Abstract: The depth of a metal object (for example) beneath the ground (for example) is measured by measuring propagation times of electromagnetic radiation travelling in two different paths within the ground, each from a transmission location via reflection from the object to a receiver location. There should be either multiple transmitter locations, multiple receiver locations or both and the spacing in the ground plane between the transmitter and receiver locations is known. By mathematically manipulating the propagation times for reflected signals travelling in different paths, dependence on the permittivity of the ground itself can be eliminated from the calculation of depth.