3D Flare Imaging
Gas bubbles can rise from the seafloor, where free gas is migrating through the sediment column. These socalled flares are clear indicators of such seepage sites, and acoustic surveying therefore supports their localaization and subsequent investigations and sampling.
To investigate the geometry and temporal variation of gas bubble streams, a datasaet above a seafloor pockmark in the Lower Congo Basin was analyzed. During seismic line surveying as well as continuous recording during geologic station work, a dense information could be gathered in the vicinity of the seep site. The beam width of the single beam echosounder Parasound (18 kHz) of +/-2° is too wide to localize the small patches of gas release directly, since the footprint of ~7% of the water depth is several hundred meters in greater water depth.
With a 3D gridding approach, lateral resolution can be improved and the localization of anomalies beyond the inherent depth-dependent footprint size of the sonar system. This allows a detailed reconstruction of gas bubble trajectories through the water column, revealing a stable gas bubble stream for the deepest 1000 m.
The bubbles are laterally shifted laterally by ∼200m in the transition zone between North Atlantic Deep Water and Antarctic Intermediate Water with a reconstructed current velocity of ∼4 cm/s. The direction of lateral shift varies throughout the ∼15 h survey possibly because of tides, internal waves and small-scale eddies at the water mass boundary.
Furthermore, a stacking of SBES water column data acquired during sampling stations showed a periodic variation in seepage intensity with periods of 6 and 8 min, which are likely related to temporally modulated gas supply to the sea floor. Singlebeam echosounder are commonly limited by their small coverage of the survey area during individual passes and their footprint size in detailed imaging of individual seep sites, especially in the deep sea. The presented gridding method for singlebeam echosounder data may facilitate the use of such data in deep water gas bubble stream identification and localization.
Wenau, Stefan, Spiess V., Keil H., Fei T. (2018) Localization and characterization of a gas bubble stream at a Congo deep water seep site using a 3D gridding approach on single-beam echosounder data. Marine and Petroleum Geology 97, 612-623, https://doi.org/10.1016/j.marpetgeo.2018.07.016. (PDF)