Electrical and electro-magnetic methods measure either natural or man-made electric and/or magnetic fields. These measurements can be taken on land, in the air, at sea, or in a borehole. From these measurements, some parameter of the subsurface is derived. The most used parameter is resistivity (or its inverse, conductivity).
All rocks conduct electricity to varying degrees. The resistance to electrical current flow is called 'resistivity' and is measured with a direct-current (DC) resistivity instrument using electrodes that are implanted in the earth. DC resistitivity surveys commonly probe to depths of several hundred meters, and are often used for groundwater and engineering studies. (Fresh groundwater is resistive, whereas brackish water is conductive). 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.
Conductivity (how well a rock conducts electricity) is the reciprocal of resistivity, and can be measured inductively without physical contact with the ground. 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 ore bodies. Such systems discovered over $10 billion worth of ore deposits in Canada in the 1960s and 1970s. Hand-held metal detectors or 'treasure hunters' are examples of electromagnetic devices.
Examples of EM methods include: Magnetotellurics (MT), Audio MT (AMT), Controlled-Source AMT (CSAMT), Induced Polarization (IP), Direct-Current Resistivity (DC), Electromagnetics (EM). Often no distinction is made between EM and electrical methods and they are referred to as either EM or electrical.
What are EM methods used for? EM geophysics are used to look for almost any resource, whether it's minerals, oil, gas, groundwater, or geothermal energy. They are used for environmental and engineering applications, such as mapping brine contamination in the subsurface, or for finding things, such as archaeological sites or spent ordinance on bombing ranges. Some applications involve more academic topics, like mapping plate boundaries and deep-seated structures.
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