A Model for Variations of Sound Speed and Attenuation from Seabed Gas Emissions

Abstract

The release of greenhouse gas, such as carbon dioxide (CO2) and methane (CH4) into the atmosphere, is a major source of global warming. As such a large emphasis has been placed on developing methods of measuring the amount of gas escaping from natural seep sites, particularly in the marine environment. Fortunately, gaseous bubbles crossing the seabed interface into the water column are relatively easily detected and quantified by passive acoustics. Here we design an active acoustic model to determine the frequency dependent changes in sound speed and attenuation in the water column due to gaseous CO2. Our approach is to formulate a numerical model simulating the propagation of sound energy at different frequencies through a mixed media incorporating a gas bubble plume with detection on a hydrophone.

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

“…While for gases possessing higher solubility, such as the CO 2 , the dimensions (height and width) of any emerging gas plume would be smaller, making the detection of leakage/seepage with an active acoustic approach more difficult. 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].…”

Broadband Acoustic Inversion for Gas Flux Quantification—Application to a Methane Plume at Scanner Pockmark, Central North Sea

“…Understanding the rate of gas release from natural ebullition sites, such as pockmarks, into the water column is a major factor in understanding the input of greenhouse gases, such as methane (CH 4 ) and carbon dioxide (CO 2 ), into the global ocean system (Greinert, McGinnis, et al., 2010; Kennett et al., 2003; Leifer & Boles, 2005; Ligtenberg & Connolly, 2003; McGinnis et al., 2006; Shakhova et al., 2010). The detection and quantification of gas flux in the marine environment have relied upon methods of passive (Berges et al., 2015; Blackford et al., 2014; Leighton & White, 2011; Li, White, Roche, et al., 2019; Li et al., 2020) and active (Greinert, Lewis, et al., 2010; Greinert et al., 2006; Greinert & Nützel, 2004; Leblond et al., 2014; Li, White, Bull, Leighton, & Roche 2019; Nikolovska & Schanze, 2007; Ostrovsky, 2003; Riedel et al., 2018; Rona & Light, 2011; Shakhova et al., 2014; Veloso et al., 2015; von Deimling et al., 2011; Westbrook et al., 2009; Xu et al., 2014) acoustics. These two methods are largely complementary with passive acoustics well suited to long‐term and local monitoring of small sites allowing quantification, whereas active acoustics survey equipment is widely available and able to detect gas over a large spatial area but is less well adapted to quantification.…”