Magnetic permeability and electrical resistivity mapping with a multifrequency airborne EM system

The algorithms currently used to generate the apparent resistivity from helicopter EM data are not reliable in highly magnetic areas. This is because magnetic polarisation currents occur in addition to conduction currents, causing the computed resistivity to be erroneously high.

A new method for computing the apparent resistivity and apparent magnetic permeability has been developed for the magnetic conductive half-space. The inphase and quadrature responses at the lowest frequency are first used to estimate the apparent magnetic permeability. The apparent resistivity is then computed for all frequencies using the quadrature responses alone.

The EM response of magnetically permeable material is much greater for the inphase component than for the quadrature component. This means that the calculation of resistivity using the quadrature component at two frequencies is less subject to error from magnetic polarisation than if the inphase and quadrature components at a single frequency are used. The method allows the EM data to be portrayed as credible resistivity maps when magnetic polarisation currents occur in addition to conduction currents.