Transient electromagnetics methods (TEM)

Brief overview

Time-domain electromagnetics TDEM (also called transient electromagnetics TEM) – is an electromagnetic sounding method widely applied for geophysical studies. It is based on the electromagnetic induction phenomenon. Primary electromagnetic field usually is generated by square or round horizontal loops. The size of loop varies from several to hundreds meters depending on measurement technique and desirable investigation depth. Signal generator connected to the loop produces electric pulses and magnetic field appears around the loop. Its change with time produces eddy currents in a geologic media due to Faraday’s law of induction.

The behavior of eddy currents propagation requires the diffusion law. Just after the switching off the current in the transmitter, the flipped current ring appears directly beneath the loop. This ring then start to plunge into the ground affecting more and more underlying rocks during the expansion. Eventually, induced currents decay completely, transformed into a heat energy. The spatial position of the current ring depends on both time and rock’s conductivity.  Rock conductivity values also determine the speed of such plunge and expansion of the current ring. Than lower the resistivity than slower the ring penetrates into the ground and disperse there. This is the idea of TEM soundings.

Eddy current generates itself the secondary magnetic field that is also changes in space and time. It is the magnetic field that produces the induction effect in the receiving magnetic antenna (or loop) placed on the ground. The intensity of the received signal as well as its time decay depends on conductivity distribution in the ground. The investigation depth can be reached by TEM technique varies from several to hundreds meters depending on measurement time, sizes of transmitter and receiver loop.

The advantages of TEM method is a consequence of its physical basement:

• The method is favorable for the detection of conductive rocks even if they are located inside the high-resistive bearing strata. For example, they are a massive sulfide body inside basic and ultrabasic rocks or massive polymetallic deposit surrounded by volcanogenic or sedimentary rocks.
• TEM is very sensitive to the conductivity variations that is why the method is suitable for the detection of very conductive rocks inside less-conductive ones.  For example, they could be sulfide bodies inside carbonic rocks, so one can detect gold-sulfide deposits in black-shale formations.
• Measurement technique does not require groundings, so it is possible to make soundings on snow (in winter) and dry sand (in a desert). It is even possible the making of helicopter-based aero-surveys.