Baltic Gas

Vast amounts of methane are formed in Baltic Sea sediments during the sub-surface degradation of buried organic matter. The methane accumulates in the seabed below the sulphate zone and, as methane is poorly soluble in seawater, methane bubbles may develop at a depth where the methane concentration exceeds saturation. The depth and extent of such shallow gas formation depends on the rate of methanogenesis, the thickness of the methanogenic deposit, and the hydrostatic pressure and thus the water depth.

Methane bubbles may be trapped in soft clayey sediments or rise towards the sediment surface. In most sediments, such rising methane bubbles dissolve again when they penetrate up into the sulphate zone where sulphate dependent anaerobic oxidation of methane takes place. However, occasionally methane bubbles also reach the sediment surface and escape from the sea floor in a diffuse or eruptive manner.

Methane seeps are observed mostly as plumes of gas bubbles rising up through the water column, recognizable by acoustic survey. Such plumes are observed in several areas of the Baltic Sea and may be responsible for an extensive methane flux into the seawater.

Mapping areas of shallow gas

The depth of the shallow gas appearance or, more precisely, the upper boundary of methane gas bubble occurrence can be easily detected by acoustic profiling even at gas quantities as little as 0.5 percent of the pore volume. Sub-bottom echo sounders such as CHIRP or Sparker systems are used to survey the seabed when mapping areas of shallow gas.

The normal echo sounder transmits high frequency sound waves down to the sea floor. Depending on the frequency, more or less of the energy is reflected by the sea floor, which enables a precise determination of the water depth with an accuracy of a few centimetres. Lower frequency sound waves penetrate deeper into the sediment and reveal variations in the hardness of the seabed due to differences in mineralogy and other geological features (sediments/rock).The seismo-acoustic data obtained with the above described methods give an acoustic cross section of the seabed. The sediments and sediment strata are seen by ‘acoustic imagery’ as a vertical pattern of shading.

Seismic instruments. A: Single channel GeoSpark 200 system. B: EdgeTech DF-1000 Digital side scan sonar system. C and D: X-Star Full Spectrum Sonar system, X-Star Tow Vehicle Model SB-0408

Different types of sedimentary deposits can be recognised by interpretation of the acoustic imagery: Sound waves penetrate relatively easily into fine grained sediments as mud, silt, and clay, while penetration depths are very limited in sand, gravel and glacial till. Gas, however, effectively absorbs the acoustic energy and consequently blanks all information from the underlying sedimentary strata. Thus by acoustic imagery shallow gas is observed as a conspicuous blanking.