Peter Whyte Walker

Parametric estimators for current excitation on a thin plate

Parametric analysis is used to predict whether induction or current channeling dominates the current excitation in a conductive body in the earth. Knowing that one mode dominates the current response permits the use of relatively simple models that account for only a single mode of excitation in place of more complicated general ones that account for both modes of the current response. This is useful both in forward modeling and in inversion. In interpretation, predicting the current excitation is useful for verifying that the assumed mode of excitation is consistent with the interpreted body. Prediction is done with a set of “current excitation ratios” that we demonstrate for a thin conductive plate in a conductive half‐space. To derive the excitation ratios, parametric theory is used to estimate the strength of the inductive and galvanic modes of the current response of the plate. The ratios then follow by dividing the inductive estimate into the galvanic one. When this ratio is less than one, induction...

Characterization of the Gamsberg deposit from electromagnetic modelling

The Gamsberg deposit, located in the Northern Cape province of South Africa, contains a major zinc resource in a complexly folded synclinal sequence that extends over several kilometers. The deposit can be treated, for the purposes of electromagnetic modelling, as a thin sheet in a resistive background medium. Because the sequence is large and folded, standard approaches to modelling using thin, plate-like bodies have been limited to representing small regions of the conductor where the current systems can be approximated to flow locally and in a plane. As a result, late time behavior, where the currents can be controlled by the non-planar shape of the conductor, has been difficult to characterize. However, recent use of SQUID sensors at Gamsberg has lowered noise levels and extended the delay times at which data can be reliably acquired over those previously acquired with coil sensors. By using a mesh-based integral equation algorithm to model these data, the curved character of the conductor can be represented, resulting in improved understanding of the character of the conductor and the mineralized zones within it.

The effect of discrete conductivity isotropy on electromagnetic surveys

The effect of discrete conductivity anisotropy on electromagnetic survey interpretation is examined with a numerical modeling study. Discretely anisotropic media are shown to mimic the characteristics of horizontally layered media, alter the apparent decay constants of conductors, and profoundly alter the amplitudes of the anomalous response. If unrecognized, the effects of discrete conductivity anisotropy on (mis)interpretation can be significant. The illustrations we present are representative of cases encountered in the Athabasca Basin in Canada.

Simple predictive discriminators for inductive and galvanic EM excitation

An electromagnetic field incident on a conductor cm stimulate current in the galvanic mode, the inductive mode or in a combination of the two. The relative strength of the modes depends on a variety of factors intrinsic to the conductor such as its dimensions, conductivity and the conductivity of the host region, The strength of the modes is also determined by the source electric and magnetic field strengths, which depend on the nature of the source, its orientation and its distance from the conductor.

Rapid joint imaging of multiple electromagnetic data sets

Summary An algorithm is presented which allows electromagnetic data acquired from multiple surveys to be imaged jointly. By jointly imaging data sets, it is possible to create an image that is more representative of the ground than could be obtained from imag es created from each data set individually. The algorithm is based on an approximation to the integral equation method, and fast enough to be used for routine interpretation on standard personal computers. An example using simulated data illustrates the effectiveness of jointly imaging a helicopter electromagnetic data with ground based slingram measurements. Rather than solving a full regularized inverse problem, I present a rapid electromagnetic imaging algorithm that is based on an approximation to the integral equation forward modeling method. The algorithm is fast enough that it can be applied routinely t o interpret the large electromagnetic data sets typical of mining exploration, and can furthermore be used to jointly image multi ple data sets. Results of the algorithm are demonstrated on synthetically generated data, illustrating the case in which helicopte r electromagnetic (HEM) data are jointly imaged with slingram ground data. The example typifies many in mining exploration, and illustrates the how the algorithm is applicable to assisting interpretation in where both airborne and ground electromagnetic data are available,. Theory In the integral equation method, the embedded structure is repl aced by an equivalent volume of induced sources, where the induced sources S, are related to the total field F through some anomalous physical property contrast, p by F = p S (1) The measured electromagnetic response of the body (or data) D, at a measurement location rm is given by the integral over the volume of the anomalous volume containing induced sources, S, weighted by a Green function G