Abstract

The most popular model for quantitative interpretation of airborne EM data is the conductive half–plane. This is a good shape for representing many long–strike conductors, but a different model is needed for interpreting localized anomalies. A conductive sphere is an obvious choice. Although the mathematical solution has been available for decades and dozens of papers have been published on the subject, no sets of type curves or interpretation diagrams for this simple model and actual EM prospecting system are yet available in the open literature.

For this paper, response profiles for an airborne EM system of “double dipole” type (coil axes in the direction of flight) have been computed from the mathematical solution using all necessary terms of the series. The anomaly profiles are of simple shape and it has been found that the profile half–width has a constant relationship with the depth to the center of the sphere. The profile half–widths along with the in–phase and quadrature anomaly amplitudes allow a direct interpretation of depth, radius, and conductivity of the sphere. Where the sphere is also permeable, interpretation of the accompanying magnetic survey profile can allow this extra parameter to be taken into account. Finally, sample AEM profiles from the New Insco deposit are interpreted. True depth as obtained from drilling lies between the depth interpreted by two extreme models, viz., a sphere and a half–plane.

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