Acoustic pulse tube studies for seafloor and sea ice greenhouse gas quantification

Best, Angus, Sahoo, Sourav Kumar, North, Laurence J., Madhusudhan, B.N., Sanford, Oliver and Clarke, Timothy (2024) Acoustic pulse tube studies for seafloor and sea ice greenhouse gas quantification. In Proceedings of the Institute of Acoustics. vol. 64, Institute of Acoustics. 8 pp . (doi:10.25144/22208).

Abstract

The acoustic pulse tube, or impedance tube, is a useful method for quantifying compressional wave properties of various materials. In the NOC context, designed an instrument suitable for studying wave propagation in porous media, especially seafloor sediments and latterly sea ice, but also more broadly applicable to ocean acoustics research. Notable design improvements over extant systems include full ocean depth pressure rating (60 MPa), the ability to extract broadband signatures over 1 – 20 kHz using a novel broadband sonar projector with a transmission line calibration/modelling approach, and sample jacket designs allowing sediment effective pressure control. An acoustic source (located at the base of a 4.5 m long, 70 mm internal diameter, stainless steel, thick-walled tube filled with water) projects a Stoneley wave that approximates a plane wave propagating up the tube through the test sample suspended halfway. Hydrophones in sidewall ports below and above the test sample record the acoustic signals from which sound wave velocity and attenuation (Q-1) frequency spectra are calculated with respective accuracies of ± 2.4% and ± 5.8% for jacketed sediments. Novel scientific results on sand samples as a function of water saturation and pressure, and on synthetic sea ice cores with different air contents, provide insights into wave propagation mechanisms for these complex porous media. Complementary shear wave velocity and attenuation measurements are provided using a separate geotechnical resonant column instrument with respective accuracies of ± 1% and ± 20%. Modified Biot theory effective medium modelling approaches have been used to quantify the volume and distribution of free gas, needed for constraining impacts of greenhouse gases in the seafloor and air in sea ice, of interest to climate research in rapidly changing polar regions. A review of novel instrument developments and scientific results will be presented.

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Contributors

Author: B.N. Madhusudhan

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