Passive and active airborne electromagnetics - separate and combined technical solutions and applicability

Airborne electromagnetic methods are divided, by primary field sources, into 'active' (with controlled primary field sources) and 'passive' (without the ability to control the primary field). Each has pros and cons related to the depth of investigation, bandwidths, sensitivity, resolution, terrain clearance requirements, and parasitic effects. Expert Geophysics Limited has developed AEM systems utilizing active and passive principles, separate and combined. The MobileMT system is an entirely passive system using a remote reference technique. The system provides low-noise broadband data extracted from natural field audio frequency (AFMAG) and a very-lowfrequency (VLF) power spectra. In addition to the passive field data, but with limited broadband, the TargetEM system measures time-domain data with an active and focused source of the primary transmitting field. The combined (active and passive) airborne electromagnetic system records broadband streaming data used to extract AFMAG, VLF, and time-domain components. The natural field data, even in a limited frequency range, is valuable in filling the gaps when the time-domain method is limited - at mapping highly resistive geological terrains, in detecting superconductors, during surveys in rugged relief conditions, and at parasitic effects appearance. In this paper, we present the combined 'active-passive' system.