Initial analysis of Long Offset Time Domain Electromagnetic (LOTEM) data
has traditionally been made in terms of early and late time apparent
resistivities. By using more than one grounded bipole as a source, a tensor
analysis technique analogous to the method used to analyse DC resistivity
multiple-source bipole-dipole surveying may be formulated. We define the
instantaneous apparent resistivity tensor' as the tensor that relates the total
electric field vectors at each instant to the steady state (DC) uniform half
space current density vectors due to each source. Instantaneous apparent
resistivity tensors calculated for a 3D resistivity model suggest that profiles
of LOTEM measurements across simple 3D structures can be used to create an image
that reproduces the main features of the subsurface resistivity. Where
measurements are distributed over an area, maps of the tensor invariants or the
tensor ellipses at different times can be made into a sequence of images which
provide a way of time slicing' down through the target structure.