INTRODUCTIONIn the traditional frequency-domain electromagnetic (FEM) methods of exploration the ground is energized by passing an alternating current (ac) through an ungrounded loop situated usually on or above the surface of the earth. The primary field of the loop will induce eddy currents in all conductors present in the earth. The secondary electromagnetic (EM) fields due to these induced currents, together with the primary EM field, are recorded with a suitable receiver at various points in space. In general, the secondary EM field at the receiver, which contains all the information regarding the underground conductors, may be several orders of magnitude smaller than the primary field. Under these conditions the separation of the measured total EM field into its primary and secondary parts is difficult.This fact led to the idea of using time-domain electromagnetic measurements (TEM), often referred to as transient EM techniques. In TEM measurements a strong direct current (dc) is usually passed through an ungrounded loop (Figure 1). At time t = 0 this current is abruptly interrupted. The secondary fields due to the induced eddy currents in the ground can now be measured with a suitable receiver in the

A variety of signal processing techniques can be used to minimize the effects of noise on linear, wideband, electromagnetic (EM) systems operating in the timedomain. All systems use repetitive waveforms with polarity reversal in alternate half-cycles. Exponential averaging or digital integration (stacking) is employed to increase signal-to-noise (S/N) ratios by limiting the noise acceptance to narrow frequency bands centered on odd harmonics of the repetition frequency, the width of the acceptance bands being inversely proportional to stacking time For certain types of nonstationary noise (e.g., occasional transients) or coherent noise (e.g., powerlines) it is possible to increase S/N ratios above those obtained by simple stacking for an equal time by use of techniques such as pruning, tapered stacking or randomized stacking. With some system waveforms and when the noise spectrum is not "white", use of preemphasis filtering in the transmitter and a corresponding de-emphasis filter in the receiver may significantly improve the input S/N ratio before stacking. Specific applications of the various techniques are discussed with reference to one particular time-domain EM system, the UTEM 3 system. By their use, improvements in S/N ratio of as much as 6 to 1 have been regularly achieved without any increase in transmitter power, depending on the nature of the local noise.

Various types of electrodes designed for the measurement of the electric field in the soil or in sea water at periods larger than one minute have been compared in a one-year experiment in Garchy, France. The experiment included more than fifty electrode pairs with liquid or absorbed electrolytes and Pb/PbCl2, Ag/AgCl, Cu/CuSO4 and Cd/CdC12 metal-ion couples. The electrode parameters were systematically measured in the laboratory and the electrodes were installed in the field to constitute 50-meter long parallel dipoles separated by 2 meters. Pairs of electrodes used for sea measurements were monitored in a salted water vessel. Fourtytwo potential differences were recorded with a sampling interval of 1 minute between May 1995 and April 1996. When electrodes are compared, large differences are observed in the long term stability as well as in the sensitivity to diurnal variations, rainfall and soil saturation. For measurements in soil, the installation method of the electrodes plays an important role. In salted water, the best performing electrode pair has a drift of the order of 0.1 mV per year. In soil, typical drifts for the best sensors are of the order of 0.2 mV per month in dry soil and 0.5 mV per month in soaked soil. Preferred electrode designs and installation methods, depending on the external conditions or the type of geophysical measurement, emerge from this experiment. In addition to the magneto-telluric field, potential variations which are not electrode or installation effects are observed and attributed to electrical sources in the soil.

When airborne electromagnetic (AEM) data is acquired as a streamed or time-series data set, the great redundancy in the data favours compression as a first step in processing. Traditional data compression schemes are time windowing and spatial averaging. An alternative, more efficient data compression scheme is to transform time or frequency domain data to time-constant tau space, which has the effect of removing the waveform dependence of the AEM response.When there are many local anomalies and a variable background, the next stage of rapid processing is to transform the response to a conductivity-depth image (CDI) to facilitate geological interpretation of the background response. Use of the full time range of recorded data, particularly the inclusion of on-time data, improves the stability of the CDI process.The final AEM data processing step for mineral explora tion is to assess the likelihood that any local anomaly corresponds to a desired economic target. This step involves the extraction of target geometry and conductivity informa tion from the AEM data. The only economically feasible route at the present time is to parameterise both the data (using inductive and resistive limits) and the model to allow inversion of the local anomalies. A fit to one or two plate like conductors can be achieved in seconds; fits to a block like body take minutes on a fast PC. A significant research challenge remains to speed up and stabilise this process. APPENDIX A: MATHEMATICAL BASIS OF ARBITRARY WAVEFORM DECOMPOSITIONThis brief summary includes some sign corrections from the article by Stolz and Macnae(1998): Representation of time-domain step and frequencydomain responseThe step function response of an isolated conductor can be exnressed as:

A wide‐band time‐domain EM system, known as UTEM, which uses a large fixed transmitter and a moving receiver has been developed and used extensively in a variety of geologic environments. The essential characteristics that distinguish it from other systems are that its system function closely approximates a stepfunction response measurement and that it can measure both electric and magnetic fields. Measurement of step rather than impulse response simplifies interpretation of data amplitudes, and improves the detection of good conductors in the presence of poorer ones. Measurement of electric fields provides information about lateral conductivity contrasts somewhat similar to that obtained by the gradient array resistivity method.