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

Richard S. Smith; Terry J. Lee

doi:10.1071/EG01113

ISSN: 0812-3985
E-ISSN: 1834-7533

The Impulse-Response Moments of a Conductive Sphere in a Uniform Field, a Versatile and Efficient Electromagnetic Model
Authors Richard S. Smith^a and Terry J. Lee^b
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^a Fugro Airborne Surveys, , 2060 Walkley Road, Ottawa, Ontario, Canada, , K1G 3P5, . E-mail: [email protected], Phone: +1-613-731-9575, Fax: +1-613-731-0453 ^b P.O. Box 1984, Canberra, , ACT, , Australia, , 2601, . E-mail: [email protected]
Publisher: Australian Society of Exploration Geophysicists
Source: Exploration Geophysics, Volume 32, Issue 2, Jun 2001, p. 113 - 118
DOI: https://doi.org/10.1071/EG01113
- Published online: 01 Jun 2001

Abstract

The impulse response moments of a conductive sphere in free space excited by a uniform magnetic field can be used to approximate the moments of a sphere in a dipolar field. The numerical computations are straightforward and the approximation is especially good for higher-order moments. The greatest discrepancy is seen on the zeroth-order moment when the radius of the sphere is large. It is possible to improve the accuracy for the zeroth-order moment by modelling the large-radius sphere (in a dipole field) as the combined response of multiple small-radius spheres (each in a locally uniform field). The small spheres are closely packed inside 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 plane with a specific orientation. This means that plate-like bodies or anisotropic spheres can also be modelled.

The third-order moment has been calculated from data acquired during a MEGATEM airborne electromagnetic survey of the Reid-Mahaffy test site. There is an anomalous response in the third-order moment that 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.

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Article Type: Research Article

Keyword(s): airborne; electromagnetic; modelling; moment; sphere

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

Abstract

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