Application of the EM-31 terrain conductivity meter in highly-conductive regimes

Over highly-conductive Earths, the maximum effective depth of investigation of the EM-31 terrain conductivity meter is reduced from its nominal value of 6 m due to the effects of skin depth attenuation. In extreme cases, depths of investigation may be as small as 2.5 m and 4 m for the vertical coplanar and horizontal coplanar coil configurations respectively. Sensitivity of the instrument to variations in the parameters of layered earth models is highly dependent on the geoelectric structure. For high values of ground conductivity the in-phase component of the response depends strongly on the subsurface electrical conductivity, and for some layered earth models may offer an increased depth of investigation in comparison with apparent conductivity (quadrature) data. The restricted measuring range of the EM-31 instrument is however a limitation on the usefulness of in-phase measurements in such cases. © 2001, CSIRO. All rights reserved.