Combining total magnetic field maps from multiple very low frequency (VLF) radio transmitters into a unified image

Electromagnetic fields from distant radio transmitters are widely used to map buried conductive objects. This geophysical method can be implemented in ground-based, airborne, boat-towed, and drone-borne configurations. For the latter approach, it is convenient to make use of the total magnetic field measurement technique, which can be performed with non-oriented receiving systems but exhibits a strong dependence of the resulting anomalies on the transmitter azimuth. In this paper, we demonstrate how the total-field data collected at several close frequencies corresponding to different radio transmitters can be combined into a unified interpretable image that is unbiased by transmitter azimuths. To derive such an image, one can use data from either a pair of orthogonal transmitters or a triplet of transmitters with significantly different azimuths. The validity of the proposed approach is demonstrated using both synthetic and field data examples.