A mode travel time based technique for geoacoustic inversions has been developed using broadband explosive sources [Potty et al., J. Acoust. Soc. Am. 108, 973–986 (2000)]. In this inversion scheme we search for various parameters including water depth, sound speed in the water column represented by Empirical Orthogonal Function (EOFs), compressional wave speed in the sediment and sub-base, thickness of the sediment and range. The sensitivity of the objective function to perturbations in these parameters is different, i.e., different parameters have different sensitivities. The most sensitive parameters can be estimated with higher accuracy compared to parameters with low sensitivities. In this study we aim to link the search process to parameter sensitivities by matching the fineness of search to sensitivities. The highly sensitive parameters will be searched with a fine scale sampling of the model space with a coarser sampling adopted for less sensitive model parameters. This condition will be incorporated into the search tool (Genetic Algorithm) used for the inversion scheme. The objective of this approach is to utilize the computational resources in an optimum manner by giving more emphasis to parameters which are more sensitive as opposed to a uniform weighting for all parameters. This approach will be applied to data from the ASIAEX-2001 and Shelf Break Primer data sets. [Work supported by ONR.]

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This paper discusses the early results from the acoustic bottom interaction experiment conducted in May–June 2001 in the East China Sea as part of the ASIAEX-2001. Explosive sources (38 g and 1000 g) were deployed in the experimental area from a Chinese Research vessel (Shi Yan 2) during the experiment. These shots are deployed in circular pattern of radius 30 km and on radial tracks. The acoustic signals from these explosive sources were received at the vertical line array hung from R/V Melville near the center of the circular pattern. The sediment properties show a sharp contrast on either side of a sediment front. The presence of high-speed sand and low speed mud–sand on either side of the front has been confirmed by core data from ASIAEX-2000. Extensive coring to determine the sediment information in more detail is planned in August 2001. The time-frequency scalograms of the received explosive signals have been constructed using wavelet analysis. This analysis yields the dispersion characteristics of the acoustic signals and clearly shows the modal arrivals. The dispersion behavior of the acoustic modes from either side of this sediment front is presented. These modal dispersion diagrams form the basis for our sediment inversion technique. [Work supported by ONR.]

Dispersive acoustic signals contain significant amounts of information concerning the shallow water environment structure. It is possible to unravel the modal propagation dispersion characteristics using a proper time-frequency analysis. Recent experiments such as ShelfBreak Primer and ASIAEX collected long range propagation data in shallow water on vertical line arrays from SUS charge explosive sources. Signals collected by the vertical arrays were processed using Morlet wavelets to observe the modal arrival structure due to group speed dispersion. Although the wavelet transform can extract the time-frequency information, it is not sufficient to get high resolution for high frequencies. The matching pursuit algorithm is an alternative approach that provides excellent spectral localization. We present an improved processing algorithm based upon the matching pursuit algorithm to analyze the data from ASIAEX-2001. This analysis yields the dispersion characteristics of the acoustic signals and clearly shows the modal arrivals, especially for higher modes. These modal dispersion diagrams from the results form the basis for long range sediment tomography. [Work supported by ONR.]

This paper discusses ongoing data analysis results from the acoustic bottom interaction experiment conducted in May–June 2001 in the East China Sea as part of the Asian Seas International Acoustics Experiment (ASIAEX-2001). Using time-frequency scalograms of broadband signals, the modal arrivals and group speed minimums (Airy Phase) of several modes are clearly observed. The structure of the Airy Phase signal is used to match the dispersion curves which forms the basis of this inversion technique. Utilizing the Airy Phase group speed minimums and corresponding pressure amplitudes of each observable mode, the sediment compressional wave speed and attenuation as a function of depth are derived. The group speed minimum for each mode provides additional information on the compressional wave speed in the modal sediment depth penetration interval. To refine the sediment parameters, synthetic and measured time series are correlated for goodness of fit and used in the inversion process. The synthetic time series is generated from the scalogram values corresponding to the times and frequencies of the calculated dispersion curves. Inverted speeds and estimated modal penetration depths are then used to develop the sediment profile. The estimated resolution is dependent on the number and frequency span of the observable modes. Estimated sediment properties from several areas are presented with verification from coring results. [Work supported by ONR.]