Magnetotellurics (MT) refers to a technique in which electrical resistivity is determined by doing measurements of electric and magnetic fields related to naturally occurring currents flowing in the ground. Typical MT frequencies are from 0.0005 Hz to 1,000 Hz. The ratio of the amplitudes of the electric and magnetic fields is used to calculate the electrical resistivity of the ground at a depth determined by the ground resistivity and the frequency of the measured signal. Higher ground resistivity and lower frequencies allow greater depth of investigation. Audio Magnetotellurics (AMT) is similar to standard MT in that it uses naturally-occurring currents, but the frequency band is limited to the audio range, generally from 0.1 Hz to 8,000 Hz. Depth of investigation for AMT is typically from 30m to 2 km.
Time-Domain EM (TDEM)
Time-domain EM is an active technique that induces small, transient eddy currents into the ground and then measures their rate of decay. The decay curve can provide information about the electrical properties of the earth materials or metal objects. TDEM is used for a variety of applications from mining to shallow metal detection. TDEM is synonymous with unexploded ordnance (UXO) detection and identification.