Gas Hydrate and Acoustically Laminated Sediments: Potential Environmental Causeof Anomalously Low Acoustic Bottom Loss in Deep-Ocean Sediments

摘要

Gas hydrates (clathrates) are solid, icelike crystalline compounds formed frommixtures of water and certain gas molecules. They are an important secondary geological attribute of deep-ocean sediments, whose presence has been detected only recently. They are stable under medium-to-high hydrospheric pressures and low temperatures typical of ocean basin near-surface sediments, and they form in blankets up to 1100-m thick at the top of the sediment column. They have been identified in virtually all ocean basins, continental shelves, and deep lakes. Because of the extreme pressure in the deep-ocean basins and the generally low temperatures at which hydrates are stable, gas produced in deep-ocean, near-surface sediment or rising into it from below, will be transformed into gas hydrate. P-wave acoustic velocity of gas hydrates appears to be in the 2.5 to 4.5 km/s range, although sediment-gas hydrate is more commonly in the 2.3 to 3.4 km/s range. The presence of hydrates and the velocity structure they impart to bottom sediments can radically alter overall geoacoustic response. Three environmental cases have been modeled for 10 Hz, 50 Hz, and 1 kHz; unhydrated sediment, hydrated sediment, and hydrated sediment with bottom-simulating reflector (BSR) caused by subjacent free gas. At frequencies below 350 + or minus 50 Hz, there is a direct relationship between reflection-loss coefficient and frequency; lower frequencies show lower loss. Above 400 Hz, the effect of gas hydrates is less, and reflection loss is less frequency-dependent. Keywords: Gas hydrate; Anomalous low-bottom loss; Abyssal sediments; Bottom-simulating reflector(BSR); Acoustic-impedance enhancement; Energy resource. (emk)