a moderate resolution. The scattering responses are also sensitive to changes in depths of burial, inclination and conductivity contrast with the surrounding half-space of the target inhomogeneity. An interpretation scheme as well as a number of characteristic detectability parameters are developed for the conventional Turam method used in mineral exploration.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1973.tb00032.x
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  • 7
    Electronic Resource
    Electronic Resource
    SOME THEORETICAL ASPECTS OF ELECTRODE POLARIZATION IN ROCKS (1963)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 11 (1963), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: A study has been made on the influence of the several variables of an electrode-electrolyte system on the different parameters usually measured in connection with the induced polarization method of geophysical exploration. Both the reaction resistance and the Warburg impedance of the system are found to vary appreciably with active ion concentration. The activation energy barrier exerts a predominant influence only on the former, whereas the salt concentration is important to a small degree in the determination of only the latter.With a simple equivalent electrical circuit to represent the complex mechanism of conduction of electrical currents through mineralized rocks, the amplitude and phase of the metal factor are evaluated at a frequency of 10 cps. An analysis is then made to determine the build-up and decay characteristics of the transient signal in the rock induced by a rectangular current pulse. All these measurable quantities are highly dependent on the salt and active ion concentrations. It is interesting to note that the build-up or decay time of the transient signal increases normally as the square of the solution resistance and inversely as the square of the Warburg impedance. The activation energy barrier exerts, practically no influence on the metal factor nor on the transient signal.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1963.tb02033.x
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