| My purpose is to develop a new geophysical tool capable of assessing offshore methane hydrate deposits; something that traditional seismological techniques have not been able to do effectively. Methane hydrate is an ice-like clathrate trapping methane compactly. Natural deposits are estimated to account for 53% of the total organic carbon in the Earth, twice as much carbon as all other fossil fuels combined. Methane is not only an immense fuel resource, it is also a powerful greenhouse gas. Clearly the detection and evaluation of the extent of sub-seafloor methane hydrate deposits is of great importance, not only to determine the potential for resource recovery but also for natural hazard assessment.; I applied a relatively new and novel technique: seafloor compliance. Wind-induced surface gravity waves create a pressure field which propagates downward to palpitate the seafloor. Compliance is the transfer function between this pressure, which is of the order of 1 Pa and the associated vertical deformation, of the order of microns. Compliance measurements are made by lowering a self-levelling gravimeter, differential pressure gauge and data logger to the sea floor, where simultaneous time series of pressure and acceleration are recorded. Sea-floor compliance data can be inverted to yield a profile of shear modulus with depth and by inference hydrate content because the shear modulus is a strong function of sediment stiffness caused by hydrate cementation.; In conjunction with Scintrex Ltd., I adapted their gravimeter for seafloor use, and experimentally ascertained the frequency calibration of both it and the differential pressure gauge. I gathered seafloor compliance data on five separate research cruises off the coast of Vancouver Island. The data provide a new estimate for shear velocity in the ocean bottom sediments of the region, something quite difficult to achieve with traditional methods, but they do not as of yet uniquely resolve the hydrate layer. I have established how best to apply the method, and that it is consistent with other available geophysical data. When inverted jointly with seismic data, compliance can produce an estimate of the total mass of a hydrate deposit. |
