Transient Electromagnetic, Physics tutorial

TEM survey parameters:

System in which primary field is not present when secondary fields are being estimated can use very high powers, and TEM systems are popular in areas where overload conductivities are high and penetration is skin-depth limited. As measurements are made when no primary field is there, transmitter loop, that may have sides of 100 m or more, can also be utilized to receive secondary field. Alternatively, smaller receiver coil can be positioned within loop. This method can be utilized in CWEM surveys only with very large transmitter loops due to strong coupling to primary field.

It is also possible to perform TEM Slingram surveys, and most commercial systems can use many different loop configurations. They vary in portability and, in detail, in sampling programs. SIROTEM may be taken as typical. It generates square-wave current with equal on and off times in range from 23 to 185 msec. Voltage in receiver coil can be recorded at 32 different times during eddy current decay, and signals can be averaged over as many as 4096 cycles.

Alternative approach is given by UTEM system, in which current with accurately triangular waveform and fundamental frequency of between 25 and 100 Hz is circulated in the large rectangular loop. In absence of ground conductivity, received signal, proportional to time derivative of magnetic field, is a square wave. Deviations from in the vertical magnetic and horizontal electric fields are seen by sampling at eight time delays. In mineral exploration, TEM data are generally offered as profiles for individual delay times. Results at short delay times are dominated by eddy currents in large volume, relatively poor conductors.

TEM depth sounding:

TEM methods were initially developed to overcome some of drawbacks of CWEM methods in mineral exploration but are now also being extensively utilized for depth sounding. In homogeneous or horizontally layered ground, termination of current flow in transmitter loop induces similar current loop or ring in adjacent ground. This current then decays, inducing further current ring with somewhat greater radius at slightly greater depth. Induced current therefore progresses through subsurface as expanding smoke ring and related magnetic fields at increasingly later times are determined by current flow (and therefore by resistivity) at progressively greater depths. TEM surveys with 100 m transmitter loops have been utilized to attain estimates of resistivity down to depths of several hundred metres, something needing arrays several kilometres in length if conventional DC methods are utilized.

If localized good conductors, whether buried oil drums or sulphide ore bodies, are present, effects of eddy currents induced in them will dominate late parts of decay curves and may prevent valid depth sounding data from being attained. Relatively minor shift in position of transmitter and receiver loops may be all which is required to solve problem.

710_TEM expanding smoke ring in layered medium.jpg

TEM and CWEM:

CWEM and TEM techniques are theoretically equal but have different benefits and drawbacks as the principal sources of noise are quite different. As noise in CWEM surveys arises largely from variations in coupling between transmitter and receiver coils, separations and relative orientations of coils should either be kept constant or, if this is not possible, should be very accurately estimated. Receiver circuitry should also be very exactly stabilized, but even so it is hard to make sure that initial 100% (for in-phase channel) and 0% (for quadrature channel) levels don't drift significantly during course of day. Very sharp termination of transmitter current gives timing reference which is inherently simpler to use than rather poorly defined maxima or zero crossings of the sinusoidal wave, and crystal-controlled timing circuits drift very little. Significant sources of noise in TEM surveys are external natural and artificial field variations. Effect of these can be decreased by increasing strength of primary field and by N-fold repetition to get a N improvement in signal-to-noise ratio. There are, though, practical limits to the methods of noise reduction.

Transmitter loop magnetic moments rely on current strengths and loop areas, neither of which can be increased indefinitely. Safety and generator power particularly set fairly tight limits on usable current magnitudes. Large loops which are essential for deep penetration are unavoidably hard to use and can be moved only slowly. Numerous repetitions are not a problem in shallow work, where virtually all helpful information is contained in first few milliseconds of decay curve, but can be time consuming in deep work, where measurements have to be expanded to time delays of as much as half a second. Furthermore, repetition rates should be adjusted so that power-line noise (that is systematic) is cancelled and not improved, and number of repetitions should be sufficient for this reason. As two coils can be superimposed in TEM survey, resolving power can be very high. TEM is therefore much more appropriate than CWEM for precise location of very small targets. Many modern metal detectors, comprising super metal detectors like the Geonics EM-63, that was developed specially to detect unexploded ordnance (UXO) at depths of few metres, utilize TEM principles.

TEM and IP:

TEM apparently resembles time-domain IP techniques. Most obvious difference is that currents in most IP surveys are injected directly in ground and not induced by magnetic fields. Though, at least one IP technique does use induction and more essential difference lies in time scales. Time-domain IP systems regularly sample after delays of between 100 msec and 2 sec, and so avoid most EM effects. There is small region of overlap, from approximately 100 to 200 msec, between two systems and some frequency domain or phase IP units are developed to work over whole range of frequencies from DC to tens of kHz to get conductivity spectra. Though, it is generally possible in mineral exploration to regard EM and IP phenomena as entirely separate and to avoid working in regions, either of frequency or time delay, where both are important.

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