Hydroacoustic methodology for detection, localization, and quantification of gas bubbles rising from the seafloor at gas seeps from the eastern Black Sea

Abstract

[1] Detailed acoustic investigation of bubble streams rising from the seafloor were conducted during R/V Meteor cruise M72/3a at a deep submarine hydrocarbon seep environment. The area is located offshore Georgia (eastern part of the Black Sea) at a water depth between 840 m and 870 m. The sediment echosounder Parasound DS-3/P70 was used for detecting bubbles in the water column that causes strong backscatter in the echographs (''flares''). Employing the swath echsounder Kongsberg EM710 flares in the water column were mapped along the entire swath width of approximately 1000 m at high spatial resolution. The exact location of the flares could be extracted manually. Subsequently, the horizontally looking sonar Kongsberg digital telemetry MS1000 mounted on a remotely operated vehicle (ROV) was utilized to quantify the flux of bubbles. A model was developed that is based on the principle of finding the ''acoustic mass'' in order to quantify the bubble flux at various seeps. The acoustic approach from the backscatter data of the ROV sonar resulted in bubble fluxes in the range of 0.01 to 5.5 L/min (corresponding to 0.037 to 20.5 mol CH 4 /min) at in situ conditions ($850 m water depth, $9°C). Independent flux estimations using a funnel-shaped device showed that the acoustic model consistently produced lower values but the offset is less than 12%. Furthermore, the deviation decreased with increasing flux rates. A field of bubble streams was scanned three times from different directions in order to reveal the reproducibility of the method. Flux estimations yielded consistent fluxes of about 2 l/min (7.4 mol CH 4 /min) with variations of less than 10%. Although gas emissions have been found at many sites at the seafloor in a range of geological settings, the amount of escaping gas is still largely unknown. With this study presenting a novel method of quantifying bubble fluxes employing a horizontally looking sonar system, it is intended to contribute to the global effort of better constraining bubble fluxes at deep-sea settings.

Molecular and isotopic partitioning of low-molecular-weight hydrocarbons during migration and gas hydrate precipitation in deposits of a high-flux seepage site

“…Active seepage during our recent sampling campaign was indicated by gas flares rising up to about 300 m below sea level (Nikolovska et al, 2008). The authors reported a maximum gas flux of 5.5 L min − 1 from single outlets, and Haeckel et al (2008) calculated an annual flux of dissolved methane of 2.45 ⁎ 10 6 mol for a field covering 0.31 km 2 of the Batumi seep area.…”

“…7), as gas hydrates are 14 bar or almost 1.6°C, respectively, below decomposition conditions. Moreover, supply of vent gas from deep reservoirs through vertical faults (Wagner-Friedrichs, 2007) appears plausible with respect to the seepage intensity observed (Nikolovska et al, 2008). Assuming that transport of vent gas through permeable conduits proceeds with high velocity and low residence time compared to diffusive transport, LMWHC and carbon dioxide of this gas type should only be slightly affected by syn-migration alterations.…”

Automated gas bubble imaging at sea floor – a new method of in situ gas flux quantification

“…Nikolovska and Waldmann presented a study quantifying underwater gas seepage using passive acoustics (Nikolovska and Waldmann, 2006). Nikolovska and co-authors also employed a horizontally looking sonar mounted onto a ROV in order to quantify gas emission offshore Georgia at the eastern part of the Black Sea (Nikolovska et al, 2008). For gas quantification and bubble analysis, acoustic methods have the clear advantage of covering large fields of observation and not interfering hydrodynamically with the volume of observation.…”

Quantifying in-situ gas hydrates at active seep sites in the eastern Black Sea using pressure coring technique

“…The simultaneous occurrence of high amounts of gas hydrates and widespread vigorous gas ebullition observed in the area (Klaucke et al, 2006;Nikolovska et al, 2008) suggests gas transport as the major supply for methane to form gas hydrate. Our model simulations confirm this hypothesis because neither diffusion nor the low upward advection velocities constrained for the Batumi area are able to transport sufficient amounts of methane to the surface sediments to allow for considerable hydrate formation.…”