Depth of penetration of frequency-domain airborne electromagnetics in resistive terrains

Abstract

The depth of investigation of a fixed-wing frequency-domain airborne electromagnetic system is studied. Starting from the definitions for plane-wave EM and a homogeneous half-space, the analysis is extended to dipole fields and a layered earth, and finally to more realistic two- and three-dimensional conductive sources. The main factors affecting the depth of penetration are coil configuration, operating frequency and signal-to-noise ratio of the AEM system. The analysis is based on forward modeling of AEM responses in the frequency domain using existing software for conductive ID, 2D, and 3D bodies. The AEM system analysed as an example is a vertical coaxial, frequency-domain system flown in a DC-3 aircraft. Summary diagrams for different model geometries are constructed from the modeling results. Based on the analysis of the modeling results at the inductive limit, the depth of penetration in resistive terrain for the AEM system studied is between 85 m and 210 m depending on the geometry and the size of the conductor. Host rock and overburden of low but finite conductivity do not cause a significant change in penetration values. The screening and the attenuation effects from conductive overburden and conductive host rock will, however, decrease the penetration depth.