Yellow Shark Spring '95: inversion results from sparse broadband acoustic measurements over a highly range dependent soft clay layer
In May 1995, SACLANTCEN performed broad-band (200-800 Hz) acoustic measurements in the Giglio basin off the coast of Italy as part of the Yellow Shark inversion experiments. In this paper, inversion of sparse, broad-band transmission loss (TL) measurements is investigated to determine bottom properties in strongly range-dependent (RD) situations commonly encountered in shallow water. The data are from the Elba-Formiche transect where water depth varies from 65 m at the acoustic projector location, to approximately 120 m at the 4-element vertical arrays deployed at ranges of 8, 16, 24, 32 and 40 km along the transect. The experimental site has a soft clay layer bottom which varies in thickness from 3 to 10 m with a sound speed less than the water column. A modal analysis including coupling effect is given to explain the frequency bands for which high TL was observed as a function of range and depth. The TL measurements were inverted by matching the RDfields with model results. Using parabolic equation modeling, bottom geo-acoustic parameters were varied in a marching search to fit the TL measured at the five ranges. The experimental results demonstrate that RD bottom properties such as sound speed and thickness of the slow clay layer can be obtained from broad-band TL measurements sparsely distributed in range and depth.