Electrical currents induced in the ground by an electromagnetic system are affected by the resistivity r, magnetic permeability m (equals susceptibility(k)+1)and dielectric permittivity e of the host rock. Resistivity calculations currently standard in airborne EM processing assume that m and e are equal to that of free space and that only r is changing. This is very often not the case, and variations in m and e can affect the calculated apparent resistivity, creating false anomalies, or masking real ones.

For example, kimberlite pipes in crystalline hosts appear as low resistivity anomalies, but also have a high magnetic permeability. The effect of the high permeability of the pipe is to reduce the in-phase component on the lower frequencies, increasing the apparent resistivity(Huang and Fraser, 1998, 1999a). This can reduce the anomaly measured over the pipe to below the local geologic noise, completely hiding the pipe.

Conversely, diabase dikes in a moderately resistive host can appear even more resistive than the surrounding rock, because of the effect of the higher permeability of the dikes. This creates false anomalies in the data, and may mask real anomalies close to the pipes. Permeability corrected apparent resistivities will calculate the correct apparent resistivity for the dikes relative to their host rocks.

High dielectric permittivity affects mostly the highest frequency measurements of highly resistive rocks, making them appear more resistive than they truly are(Huang et al, 1998, Huang and Fraser, 1999b). By calculating the apparent resistivity with the permittivity, it is possible to produce a more accurate measure of the rock resistivities.

Fugro Airborne Surveys has pioneered the application of these improved apparent resistivity calculations, including the combined calculation of the resistivity, susceptibility and permittivity to produce separate maps of all three rock parameters, each corrected for its dependence on the others.