Thermal and visual time-series at a seafloor gas hydrate deposit on the Gulf of Mexico slope

Abstract

Ambient temperature is a critical factor determining the stability of gas hydrate deposits on continental margins. To study this process directly under varying conditions, a monitoring array comprising a time-lapse camera and in-situ temperature probes was deployed at a hydrocarbon seep known as Bush Hill, where gas hydrates deposits are exposed at the seafloor in a water depth of 570 m. For intervals of 91 days and 30 days, the digital camera recorded several daily images of a prominent gas hydrate mound consisting of structure II gas hydrate. The temperature probes were constructed with one autonomous thermistor at each end of a 50-cm PVC wand and recorded temperatures with precision of better than 0.1 °C at 30-min intervals over 327 days. One probe was implanted with a tight seal into a drill hole about 7 cm deep in the top of the gas hydrate mound. The second was inserted about 50 cm deep into the adjacent sediments. For each probe, the top thermistor recorded the ambient water temperature, while the bottom thermistor synoptically recorded the internal temperature of the hydrate or sediment. The bottom water temperatures ranged from 6.64 to 9.73 °C with a mean of 7.90 °C and standard deviation of 0.437. Photographic results showed no dramatic changes in the size, shape or gas venting from the mound. By comparing the temperatures recorded at the tips of the probes with the synoptic water temperature, we estimate that thermal diffusivity for the gas hydrate deposit was 7.14·10 −8 to 8.33·10 −8 m 2 s −1 and was 1.77·10 −7 to 3.01·10 −7 m 2 s −1 for the sediments at the sites where temperatures were recorded. The diffusivity measured in gas hydrate was lower than that measured under laboratory conditions. Stability of gas hydrate in this setting is not likely to be affected by short-term changes in bottom water temperature within the range observed.