This article discusses inversions for bottom geoacoustic properties using broadband acoustic signals obtained from explosive sources. 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 SUS signals were analyzed for their time-frequency behavior using wavelets. The group speed dispersion curves were obtained from the wavelet scalogram of the SUS signals. A genetic algorithm (GA) was used for the inversion of sound speeds in the water column and compressional wave speeds in the sediment layers. The variations in the sound speeds in the water column were represented using empirical orthogonal functions (EOFs). A range-independent normal mode routine was used to construct the replica fields corresponding to the parameters. Comparison of group speeds for modes 1 to 9 and for a range of frequencies 8 to 200 Hz was used to arrive at the best parameter fit. An efficient hybrid optimization scheme using the GA and a Levenberg-Marquardt algorithm is presented. Linear perturbation methods were also used to "fine tune" the inversions and to obtain resolution and variance estimates. Analysis was also done to compute the degree of convergence of each of the parameters by explicitly calculating the Hessian matrices numerically. A posteriori estimation of mean and covariance was also done to obtain error estimates. Group speeds for the inverted sound speed fields provide an excellent match to the experimental data. The inverted sediment compressional speed profile compares well with in situ measurements.
This paper presents single receiver geoacoustic inversion of a combustive sound source signal, recorded during the 2017 Seabed Characterization Experiment on the New England Mud Patch, in an area where water depth is around 70 m. There are two important features in this study. First, it is shown that high-order modes can be resolved and estimated using warping (up to mode number 18 over the frequency band 20-440 Hz). However, it is not possible to determine mode numbers from the data, so that classical inversion methods that require mode identification cannot be applied. To solve this issue, an inversion algorithm that jointly estimates geoacoustic properties and identifies mode number is proposed. It is successfully applied on a range-dependent track, and provides a reliable range-average estimation of geoacoustic properties of the mud layer, an important feature of the seabed on the experimental area.
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.
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