Abstract: Apparatus is provided which may be made portable and is calibrated to provide direct readings of terrain conductivity. Transmitter and receiver coils are separated by less than one tenth of the electrical skin depth in terrain having a maximum conductivity to be measured, at the frequency of alternating current signal applied to the transmitter coil, and the signals picked up by the receiver coil are processed to cancel received signal components in phase with the transmitted signal and amplify a received quadrature component so as to provide a direct indication of ground conductivity. The in-phase component received direct from the transmitter coil is cancelled by a preset equal and opposite component, and the in-phase component of the secondary signal received from the terrain is substantially cancelled by a form of negative feedback, in order to avoid swamping of the wanted signal.