Mapping free gas surfaces in Baltic Sea sediments

Sediment echo sounder data are routinely collected during research cruises with a geophysical or geological programme. This has resulted in the accumulation of huge amounts of sub-bottom data over the last decades (more than 30,000 survey lines of variable length in the Baltic Sea for the IOW alone), which are often not further interpreted. On the other hand, the value of these datasets for research and industry is increasing, as new regulations (e.g. related to the establishment of marine protected areas) and offshore infrastructure (e.g. wind farms) often prohibit the collection of new survey data. Current topics of interest in the Baltic Sea include the reconstruction of the Late Pleistocene to Holocene palaeogeography of the Baltic Sea and the identification of fluid flow and free methane in the subsurface. The latter can be used to assess the potential release of methane from carbon-rich sediments accumulating in the Baltic Sea basins by identifying the extent, depth and temporal variation of free gas surfaces (e.g. due to seasonal effects and changing wind conditions) in sediment echosounder data. Free gas in the subsurface and water column is readily identified in sediment echosounder and low-frequency multibeam echosounder data due to the increase in acoustic impedance between water-saturated sediments and gas.  Information on methane release is needed to assess the suitability of natural sediments in the Baltic Sea basins as a long-term carbon sink. The methane reservoirs in the southern Baltic Sea are related to various sedimentary and tectonic situations. The frequent generation of methane due to organic carbon accumulation, which is released into the water column and potentially the atmosphere, would have an antagonistic effect on carbon burial. Due to the large amount of data available, a deep learning model (U-Net) is trained on sediment echo sounder data from the Arkona Basin in the southern Baltic Sea to identify free gas surfaces and their depth below the surface. The parameters of the free gas surfaces are related to the thickness of the Late Pleistocene and Holocene sedimentary units (Baltic Ice Lake, Ancylus Lake and Littorina Sea). Repeated lines of sediment echo sounder data allow assessment of changes in spatial extent and depth of free gas through time.