This paper summarizes evidence of a nonlinear frequency dependence of attenuation for compressional waves in shallow-water waveguides with sandy sediment bottoms. Sediment attenuation is found consistent with α(f)=α(fo)∙(f∕fo)n, n≈1.8±0.2 at frequencies less than 1kHz in agreement with the theoretical expectation, (n=2), of Biot [J. Acoust. Soc. Am. 28(2), 168–178, 1956]. For frequencies less than 10kHz, the sediment layers, within meters of the water-sediment interface, appear to play a role in the attenuation that strongly depends on the power law. The accurate calculation of sound transmission in a shallow-water waveguide requires the depth-dependent sound speed, density, and frequency-dependent attenuation.
Experiments have been conducted near the site of AMCOR Borehole 6010 on the New Jersey Shelf to evaluate propagation predictability in sandy shallow-water environments. The influence of a nonlinear frequency dependence of the sediment volume attenuation in the uppermost sediment layer at this location is examined. Previously it was determined that a frequency power-law exponent of 1.5 was required for the best modeling of experimental results over the band 50-1000 Hz. The approach here references the attenuation to an accepted value at 1 kHz and makes extensive comparisons between measurements and calculations, to determine a power-law exponent of 1.85± 0.15.
Experiments were conducted near the site of AMCOR Borehole 6010 on the New Jersey Shelf to characterize propagation predictability. The importance of a non-linear power-law frequency dependence of the sediment volume attenuation in the uppermost sediment layer is demonstrated. One metric of transmission loss variation with range is an effective attenuation coefficient that can be extracted from measurements and calculated with the parabolic equation. Previously it was found [W. M. Carey and R. Evans, J. Oceanic Eng., 23] that a power exponent of 1.5 modeled the measurements. The present approach uses 1 kHz as an attenuation reference frequency and employs different parameter ranges and optimization criteria. For 400–1000 Hz, this procedure leads to a power exponent in the range 1.7–2.0, which is consistent with other sand-silt regions. The estimates are robust with respect to variations in the water and sediment sound-speed profiles and the sediment layer thickness. The influence of measured range dependence in the sound speed and bathymetry is examined. Estimates of signal time spread are calculated, which can also be obtained for recent experiments in the same ocean region. [Work partially supported by ONR]
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