Abstract: The secondary signals induced in the receiver coils of two or three transmitter-receiver coil-pairs flown over terrain to be surveyed are combined to provide additional data relating to the terrain. One of the coil-pairs is arranged in a whaletail configuration, and the other or others in standard and/or fishtail configurations. The inphase and/or quadrature secondary whaletail and standard or fishtail signals are combined in such a manner as to cancel responses due to conductive overburden and distributed magnetite, and the whaletail and standard (or fishtail) signals are compared to differentiate anomalies caused by thin and thick steeply dipping conductive layers.