These measurements can be taken onland, in the air, at sea, or in a borehole. From these measurements, some parameter of the subsurface is derived.
All rocks conduct electricity to varying degrees.
DC resistitivity surveys commonly probe to depths of several hundred meters, and are often used for groundwater and engineering studies. The Megger earth tester shown below is an example of a hand-held resistivity instrument developed for measurement of the the earth grounding resistance for power supplies. Such instruments are called 'electromagnetic' and are based on measurement of the time-varying magnetic field in the frequency range 10 hertz to several hundred kilohertz. The early airborne electromagnetic system shown below was developed for searching for highly-conductive metallic orebodies. Hand-held metal detectors or 'treasure hunters' are examples of electromagnetic devices.