Total-field Rotational Invariant for Multi-transmitter VLF-EM Mapping

Radio signals from powerful VLF transmitters are widely employed for near-surface geological mapping using either multi-component or total-field EM measurements. Total-field technique is less informative but is more simple, robust and accurate for mobile VLF-EM systems (e.g., mounted on cars, aircrafts or drones). A serious drawback of the approach is that VLF-EM maps emphasize only conductive structures with dominant strikes in the direction of the transmitter, i.e., the anomalies depend on the transmitter choice. To overcome this problem, we propose a new VLF-EM parameter, which depends only on the subsurface conductivity and could be derived using total-field magnetic data from any 3 transmitters with significantly different azimuths.