Abstract: The invention relates to the obtaining of highly diagnostic information concerning the distribution of electrical conductivity (resistivity) in the ground between two points by creating an electromagnetic field whose frequency can be varied over a desired range at one of the points and detecting and comparing at the other of the points, at a plurality of transmitted frequencies, components of the magnetic and/or electric field which are in phase and out of phase with a reference signal derived electromagnetically at the second point from the transmitted field whereby no cable or other physical connection between the two points are required. The invention also relates to apparatus by means of which the electromagnetic field is created, the reference signal electromagnetically derived and the in phase and out of phase magnetic and/or electric field components detected.