Conductivity and Susceptibility Mapping Using Broadband Electromagnetic Sensors

The apparent conductivity and apparent magnetic permeability are derived from broadband electromagnetic data using a conductive and magnetic half-space model. The apparent permeability is first estimated from the inphase and quadrature (or amplitude and phase) data at a low frequency. This is followed by the computation of the apparent conductivity at each frequency, using the precomputed apparent permeability. The apparent conductivity may be computed from broadband data using five different algorithms. In general, three of the five methods yield a unique solution; the three yielding a unique solution are the apparent conductivity defined from inphase, phase, or phase-amplitude. A suite of synthetic data based on two-layer models is presented and it is concluded that the phase-amplitude approach is the preferred method. The apparent permeability is virtually independent of the conductivity contrast at low induction numbers. However, the permeability contrast affects the apparent conductivity, especially when a resistive layer overlays a conductive basement. The field data examples show the usefulness of the broadband electromagnetic data and the resultant frequency-dependent conductivity-permeability maps for characterizing complex environmental sites.