Abstract: In this paper, we investigated the resolution of marine Controlled-Source Electro-Magnetic (CSEM) data for thin high resistivity gas hydrate layer. We introduced the term of detectivity that incorporates both the amplitude and phase information in this discussion. The analyses of the 1D CSEM data show that in deep water environment, the three diagnostic tools (the normalized amplitude ratio, the effective anomaly and the detectivity) can give a reliable diagnostic signature of gas hydrate. In shallow water, however, both the normalized field and the effective anomaly are poor diagnostic tools, heavily affected by air waves. The detectivity is hardly affected by air waves. For all water depths, it is a good diagnostic tool for a thin reservoir of high resistivity.