Natural gas hydrates are ice-like solids that occur worldwide in seafloor sediments typically along active continental margins. They consist of gas molecules, mainly methane, contained in a cage-like or clathrate structure of water molecules. They form offshore under low temperature and high pressure conditions, typically in the uppermost few hundred metres of sediment in a water depth exceeding about 500m. The global abundance of methane frozen in hydrate could exceed the amount of all other known fossil hydrocarbon resources. Gas hydrates are a source of a large volume of methane gas and are recognized as an important possible future energy resource and a hazard to conventional exploration for petroleum.

Marine controlled source electromagnetic(CSEM)methods, sensitive to variations in resistivity, here become an important and valuable tool in the detection of offshore hydrate. Hydrate is a resistive target. It increases the formation resistivity of a sediment layer if it forms in sufficient quantity to block previously interconnected fluid filled pore spaces.

CSEM methods depend on a simple concept of physics. if a time varying EM field is generated at or near the seafloor, then eddy currents are induced in the sea water and subjacent crust in accordance with Faraday's law. The outward progress of the currents with time depends on range and electrical resistivity of the surroundings. Measurements at a remote location of the electric and magnetic fields associated with the eddy currents may be inverted for the crustal resistivity structure including local anomalous concentrations of hydrate.

CSEM systems have been used to assess hydrate on the convergent Cascadia Margin, to the west of Vancouver Island, British Columbia. The area is the focus of intensive studies on gas hydrates which have been identified on seismic sections, by direct sampling and from the analyses of cores and logs collected by the Ocean Drilling Programs. The results are most encouraging with very large amounts of hydrate identified. Some samples have been trawled from outcrops on the sea floor.

The deposits are presently being monitored as a function of time-in this case several years. the objective is the understanding of how hydrate is formed and how it evolves. We installed specialized apparatus on the seafloor which is locally powered and which can communicate interactively with a remote base station. Neptune Canada a new cabled observatory network provides the necessary infrastructure.