Acoustic investigation of cold seeps offshore Georgia, eastern Black Sea

Several gas seeps and near-surface gas hydrate deposits have been identified in 850–900-m water depth on the continental slope offshore Batumi, Georgia (eastern Black Sea) using deep-towed high-resolution sidescan sonar data. The seeps are located on a ridge named Kobuleti Ridge separating two canyons: the Supsa canyon north of the ridge and the deeply incised central canyon south of it. The southern wall of this canyon shows signs for additional gas seeps. Gas seeps are shown by acoustic anomalies in the water column on raw sonar records and as high backscatter intensity areas on processed data. The seeps on Kobuleti Ridge are characterised by carbonate deposits at the centre and a much wider area where finely disseminated gas hydrates are present. Fractures of a NW–SE direction are present at the seep sites and are probably related to the formation and decomposition of gas. Individual sites of gas emission apparently exert their influence for a circular area of up to 40 m in diameter. Gas geochemistry from gravity cores shows high gas content and a mixture of biogenic and thermogenic gases together with the presence of gas hydrates. The seeps offshore Georgia are different from other known cold seeps in the Black Sea such as shallow water seeps of biogenic gas and deep water mud volcanoes. They are located in deep water within the zone of gas hydrate stability, lack significant relief and are characterised by active gas emission and the absence of mud volcanism.

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