Shear wave velocity and attenuation in the upper layer of ocean bottoms from long-range acoustic field measurements.

Several physics-based seabed geoacoustic models (including the Biot theory) predict that compressional wave attenuation alpha(2) in sandy marine sediments approximately follows quadratic frequency dependence at low frequencies, i.e., a2 approximate to kf(n) (dB/m), n = 2. A recent paper on broadband geoacoustic inversions from low frequency (LF) field measurements, made at 20 locations around the world, has indicated that the frequency exponent of the effective sound attenuation n approximate to 1.80 in a frequency band of 50-1000 Hz [Zhou et al., J. Acoust. Soc. Am. 125, 2847-2866 (2009)]. Carey and Pierce hypothesize that the discrepancy is due to the inversion models' neglect of shear wave effects [J. Acoust. Soc. Am. 124, EL271-EL277 (2008)]. The broadband geoacoustic inversions assume that the seabottom is an equivalent fluid and sound waves interact with the bottom at small grazing angles. The shear wave velocity and attenuation in the upper layer of ocean bottoms are estimated from the LF field-inverted effective bottom attenuations using a near-grazing bottom reflection expression for the equivalent fluid model, derived by Zhang and Tindle [J. Acoust. Soc. Am. 98, 3391-3396 (1995)]. The resultant shear wave velocity and attenuation are consistent with the SAX99 measurement at 25Hz and 1000 Hz. The results are helpful for the analysis of shear wave effects on long-range sound propagation in shallow water. (C) 2012 Acoustical Society of America. [http://dx.doi.org/10.1121/1.4765078]