Shallow gas in the central part of the Korea Strait shelf mud off the southeastern coast of Korea

Shallow gas in the central part of the Korea Strait shelf mud (KSSM) off SE Korea, revealed by Chirp and sparker profiles, is associated with 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 in sparker data. Assuming that the entire mud column below the acoustic blanking is gas-charged, the velocity effect of gas predicts that the velocities of the gas-charged sediments range from about 950 to over 1200 m/s, which are greater than the in situ velocities (<800 m/s) measured using the Acoustic Lance. Alternatively, assuming that the velocity of gas-charged sediments is 800 m/s, about 30–70% of the mud below the acoustic blanking are gas-charged.Gas samples from the cored sediments consist almost exclusively of methane and their δ13C 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 the average porosity of the mud is 75%, we estimated the total volume of combined dissolved and gas-phase methane in the study area to be 5.44×106 m3 at 15 °C and 1 atm.