Sensitivity analysis of marine controlled-source electromagnetic methods to a shallow gas-hydrate layer with 1D forward modeling

Abstract

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.