A noise impact assessment model for passive acoustic measurements of seabed gas fluxes

Abstract

Accurate determination of seabed gas flux is important for understanding natural processes as well as giving confidence that the size of any leaks from marine infrastructure can be properly assessed. Acoustic methods for flux determination require a relatively quiet underwater environment, and can fail when there is too much noise from other natural or anthropogenic sources. This study applies an acoustic monitoring example of seabed gas leakage in terms of sound level intensity, to statistically assess and minimize the impact from oceanic noise on seabed acoustic experiments which require relative quiet environment. It addresses the question: how far from a source of radiated ambient noise does a recording hydrophone and location of seabed gas need to be so that acoustic methods for remotely determining gas flux are successful. We develop a model to assess impacts of ambient noise under various conditions, incorporating sound/noise sources (seabed gas leaks, sea surface agitation and shipping) and underwater acoustic propagation. The reliability of the model is tested by comparing measured seabed ambient noise in the central North Sea, and the robustness of it is verified by presenting statistical outliers and a receiver operating characteristic (ROC) curve. A range of scenarios are presented for several gas flow rates, which show the threshold of detection when the recording hydrophone is at different distances from the location of seabed gas escape, and competing noise sources (including shipping and surface waves).

Natural seabed gas leakage -- variability imposed by tidal cycles

“…Thus, the probability density function (PDF) of bubble equilibrium radius p R0 b as well as the number of bubbles for each size are obtained, and the gas flow rate F [L/min] is then computed. Due to the short distance from the hydrophone to the seep site (2 m), here we have not consider underwater multipath propagation [12], [37]- [40], e.g., sea surface reflection, and apply the spherical spreading of the bubble sound in the acoustic channel.…”

“…However, the possibility of leakage from a CCS site is an ongoing concern, and multiple strategies for monitoring and quantification of such leaks have been presented [5]- [9]. Among these strategies, passive acoustic monitoring has been presented as one of the feasible techniques [6], [10]- [12].…”

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

“…The radius r(t) in Eq. (9) and the height h(t) in Eq. 12control the vertical extent of the plume axis.…”

“…In recent years, intensive research efforts have been launched by acousticians and oceanographers geared towards measurements of seabed gas emissions, both in controlled release (e.g., carbon dioxide (CO 2 )) [1]- [9] and natural seeps (e.g., submarine methane (CH 4 )) [10]- [13]. While these measurements are relatively easily conducted via optical and passive acoustic methods [9], active acoustic techniques are more complicated, with information on the variations of sound speed and attenuation resulted from the seabed gas emissions is still scarce.…”

Optical Flow-Based Detection of Gas Leaks from Pipelines Using Multibeam Water Column Images

“…Many types of MBESs can collect water column image (WCI) data, which carry backscattering signals of scatters from the transducer to the seabed. The images can be used to detect artificial or natural structures in water columns, such as gas bubbles rising from seep sites [2][3][4][5][6][7][8] or gas pipelines [9], shipwrecks [10], fish schools [11], in addition to serving as a reference for the quality control of multibeam bathymetric data.…”