Seismic imaging of shallow gas off the southeastern coast of Korea

摘要

Shallow gas in the central part of the Korea Strait shelf mud (KSSM) off SE Korea, revealed by sparker and Chirp, is expressed as acoustic blanking, acoustic turbidity, seepages, and plumes in the water column. The low velocity of gas-charged sediments caused velocity pull-downs below the acoustic blanking and acoustic turbidity. Gas samples from the cored sediments consist almost exclusively of methane and their δ~(13)C values (-77 to -84 per mil) indicate biogenic gas. Abrupt seaward termination of the acoustic blanking at depth of about 102 m suggests that the pore water at this depth completely dissolves the methane. Assuming that the amount of methane dissolved in pore water at this depth represents the average concentration of methane and that porosity (75%) is constant throughout the KSSM, we estimated the total volume of combined dissolved and gas-phase methane in the study area to be 5.43 x 10~6 m~3 at 15°C and 1 atm.