Visualisation of Tensor Time Domain Electromagnetic Data

Abstract

-Long Offset Time Domain Electromagnetic (LOTEM) measurements traditionally use a single current source.By using a second source, a tensor analysis technique analogous to that used in DC resistivity multiple-source bipole-dipole surveying, is possible. An instantaneous apparent resistivity tensor is defined as the relationship between the time varying (total) electric field and the DC half space current density vectors due to each source. If the sources are dipoles the three coordinate invariant apparent resistivities of the tensor are independent of source orientation. For a uniform half space, one of the invariants is virtually constant in time, deviating from the half space resistivity by a maximum of 6%. This method provides a way in which the complicated data set obtained during a tensor LOTEM survey can be presented in a compact and intelligible form, and has many advantages over conventional methods of analysing LOTEM data particularly where the resistivity distribution is three dimensional (3D). Results from a 3D resistivity model of an idealised geothermal reservoir and outflow structure are used to illustrate the power of this analysis.