Acoustic methane seepage quantification model design, experiments and deep-sea application

Detailed acoustic measurements of methane bubble distribution and plume dynamics were conducted during the Meteor M72/1 research survey in a deep marine hydrocarbon seep environment. This is located within the Dnepr paleodelta (north-west Black Sea) at a water depth of up to 2000 m. Two sonar systems were employed; one is a PARASOUND echosounder (used for detection and localization of the seepage points at the seafloor) and the second is a horizontally looking sonar mounted on a remotely operated vehicle (ROV), used for gas flux quantification. The evolution of several bubble plumes was followed during their rise through the water column. It was detected that the dissolution of methane had a significant impact on the acoustic backscatter, especially in the region below the gas hydrate stability zone. Strong methane seepage flows were observed with volume fluxes ranging from 150 mm³/s to 600 mm³ /s, measured from single outlets at the seafloor and rising up to 200 m bellow the water surface, thus demonstrating the rapid exchange of gases within the water column. A comparison with experimental data shows good agreement with predicted gas volume fluxes.