Full transmitter waveform transient electromagnetic modeling and inversion for soundings over coal measures

Abstract

Modeling of transient electromagnetic soundings requires consideration of the full bipolar transmitter waveform in order to obtain full measurement accuracy from field instruments. The full‐waveform transient may be computed using existing impulse‐response forward algorithms followed by a convolution procedure with truncation and residual estimation after an even number of terms, which results in average increases in run time of about 80 percent. Simple calculations based on approximations for the behavior of transient decay over (a) a conductive layer over a resistive basement, (b) a homogeneous half‐space, and (c) a resistive layer over a conductive basement show that the effect of the full waveform is greatest at sample times near the end of the transmitter off‐time, and for the case of a conductive basement. Failure to consider the full waveform is expected to yield errors in the amplitude of the computed transient in the range of 4 to 100 percent or more, depending upon the sample time and type of earth model. Examples of soundings over moderately conductive coal measures and highly conductive cindered coal show that failure to use the full waveform yields depths and resistivities biased to erroneously high values. The bias is small in the former case, but produces substantial errors in the latter case. Depth to the electrical basement of 300 to 400 m may be estimated to accuracies of approximately 5 percent by computer inversion to three‐layer or four‐layer models.