Sediment echo types and acoustic characteristics of gas-related acoustic anomalies in Jinhae Bay, southern Korea

Abstract

Analysis of Chirp profiles from Jinhae Bay, southern Korea, revealed three sediment echo types (echo type I–III) and acoustic anomalies associated with shallow gas such as acoustic blanking and turbidity. Echo type I, characterized by low-to-high amplitude internal reflectors, almost parallel to the seafloor and seen away from the Gadeog Channel, probably represent mud from the Nakdong River that moved into the area by bayward transport. Echo type II, characterized by truncation at the seafloor and seen only in the Gadeog Channel, is interpreted to be tidal-channel deposits. Echo type III is acoustically transparent and interpreted as distal prodelta deposits. The acoustic blanking and turbidity were further divided into AB-I-III and AT-I-II, respectively. AB-I has a very strong, shallow top reflection that is parallel to the seafloor, crosscutting the stratigraphy. Depths of AB-I may correspond to the sulphate-methane transition that remains shallow probably due to large supply of methane. The top reflection of AB-II is of high amplitude and follows the bedding planes, probably controlled or limited by lithology. AB-III is characterized by a very irregular or highly diffractive and strong top reflection, which may be due to very closely-spaced patches of high-concentration gas. AT-I exhibits a weak and highly irregular top reflection, suggesting low gas concentration. AT-II, comprised of isolated weak diffractions, is likely due to isolated accumulations of low concentration gas. C/N ratios (6.8–8.6) of the organic matter in the cored sediments in the study area suggest mixed marine and terrestrial sources.