Acoustic Determination of Methane Hydrate Dissociation Pressures

The unique nature of the molecular structures of gas hydrates results in curious acoustic properties which have yet to be adequately characterized. Understanding the acoustic behavior of hydrates in liquids, in bubbly liquids, and in sediments containing liquids and/or gas is vital for surveying their location using seismic or echosounding techniques and may become a key tool for monitoring hydrate dissociation and its possible link to climate change. Acoustic properties of gassy substances are known to have a strong dependence on excitation frequency; however, tabulated values of hydrate sound speeds are most often measured at high frequencies (>200 kHz) despite modern location methods which use frequencies below 100 kHz. This presentation details a laboratory experiment in which the dissociation pressures of natural structure I and structure II methane hydrate samples were determined by measuring their low-frequency acoustic velocity in a liquid as a function of hydrostatic pressure. [Work supported by The United States Office of Naval Research.]