The moments of a sphere in a uniform field, a versatile model

Richard S. Smith; Terry J. Lee

doi:10.1071/ASEG2001ab133

ISSN: 2202-0586
E-ISSN:

oa The moments of a sphere in a uniform field, a versatile model
Authors Richard S. Smith^a and Terry J. Lee^b
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^a Fugro Airborne Surveys, , Canada, [email protected] ^b Canberra, , Australia, [email protected]
Publisher: Australian Society of Exploration Geophysicists
Source: ASEG Extended Abstracts, Volume 2001, Issue ASEG2001 - 15th Geophysical Conference, Dec 2001, p. 1 - 4
DOI: https://doi.org/10.1071/ASEG2001ab133
- Published online: 01 Dec 2001

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, however, the zeroth-order moment is not commonly used for interpreting airborne electromagnetic data.

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 also 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.

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References

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Article Type: Research Article

Keyword(s): airborne; electromagnetics; modelling; moments; sphere

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