Seismic indicators of gas hydrate and associated gas in the Ulleung Basin, East Sea (Japan Sea) and implications of heat flows derived from depths of the bottom-simulating reflector

Analysis of multi-channel seismic reflection data from the Ulleung Basin, East Sea (Japan Sea), reveals various seismic indicators of gas hydrate and associated gas, including the bottom-simulating reflector (BSR), enhanced reflections below the BSR, and seismic chimneys. The recent recovery of massive gas hydrate by drilling, together with these seismic indicators, strongly suggests favorable conditions for the formation of gas hydrate in the area. The BSR is most common and of a wide range of amplitude and continuity. Seismic chimneys, characterized by columnar zones of amplitude reduction, terminate mostly within the gas hydrate stability zone (GHSZ), but a few extend to pockmarks or mounds on the seafloor. Seismic chimneys with seafloor expressions probably represent vertical vents for gas originating from below the GHSZ. The increase in the degree of amplitude reduction in seismic chimneys with increasing seismic frequency may be due to seismic attenuation by gas bubbles trapped in hydrate-filled/coated fractures in the GHSZ. BSR-derived heat flows range from about 65 to over 115 mW/m2, comparable to those from direct measurements. Heat flows in the northern part of the study area are very high for the age of the basin, suggesting hotter than normal mantle temperature. The area of high (> 105 mW/m2) heat flows also coincides largely with the distribution of the presumed incipient oceanic crust, formed during the earliest stage of seafloor spreading.