A computer program has been developed to evaluate electromagnetic (EM) responses for a one-dimensional (1D) model with multiple source and receiver dipoles that are finite in length. Using the 1D code, we conducted sensitivity analysis of marine controlled-source EM methods to a gas-hydrate layer in the shallow section. In this study we used a normalized amplitude and amplitude difference of EM fields simultaneously in determining the detection capability of the hydrate layer. The field amplitude must be normalized by the one for the corresponding background model without the hydrate layer. The normalized amplitude can be numerically large, but if the field amplitude is smaller than the threshold, it would be misleading and therefore is useless. From these numerical experiments, we found that there are plenty of useful offset ranges and frequencies where amplitude difference is large enough to detect the target layer. Furthermore, an effect of air waves is almost absent in amplitude difference.

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Time-domain/transient electromagnetic (TEM) survey measures changes in the electromagnetic (EM) field with respect to time, after an artificially generated electric or magnetic field is discontinued. Therefore, only secondary magnetic fields are measured that are free from the primary magnetic fields. Theoretically, TEM measurements are frequency responses that provide information on the conductivities of shallow to deep sub-surfaces. TEM data, normally stacked, are corrected during data processing. TEM measurement responses for synthetic models can be numerically computed using the finite difference, finite element, and finite volume methods. In this review, initially we have discussed the basic theory of the TEM method and TEM survey systems by classifying them into loop and ground-wire transmitters. Subsequently, we discussed data processing, modelling, and inversion. Finally, we discussed field survey cases from various application fields by classifying them into land, marine, and airborne TEM systems.

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