Abstract
This article discusses inversions for bottom geoacoustic properties using broadband acoustic signals obtained from explosive sources. Two different inversion schemes for estimating the compressional wave speeds and attenuation are presented in this paper. In addition to these sediment parameters, source-receiver range is also estimated using the arrival time data. The experimental data used for the inversions are SUS charge explosions acquired on a vertical hydrophone array during the Shelf Break Primer Experiment conducted south of New England in the Middle Atlantic Bight in August 1996. The modal arrival times are extracted using a wavelet analysis. In the first inversion scheme, arrival times corresponding to various modes and frequencies from 10 to 200 Hz are used for the inversion of compressional wave speeds. A hybrid inversion scheme based on a genetic algorithm (GA) is used for the inversion. In an earlier study, Potty et al. [J. Acoust. Soc. Am. 108(3), 973-986 (2000)] have used this hybrid scheme in a range-independent environment. In the present study results of range-dependent inversions are presented. The sound speeds in the water column and bathymetry are assumed range dependent, whereas the sediment compressional wave speeds are assumed range independent. The variations in the sound speeds in the water column are represented using empirical orthogonal functions (EOFs). The replica fields corresponding to the unknown parameters were constructed using adiabatic theory. In the second inversion scheme, modal attenuation coefficients are calculated using modal amplitude ratios. The ratios of the modal amplitudes are also calculated using time-frequency diagrams. A GA-based inversion scheme is used for this search. Finally, as a cross check, the computed compressional wave speeds along with the modal arrival times were used to estimate the source-receiver range. The inverted sediment properties and ranges are seen to compare well with in situ measurements and historical data.
Highlights
Acoustic propagation in shallow water is greatly influenced by the properties of the bottom
In inversion scheme I, sediment compressional wave speeds are estimated using a hybrid inversion scheme based on the dispersion behavior of broadband acoustic propagation
When a broadband acoustic source is used in shallowwater waveguide, the acoustic propagation exhibits dispersion effects
Summary
When a broadband acoustic source is used in shallowwater waveguide, the acoustic propagation exhibits dispersion effects. Turning to the estimation of medium attenuation, Tindle1982͒ and Zhou et al ͑1987͒ used modal amplitude ratios to extract modal attenuation coefficients Using this approach they were able to model the nonlinear frequency dependence of the acoustic attenuation in the upper sediment layer in a shallow-water location in the Yellow Sea. Rajan, Frisk, and Lynch1992͒ estimated modal attenuation coefficients using various methods based on the pressure field or its Hankel transform. An inverse scheme for attenuation is presented which utilizes the compressional wave-speed values determined using the dispersion-based inversion. This scheme calculates the modal attenuation coefficients based on modal amplitude ratios and transmission loss data.
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