Feasibility of a Time-domain Electromagnetic Survey for Mapping Deep-sea Hydrothermal Deposits

Marine controlled-source electromagnetic (CSEM) surveying has already become a popular tool for exploration of hydrocarbon deposits. In time-domain EM (TDEM) measurements, secondary fields which contain information on conductive targets such as hydrothermal mineral deposits in the seafloor can be measured in the absence of strong primary fields. The TDEM system is useful to the development of compact, autonomous instruments which are well suited to submersible-based surveys. In this study, we investigate the possibility of applying an in-loop TDEM system to the detection of marine hydrothermal deposits through a one-dimensional modeling and inversion study. The feasibility study showed that TDEM responses are very sensitive to a highly conductive layer. Time-domain target responses are larger and appear earlier in horizontal magnetic fields than in vertical ones. An inverse problem is formulated with the Gauss-Newton method and solved with the damped and smoothness-constrained least-squares approach. The test example for a marine hydrothermal TDEM survey demonstrated that the depth extent, conductivity and thickness of the highly conductive layer are well resolved.