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

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.