QUANTIFYING THE AMOUNT OF GAS HYDRATE BENEATH THE CENTRAL SCOTIAN SLOPE FROM THE SEDIMENT VELOCITY STRUCTURE

Pressure-temperature conditions present in sediments on the Scotian slope, for water depths greater than approximately 500 metres, will cause water and dissolved methane in sufficient concentration to form gas hydrate. Increases in bottom water temperature or decreases in relative sea level can cause gas hydrate to dissociate, releasing water and methane. This release may be an important contributor to global warming due to increasing the amount of greenhouse gas, or to slope instability due to increasing the sediment pore pressure. The base of the gas hydrate stability zone is primarily determined by the sediment temperature, and therefore approximates the shape of the seafloor. This boundary is generally indicated by the occurrence of a bottom simulating reflector (BSR) resulting from a sharp acoustic impedance contrast between the high velocity hydrate and low velocity free gas. However, the presence of a BSR does not determine the concentration of hydrate in the sediment, nor does the lack of a BSR indicate the absence of gas hydrate. An additional approach is required to directly measure variations in the velocity structure of the sediment, which can be attributed to changes in the concentration of gas hydrate . In August 2002, an area on the Central Scotian slope was investigated where gas hydrates were expected. The sediment was imaged with single channel reflection seismics and velocity measurements were determined from wide-angle seismic reflection/refraction data recorded on three ocean bottom seismometers (OBS). One of the OBS locations displays a prominent BSR in the single channel reflection seismic record, while the other locations do not appear to. Levels of gas hydrate concentration will be derived from velocity models of the OBS data at each site. These results will help to better quantify the amount of gas hydrate that may be present beneath the Scotian slope.