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

Abstract

Over highly-conductive earths, the maximum effective depth of investigation of the EM-31 terrain conductivity meter is reduced from its theoretical value of 6 m at low induction numbers. 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 the depth of the overburden-basement boundary in two-layered earth models is highly dependent on the geoelectric structure. Over highly-conductive earths, the in-phase component of the response is depends strongly on the subsurface electrical conductivity. The restricted measuring range of the EM-31 instrument is however a severe limitation on the usefulness of in-phase measurements in such cases.