Signal processing apparatus for frequency domain geophysical electromagnetic surveying system

Apparatus for processing signals in the frequency domain from a geophysical electromagnetic survey of the type in which terrain to be surveyed is subjected to a primary magnetic field by a transmitter energized by a cyclical current waveform such that the field is subject to abruptly commencing interruptions at regular intervals, and a receiver coil is responsive to secondary fields induced in the terrain, during the interruptions, the apparatus comprising a receiver producing secondary signals representative of the magnitude of the currents induced in the coil by the secondary fields, switching means configured to pass only such secondary signals, corresponding to currents induced in the receiver coil, as occur during regular predetermined intervals within the interruptions, a source of a reference signal in synchronism with said cyclical current waveform, means to control said switching means responsive to said reference signal, a narrow band filter receiving the output of the switching means and tuned to accept the fundamental frequency of the cyclical current waveform, at least a first synchronous detector receiving the output of the filter together with said reference signal whereby to detect the filter output in relation to the phase of the primary current waveform on a detection axis selected to recover the in-phase component of the secondary signal at said fundamental frequency.