Airborne EM skin depths

Skin depth is an electromagnetic (EM) scale length that provides a measure of the degree of attenuation experienced by a particular frequency of an EM system. As has been discussed in the literature, skin depth is not a complete measure of the depth of investigation, but the two may be related. Frequency-domain airborne EM systems employ pairs of transmitter and receiver coils that use a frequency range from several hundred hertz to over 100 kHz. For elevated dipoles, both geometrical and frequency-dependent attenuation of the induced fields must be considered. For airborne EM systems it is possible to define a skin depth based only on the electric field induced by the transmitter. A vertical decay scale length, here defined from the at-surface position of maximum electric field, enables the same skin-depth estimate to be obtained for both cases of vertical and horizontal dipolar excitation. Such dipolar skin depths associated with towed-bird and fixed-wing airborne systems are studied in relation to frequency, conductivity and sensor elevation. Dipolar skin depths are found to be much smaller than their plane-wave counterparts except at high frequency (>50 kHz) and in combination with high conductivity. For the majority of airborne systems the influence of altitude on skin depth is highly significant. Dipolar skin depths increase with increasing sensor elevation. Low frequencies display the greatest sensitivity. At low elevation (<40 m), geometrical attenuation dominates the behaviour of the skin depth. The study indicates that typical low-altitude airborne surveys provide vertically compact assessments of subsurface conductivity, well suited to near-surface, environmental applications.