Technical Papers - Airborne Electromagnetics

The Impulse-Response Moments of a Conductive Sphere in a Uniform Field, A Versatile and Efficient Electromagnetic Model

Richard S. Smith, Fugro Airborne Surveys, Ottawa, Canada, and Terry J. Lee, Canberra, Australia.

Abstract

The moments of a sphere in a uniform field can be used to approximately model the moments of a sphere in a dipolar field. The numerical computations are trivial and the approximation is especially good for higher-order moments. The greatest discrepancy is seen on the zeroth-order moment at large radii. It is possible to reduce the discrepancy for the zeroth-order moment by modelling the large-radius sphere (in a dipole field) with multiple small-radius uniform-field spheres. The small spheres are placed to fill the volume of the larger sphere. The discrepancy can be reduced to less than 15 % in this manner.

The sphere in a uniform field can also be used to approximate the response of a body that has its currents constrained to flow in a specific dip plane. This means that plate-like bodies or anisotropic spheres can be modelled.

The third-order moment of MEGATEM data acquired over the Reid-Mahaffy test site shows an anomalous response than can be modelled by a sphere at 170 m depth with a conductivity of 15 S/m and a radius of 40 m. The currents flowing in the sphere are constrained to flow in a vertical plane. This model is consistent with the geology of the area and a hole drilled to test the anomalous zone.

Published in Exploration Geophysics (2001) 32, 113-118.

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