Acoustic imaging of natural gas seepage in the North Sea: Sensing bubbles controlled by variable currents

Abstract

Natural seepage from the seafloor is a worldwide phenomenon but quantitative measurements of gas release are rare, and the entire range of the dynamics of gas release in space, time, and strength remains unclear so far. To mitigate this, the hydroacoustic device GasQuant (180 kHz, multibeam) was developed to monitor the tempo‐spatial variability of gas bubble releases from the seafloor. GasQuant was deployed in 2005 on the seafloor of the seep field Tommeliten (North Sea) for 36 h. This in situ approach provides much better spatial and temporal resolution of seeps than using conventional ship‐born echo sounders. A total of 52 gas vents have been detected. Detailed time series analysis revealed a wide range of gas release patterns ranging from very short periodic up to 50 min long‐lasting events. The bulk gas seepage in the studied area is active for more than 70% of observation time. The venting clearly exhibits tidal control showing a peak in the second quarter of the tidal pressure cycle, where pressure drops fastest. The hydroacoustic results are compared with video observations and bubble flux estimates from remotely operated vehicle dives described in the literature. An advanced approach for identifying and visualizing rising bubbles in the sea by hydroacoustics is presented in which water current data were considered. Realizing that bubbles are moved by currents helps to improve the detection of gas bubbles in the data, better discriminate bubbles against fish echoes, and to enhance the S/N ratio in the per se noisy acoustic data.

Passive acoustic quantification of gas fluxes during controlled gas release experiments

“…11, a correlation also noted in the time lapse photography and pCO 2 data . This variability with changing hydrostatic pressure is noteworthy and diverse for marine seeps (Schneider von Deimling et al, 2010;Leifer and Boles, 2005a,b;Torres et al, 2002;Heeschen et al, 2005Heeschen et al, , 2003Hsu et al, 2013;Boles et al, 2001). Here, the variability with tidal height is noticeable in the bubble generation rate distributions « (Fig.…”

“…Various authors (Schneider von Deimling et al, 2010;Leifer and Boles, 2005b;Torres et al, 2002;Heeschen et al, 2005Heeschen et al, , 2003Hsu et al, 2013;Boles et al, 2001) have noted a range of relationships between their measures of gas flux and tidal cycle in natural seeps, though no statistical analysis of the correlation (as done above) has previously been undertaken. The results from earlier authors range greatly.…”

“…The results from earlier authors range greatly. For natural seeps at a depth of 70 m, using a single frequency active sonar, Schneider von Deimling et al (2010) reported that the greatest flux follows high tide after a 90 • phase delay, which Leifer and Boles (2005a) suggest could be due to effects associated with the diffusion of gas in the sediments. Conversely, for natural seeps at a depth of 67 m, using seep tents, Boles et al (2001) reported that greatest gas fluxes occurred at low tide, which they attributed to the activation/de-activation of individual seeps.…”

“…In addition, sonar systems (e.g. scientific echosounders) hold the potential to produce quantification of gas flux (Hornafius et al, 1999;Caudron et al, 2012;Greinert and Nutzel, 2004;Schneider von Deimling et al, 2010;Ostrovsky, 2003;Ostrovsky et al, 2008;Nikolovska and Schanze, 2007;Shakhova et al, 2014). For the purpose of long term monitoring (e.g.…”

The Geomorphology and Nature of Seabed Seepage Processes

“…Within the central and northern North Sea, there are three, fairly well-studied methane seep locations: the Tommeliten seep area (56°29.90' N, 2°59.80'E) (Hovland and Sommerville, 1985;Hovland and Judd, 1988;Niemann et al, 2005;Wegener et al, 2008;Schneider von Deimling, 2010), the Scanner pockmark seeps (58°28.5' N, 0°96.7'E) (Hovland and Sommerville, 1985;Hovland and Thomsen, 1989;Dando and Hovland, 1992), and the Gullfaks seeps (61°10.1' N, 2°15.8'E) (Hovland, 2007;Wegener et al, 2008). Although each of them are located in different geological settings, they have one main aspect in common: -they occur as continuous macro-methane seeps (Fig.…”