Electromagnetic methods (EM)
In Time-domain electromagnetic (TDEM) methods the transmitter current is a modified symmetrical square wave. Periodically, the transmitter current is abruptly turned-off to zero.
A typical TDEM resistivity sounding survey configuration is composed by an outer square loop laid on the ground and connected to the transmitter. The side length of the loop is approximately equal to the maximum depth of exploration. A second loop (usually) at the center of the transmitter coil is connected to the receiver.
When the transmitter current ceases, a short-duration voltage pulse is induced into the ground. This voltage causes a loop of current to flow in the immediate vicinity of the transmitter wire. Due to the finite resistivity of the ground, the amplitude of the current starts to decay. This decaying current induces (again) a voltage pulse that causes more current to flow at greater depth. This phenomenon is repeated until the voltage vanishes completely. The time variation of the current is measured indirectly at the receiver loop observing the decaying magnetic field. Thus, measuring the variation of the voltage at the receiver coil it is possible to estimate the electrical resistivity distribution with depth.