In Situ Acoustic Wave Attenuation Measurements of Ocean Water

P-wave attenuation in water in the industry-seismic bandwidth is usually regarded as so low that it’s ‘ignored’, and, if quantified, given semi-arbitrary frequency-independent Q values (e.g. 103, 104) when standard acoustics models give 108-1011: hence, we measured in-situ effective attenuation from decay of direct P wave amplitudes along conventional towed streamers, and from comparison of primary and multiple seabed reflections. Neither experiment, with any dataset we used, delivered expected values. The direct wave gave Qeff increases from ~80 to 180 over 30-70Hz, and from ~30 to 90 over 20-90Hz. The primary-multiple comparison gave Q-1eff=-0.0015±0.0042 i.e. Q=-663, nominally consistent with the standard model of intrinsic attenuation but nevertheless indicating a large but negative Q. Apparent attenuation dominates for both horizontal (direct-wave) and vertical (reflection) ray-paths, raising questions about Q values to use in quantitative modelling and analysis, and implying a more insightful appreciation is needed of acoustic wave propagation in ocean water. Of various reasons that could produce apparent attenuation, to date we have investigated quantitatively dispersive sea-floor reflectivity. We find that an intrinsic Q of ~15-30 in the sea-floor sediment is a credible explanation, in whole or part, for the apparent attenuation that we observe on the vertical raypaths.