An assessment of the accuracy of boundaries picked by AEM sounding, with application to mineral exploration and salinity mapping

James Macnae; Ashley Grant; Richard Lane

doi:10.1071/EG03041

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

An assessment of the accuracy of boundaries picked by AEM sounding, with application to mineral exploration and salinity mapping
Authors James Macnae, Ashley Grant and Richard Lane
Publisher: Australian Society of Exploration Geophysicists
Source: Exploration Geophysics, Volume 34, Issue 1-2, Mar 2003, p. 41 - 45
DOI: https://doi.org/10.1071/EG03041
- Published online: 01 Mar 2003

Abstract

The vertical distribution of conductivity in the ground exhibits varied characteristics. Abrupt changes may occur at geological unconformities, and gradational changes are common with saturation and salinity gradients, or with clay content through the weathering profile.

This paper describes a rapid, automatic method of picking probable vertical locations for abrupt changes in conductivity using the conductance-depth curve derived in an approximate conductivity-depth transform of airborne EM data. The method is based on identifying the location of slope changes on a cumulative conductance vs. depth plot. The location of probable layer boundaries shows good agreement with synthetic data when the conductance contrast is adequate and boundaries are well separated. Correspondence of structures picked on separate x and z components increases the confidence of interpretation on a conductivity-depth image (CDI).

In field data, the quantitative usefulness of including probable layer boundaries on a CDI display is yet to be determined, qualitatively however it appears to provide very useful information to aid in the assessment of CDI section accuracy. It is also very useful where conductivity contrasts are poorly imaged by the limitations of colour bar or greyscale choice.

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References

Hunter, D., and Macnae, J., 2001, Subsurface conductivity structures as approximated by conductivity-depth transforms: ASEG 15th Geophysical Conference and Exhibition, Brisbane: Extended Abstracts.
Lane, R., Heislers, D., and McDonald, P., 2001, Filling in the gaps - validation and integration of airborne EM data with surface and subsurface observations for catchment management - an example from Bendigo, Victoria, Australia: Exploration Geophysics, 23, 225-235.
Macnae, J., 2003, Improving the accuracy of shallow depth determinations in AEM sounding: ASEG 16th Geophysical Conference and Exhibition, Adelaide: Extended Abstracts.
Macnae J., and Lamontagne Y., 1987, Imaging quasi-layered conductive earths by simple processing of TEM data: Geophysics, 52, 545-554.
Macnae, J., Smith, R., Polzer, B., Lamontagne, Y., and Klinkert, P., 1991, Conductivity-depth imaging of airborne EM step-response data: Geophysics, 56, 102-144.
Wolfgram, P., Christensen, N., and Sattel, D., 2003, Approximate 2D inversion of AEM data: ASEG 16th Geophysical Conference and Exhibition, Adelaide: Extended Abstracts.
Xie, X., Macnae, J., and Palmer, D., 1997, Rapid automated determination of shallow velocity-depth structure using first breaks and the generalised reciprocal method: Exploration Geophysics, 28, 170-177.

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Article Type: Research Article

Keyword(s): AEM; CDI; Conductivity; layer boundary; sounding

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An assessment of the accuracy of boundaries picked by AEM sounding, with application to mineral exploration and salinity mapping

Abstract

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