Inversion of range-dependent geoacoustic properties in South China Sea ASIAEx01 experimental site

Abstract

Matched-field inversion of range-dependent geoacoustic properties is studied using broadband (50–200 Hz, 240–260 Hz, and 550–600 Hz) acoustic data collected in the South China Sea during the ASIAEx01 experiment. Range-dependent sediment sound-speed and attenuation profiles are inverted using a global optimization scheme based on genetic algorithms to minimize an objective function defined by the Bartlett processor output. For the forward model, an efficient coupled normal mode model is used to calculate broadband acoustic fields incorporating range-dependent bathymetry and sediment layers thickness obtained by chirp sonar surveys. Inversions were performed starting from a range-independent region with a relatively short source/receiver distance. Additional geoacoustic profiles were inverted by incorporating the previously inverted profiles as the source/receiver distance was increased. The results obtained at three different frequency bands are in good agreement, especially when the range-dependency of bathymetry and sediment layers thickness is included in the inversion. [Work supported by ONR.]

An Estimate of the Bottom Compressional Wave Speed Profile in the Northeastern South China Sea Using “Sources of Opportunity”

“…1, labeled "Fishing Bomb." The other source of interest is denoted by the red line labeled "J-15-3 LFM track," which shows the tow path that Turgut [1] considered in obtaining his bottom inverse results. There are two other source sites noted in Fig.…”

“…The two that most interest us here are the chirp sonar images, obtained by Schock, and the J-15-3 towed source bottom inversions, performed by Turgut [1]- [3].…”

“…This was done using Turgut's bottom model [1], the water column sound speed profile (Fig. 5) from the experimental site (which was, aside from internal wave activity, spatially and temporally constant for the ASIAEX SCS site), and the bathymetry along the track to the explosion, shown in Fig.…”

“…The general properties of the observed signal, the localization of the source position, and the extraction of the modal group velocity curve are discussed in Section III. Section IV discusses the a priori information that we put into our inverse, specifically the work by Turgut [1], Schock [2], [3], and Bartek and Liu [4]. Section V discusses our inverse methods and results, stressing the compressional wave speed profile in the bottom.…”

“…To obtain a bottom model, three experimental acoustic activities were carried out in ASIAEX: 1) a series of source tows, using a J-15-3 source to transmit signals in the 50-260 Hz range, to a moored vertical and horizontal array receiver [1]; 2) chirp sonar surveys along several tracks in the ASIAEX SCS experimental region [2], [3]; and 3) a watergun seismic survey of the overall geology of the ASIAEX SCS site [4]. However, none of these activities was able to provide a low-frequency horizontally propagating signal, such as one obtains from standard "Source of Underwater Sound" (SUS) charge surveys.…”