In electrical prospecting, an inductive method in which the current flows through an insulated copper cable connected to a source of alternating current and run along the surface in a rectangle 1 mile by 1/2 mile (1.6 km by 0.8 km) in dimensions. A series of transverse profiles are laid out perpendicular to and crossing the cable, and the magnetic part of the electromagnetic field is measured at discrete points along the profiles by special search coils consisting of several hundred turns of wire. The magnitude and direction of the induced field observed by the coils can be related to the inductive effect of the subsurface material directly below.