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Sediment waves: geohazard or geofeature?

Putans V.A.

A large-scale (generally tens of meters to a few kilometers wavelength and several meters high), undulating, depositional bedforms generated beneath a current flowing at, or close to, the sea floor, are called sediment waves. Worldwide investigations of last decade revealed these geomorphologic features in different geological and paleogeological environments, including ones connected with oil, gas, and gas hydrates fields. The revealed facts that sediment waves a) can be not only confused with but be complicated by creep processes, b) are evidence for present or past active environment (turbid flows, trigging, gas eruption), c) due to lithologically sorted material can be small-scale reservoirs both for free gas and fluids; lead to assumption that presence of these geofeatures may be direct indicator of “geological state that represents or has the potential to develop further into a situation leading to damage or uncontrolled risk” e.g. geohazard. Due to recently increased interest to gas hydrates and awareness of ecological consequences of hydrocarbon exploration, special attention is paid to correlation between sediment waves, gas escaping structures and bottom-simulating reflectors (BSR) of gas hydrates. Processing of seismoacoustic data and analysis of acoustic field anomalies are primary methods of geophysical survey. Re-interpretation and comparative analysis of data by P.P.Shirshov Institute of oceanology RAS with well-presented gas hydrate fields resulted in a) finding positive correlation between sediment waves and BSR in special geological environment, b) classification of sediment waves/gas escape relations, c) identification of several unclassified acoustic anomalies in Central Caspian sea as possible shallow BSRs nearby newly-opened mud volcano and gas chimneys zone.