Abstract

Electromagnetic (EM) methods measure the distortions of a primary field which are caused by a sub-surface conductor. The resultant field is recorded as a function of frequency or of time, depending upon the harmonic or transient nature of the primary field. The two different types of measurements thus recognized are frequency domain (or continuous-wave) and time domain (transient or TEM) methods. Interpretation of EM data is possible by comparing field response with the analytic or experimental response of a heuristic model. Most of the interpretive developments have been done for the frequency domain technique, which is mathematically more tractable than the TEM technique when we consider generalized models possessing a conductive halo, over-burden, or host rock. For such models, TEM response is more easily obtained from analog model experiments (e.g., Velekin and Bulgakov, 1967; Palacky, 1975). Response curves thus obtained, however, are dependent upon the shape of the excitation pulse which varies among the different transient EM systems available; e.g., the Input system (Barringer, 1962) uses a 1.1 msec half-sine pulse, the Crone pulse EM system (Crone, 1975) uses a linear ramp pulse with 1.4 msec rise time, while the Russian MPP01 system (Velekin and Bulgakov, 1967) uses a 15 msec square step pulse.

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