The acoustic remote sensing of subsea gas leakage, applied to the monitoring of underwater gas discharges from anthropogenic and natural sources, is becoming increasingly important. First, as the oil and gas industry is facing increasing regulation, there is a need to put more control in the industrial process and to assess the impact on the marine environment. The applications are diverse, including: early warnings of "blow-out" from offshore installations, detection of leaks from underwater gas pipelines, gas leakage detection from Carbon and Capture and Storage facilities (a process aimed at mitigating the release of large quantities of CO2 in the atmosphere), and seabed monitoring. Second, this technology has a role to play in oceanography for a better understanding of natural occurrences of gas release from the sea floor such as methane seeps. This is of major importance for the assessment of the exchange of gas between the ocean and the atmosphere with application to global warming. All those phenomena involve the formation and release of bubbles of different sizes. These are strong sources and scatterers of sound. Within this context, this thesis draws on a two part study. The first part experimentally addresses the accuracy of a passive acoustic inversion method for the quantification of gas release. Such a technique offers the advantage of lower power requirements for long term monitoring. It is common practice for researchers to identify single bubble injection events from time histories or time frequency representations of hydrophone data, and infer bubble sizes from the centre frequency of the emission. This is well suited for gas release at a low flow rate, involving solitary bubble release. However, for larger events, with overlapping of bubble acoustic emissions, the inability to discriminate each individual bubble injection events makes this approach inappropriate. Using an inverse method based on the spectrum of the acoustic emissions allows quantification of such releases with good accuracy. The inverse scheme is tested using data collected in a large test tank and data collected at sea during the QICS (Quantifying Impacts of Carbon Storage) project. The second part of the thesis addresses the problem of quantifying gas releases using active acoustics. Single beam echosounders are commonly used instruments in fisheries acoustics. When investigating gas release from the seafloor, they are frequently employed to study the spatial distribution of the gas releases. However, few studies make use of these data to quantify the amount of gas being released. Here, using the common multi-frequency ability of these systems, an inverse method aimed at determining gas volumes is developed. This is tested against simulated data and the method shows good performances in scenarios with limited data sets (data collected at limited number of frequencies). Then, using data collected at sea from methane seeps to the west of Svalbard (from two research cruises), the method is applied and compared to independent measurements of gas fluxes.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:665124 |
| Date | January 2015 |
| Creators | Berges, B. J. P. |
| Contributors | Leighton, Timothy |
| Publisher | University of Southampton |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://eprints.soton.ac.uk/379746/ |
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