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Nowadays the transient electromagnetic method (TEM) is widely used in ground surface exploration, due to its ease of operation, high precision, large information and sensitivity to low resistance bodies.In the last two years, TEM has begun to be used in underground studies to detect water filled structures ahead of the tunnel.Because of the excellent resolution of low resistance bodies and the secondary field responses directly received by the antenna, TEM has a bright future in tunnel prediction.During a tunnel survey, the scale length of the survey field is so small that it is not possible to lay a large enough loop.Redesigning the optimal survey configuration of TEM is important for improving the detection precision of TEM soundings.In this paper, we introduce a TEM array with multi-aperture survey configuration in which a large aperture single transmitter loop was substituted with several relatively small aperture TEM arrays.Then we study the primary and secondary TEM fields to find a coherent multi-source TEM field.Furthermore, a coherent stack of multiple transmitter sources can improve the intensities of the primary and secondary field.It is shown that the multiple small-aperture TEM system can improve the secondary field response by nearly 31%.A tunnel forecasting TEM system has been developed and a case study shows that it is an effective and successful method for exploring and predicting unfavorable geological structures ahead of the tunnel wall during tunnel drilling.
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Guo et al. (2010) studied this question.
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