Natural seabed gas leakage -- variability imposed by tidal cycles

Abstract

The likelihood of leakage from sub-seabed Carbon Capture and Storage (CCS) sites has been debated since geological storage was proposed as an effective option to remove greenhouse gas emissions from the climate system. Within the marine environment, passive acoustics has been presented as a feasible way for detecting and quantifying any such leakage. When determining estimates of gas escape across the seabed, the influence of dynamic environments, introducing natural variations in seepage rates must be considered, including tidal cycles. Panarea, Sicily, is the location of a series of natural marine CO2 gas seeps and provides an excellent test bed to investigate variations of natural seabed gas leakage across a tidal cycle. A multivariate statistical approach was used to recognize the relationship when gas leakage is dominated by natural forcing. We show that the tidal height correlates negatively with the bubble sound power spectral density, the gas flux, and the bubble size. The strength of the correlation can vary significantly for different investigated time periods of observation, showing sensitivity of tidal influence. Our results corroborate evidence that natural migration of CO2 through the seabed is moderated by tidal cycles.

Passive acoustic monitoring of a natural CO2 seep site – Implications for carbon capture and storage

“…Panarea, located in the southern Tyrrhenian Sea, is an island within the 200 km long Aeolian Arc, parallel to the continental slope of north coast of Sicily and western coast of Calabria [23,30,32,33,34,35,36].…”

“…Further, accurate quantification using active methods requires the use of sonar(s) with a broad range of frequencies, typically in the kHz range for exciting mm-scale bubbles [22]. An alternative approach is to adopt passive acoustic methods to investigate the gas seepage from subsea installations [7,10,23]. The most common passive acoustic approach is to measure acoustic sound radiated from gas seeps at a reference point which is close to the leak location [6,10,24].…”

Underwater radiated noise from hydrofoils in coastal water

“…In this study, the acoustic recorder was positioned in the middle of the water column and may not be necessarily representative of the levels throughout the entire water column (McKenna et al, 2012). The noise levels recorded by it may differ from water column properties during each ship passage (Jensen et al, 2011), the seasonal variations and levels recorded at different range and depth, and the sea currents (Li, 2017;Li et al, 2019b). The modelling of the acoustic propagation channel is helpful to find the noise source level of a vessel, and also can be used to make comparison between events at different ranges.…”

Passive acoustic localisation of undersea gas seeps using beamforming

“…Recent technology developments for such monitoring include innovative methods in terms of acoustics, imaging, optics, chemistry, biology and geophysics Shitashima et al (2015); Li et al (2019b,a); Jenkins et al (2012); Stalker et al (2012); Zoback and Gorelick (2012); Johnson et al (2009); Roberts et al (2017). Acoustics is a key technology to achieve this, with roles for both active Nikolovska et al (2008); Leblond et al (2014); Leifer and Tang (2007); von Deimling et al (2011) and passive Berges et al (2015); Li et al (2019a) methods. Both approaches have been used to understand gas seeps in the seabed subsurface and in the water column, and complement each other with active methods being effective as surveying tools for detecting and localising gas in the water column, and passive techniques being well suited to long-term monitoring of small areas, providing estimates of gas flux and bubble size distributions von Deimling et al (2010); Hovland and Sommerville (1985).…”