Acoustic discrimination between biological objects and gas bubbles in the sea

Natural gas seeps have been observed by man since the ancient times from a variety of sources and at scales ranging from dramatic events such as erupting volcanos to much more humble bubbles rising towards the surface of a shallow lake. With improving technologies to access and observe marine environments, ocean seabed gas seeps have been found to be common. Sources of these natural subsea seeps are diverse, including but not limited to, gas-containing fluid vents, ?melting? methane hydrates, pockmarks, and mud volcanos which are responsible for some of the truly enormous subsea gas vents. More recent phenomena are manmade seabed gas seeps from subsea natural gas extraction installations (e.g. rupture in the transportation pipe) and proposed carbon dioxide storage sites in geological structures below the sea floor. Many of these geographically widespread gas seeps are subject to research and monitoring, often because of the potential influence and contribution to atmospheric gas composition and long-term climate change. One of the best ways to detect a gas bubble rising in the water column at a substantial range is via acoustic methods, echo sounders and sonars. These are arguably some of the best observation tools for rapid and cost effective coverage of large water volumes. Scientific echo sounders, for example, are rather sensitive tools, which can be calibrated to a high degree of precision and are widely used for fish stock monitoring. However, there are other objects in the sea than bubbles that can cause significant acoustic backscatter. Countless species of fish and planktonic organisms dwell in the world?s oceans. The acoustic backscattering properties of these animals are also used for target identification purposes, with a reasonable degree of success for some schooling species. However, interpretation of acoustic recordings and observed object identification is often challenging. For example, from an acoustical point of view, fish that possesses a gas-filled swim bladder reflect sound in a rather similar manner as a gas bubble of similar dimensions. For seabed-originating gas bubble seeps recorded on echogram, question may rise is the cloud of objects in the water column a cloud of bubbles or a ?cloud? of fish, fish school? Fish without swim bladders and planktonic organisms sometimes may also bear similarity to gas bubble plumes when observed by echo sounders. This particular study was part of continuing work on one of the main challenges to using active acoustics to observe and monitor gas bubble plumes ? target identification, with gaseous bladder-bearing fish as potentially the most similar target in the marine environment. This challenge was addressed by conducting measurement experiments on representative biological objects and gas bubbles. The lesser sandeel (Ammodytes marinus) was investigated as an example for fish with no swim bladder. The acoustic backscattering properties of saithe (Pollachius virens) were studied in more detail and used as an example of a gas-filled swim bladder bearing fish. The acoustic backscattering properties of induced methane, carbon dioxide and air gas bubble plumes were also investigated in detail at several discreet echo sounder frequencies (70-333 kHz). Significant differences in acoustic echo strength across a range of echo sounder frequencies were measured between gas bubble plumes and fish with large swim bladders. Based on the readily available research and that defended in this doctoral thesis, it is suggested that behavioural and acoustic backscattering differences can be used to separate gas bubble plumes from the most common biological targets, plankton and fish. Gas-filled swim bladder bearing fish are the most similar biological acoustic targets to the gas bubble plumes. Schooling and swim bladder bearing fish that are somewhat larger in size (>10 cm) can be separated using the acoustic frequency response information (echo signature over a range of acoustic frequencies). Smaller, but abundant swim bladder bearing fish, such as members from Myctophidae and Sternoptychidae, can be difficult to separate acoustically from a single gas bubble, because of their small, round, bubble-like swim bladders. However, the behaviour of such fish assemblages is substantially different from the gas bubble plumes. Using both backscattering frequency response and behaviour traits (at one time instance and over time) are likely to give the best chances for acoustic-based detection and identification of seabed gas bubble plumes.

Acoustic methods for underwater studies have the necessary potential for monitoring and quantification of natural and man-made gas bubble seeps in the sea. Advanced echo sounders can offer better observation resolution in time and space than any other cur rently available method for gas seep studies. Therefore, echo sounder technology is chosen by Metas to be used for development of new gas seep detecting unit to be manufactured and commercially sold. The proposed PhD project will aim to deliver lacking sc ientific knowledge needed for successful development of this product, while in the same time PhD thesis will be prepared and defended. Gas bubbles, observed by echo sounder, can often look very similar to fish or zooplankton. There are little to none res earch done showing how to acoustically discriminate biological objects and gas bubbles occurring together in the sea. Selected fish species, recognised as representative for the purpose, will be observed using advanced scientific echo sounders, both unde r controlled environment of a fish cage and in the open sea. Small population of caged saithe and mackerel will be investigated with dorsally and ventrally observing echo sounders and horizontally looking stereo camera system for fish orientation and leng th measurements. Similar experimental setting will be used to observe controlled gas bubble releases in large, deep water-filled bag, enabling quantification of both acoustic echo and behaviour properties of gas bubbles. The collected data will be scrutin ized in order to find best available acoustic echo and behaviour based discriminators between biological objects and gas bubbles. The ideas will be further tested using in situ data on gas bubbles and fish/zooplankton occurring together in the water colum n. Most important challenge will be to recognise differing, but stable and quantifiable, acoustic echo and behaviour properties of biological objects and gas bubbles, as observed by echo sounders only.