A geophysical exploration technique for investigating the subsurface distribution of rock and mineral formations.

Different rocks have different electrical resistivity. By introducing low frequency or direct electric current into the ground, usually via spike electrodes, and measuring the resulting voltage difference with a second electrode pair, a rough picture of the underlying structure can be achieved.

In practical terms, this can be a little difficult to achieve. A direct electric current over a large area is not easily achieved, but the most common method of doing this is as follows:
Dig a fairly large hole, around 1 to 2 metres deep. Into this, place a large metal spike: a crowbar will work well in this situation. Wrap the end of it liberally in tinfoil, and refill the hole.
Attach a length of copper wire (preferably heavy gauge) to the end of the metal bar. Walk a few kilometres in the direction you wish to map. Dig another hole, repeat the procedure.
Hook both ends of the copper wire up to a generator. Now you have a fairly large current flowing through the ground between the two poles. To gather data, simply walk along the route between the two poles, using an ordinary voltmeter and probe to check the current flowing through the earth at periodic intervals.

This method is widely used in hydrogeology, engineering geology and archaeology.