Preliminary Analysis of the Applicability of Adiabatic Modes to Inverting Synthetic Acoustic Data in Shallow Water Over a Sloping Sea Floor

Abstract

The May 2001 Geoacoustic Inversion Techniques Workshop provided synthetic transmission loss (TL) data for four cases with range-dependent shallow-water all-liquid environments. In two of these cases ("0" and "1"), the sea floor has constant slope and the geoacoustic model (GAM) is range independent. Cost functions have been computed using a new adiabatic-mode TL algorithm (which uses an exact velocity boundary condition at the sloping sea floor), as one parameter in the GAM is varied. Two frequencies (80 and 220 Hz) were selected. In case 0, the sea-floor slope is 0.0183 and the GAM comprises an inhomogeneous layer over a basement. The sea-floor sound-speed was selected as the variable parameter. The resulting cost minima at 80 and 220 Hz are displaced from the actual sound speed by 2.3 and 3.4 m/s, respectively. In case 1, the sea-floor slope is 0.012 and the GAM comprises one homogenous layer, five inhomogeneous layers, and a basement. The selected parameter was the sound-speed in the homogeneous layer. The corresponding cost minima are displaced by -1.2 and +1.1 m/s. The relative values of these four errors indicate that mode coupling increases with sea-floor slope and that there may be a dependence on frequency at the greater slope.