CAREER: Development of a broadband acoustic system for quantifying the flux of free gas in methane seeps

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In this CAREER proposal the PI requests funding to develop a broadband acoustic system to quantify the depth dependent bubble size distribution and gas void fraction in methane seeps, and to measure the flux of methane gas escaping the seabed and rising through the water column. Inversion of acoustic measurements for these quantities will exploit the mechanical resonance of bubbles by observing the bubbles response at a wide range of frequencies (1-100 kHz). The acoustic system will employ a constant beamwidth transducer for much of its frequency range (10-100 kHz) in order to accurately treat bubble plumes with different size distributions that are in close spatial proximity, as is sometimes observed to happen. The system will be designed to integrate into a deepwater remotely operated vehicle (ROV) and to function at full ocean depth. A second operational mode will be established where the system can be towed from a ship of opportunity as part of a more rapid, wide-scale acoustic survey strategy. The development and use of a wideband acoustic system can directly address hypotheses about the fate and evolution of methane gas bubbles escaping the seabed. The use of the proposed system has the potential to provide transformative results including 1) refined estimates of the global flux of methane escaping the seabed, 2) increased understanding of the oceans capacity as a methane sink, and 3) estimates of the global flux of methane reaching the atmosphere from seabed seeps.Broacer ImpactsThe data provided by the proposed instrument could be of considerable benefit to the wider scientific community. Methane seeps are thought to affect ocean chemistry, ocean microbial life and atmospheric chemistry, with consequences for ocean productivity, radiation balance in the atmosphere and paleoclimate studies. Basic information on these seeps is lacking, and the proposed instrument could make a significant contribution to plugging this hole in our understanding. Several undergraduate and graduate students would benefit from the experience of helping with the development and testing of a novel device. The idea of having undergraduates do the development work for the schools echo-sounder kit is excellent. The outreach component would reach the pupils at several local schools, and has the written support of both local teachers and the New Hampshire Sea Grant. The collaborations with industry, Sea Grant, and other agencies provides evidence that the project will have a wide spread and lasting impact and that it is perceived by many other scientists as an important problem and a good approach to the problem.