Summary

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 three-dimensional 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 two-dimensional modeling, 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, all interaction terms within the coupling matrix must also be calculated in full rather than approximated by point sources representing each element. The purpose of this paper is to extend these results to the more realistic and practical situation of a three-dimensional resistive sheet buried in a multilayered earth. As an example of the use of the algorithm, the marine controlledsource electromagnetic response of a simple anticlinal hydrocarbon reservoir underlying near surface resistors such as shallow gas or gas-hydrate deposits is modeled. 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 modeling and interpretation.

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