Modeling of low-frequency sound propagation in the Baltic

Geoacoustic inversion techniques are applied on transmission loss (TL) data recorded in an area east of Gotland, to investigate how well the experimental data can be reproduced by a numerical sound propagation model. The data were recorded during the BAROC field trials performed by FOI and FWG in May 2002. The seabed parameters were estimated by minimizing the difference between simulated and experimental data with a genetic algorithm, using the parabolic wave equation as wave propagation model. It is found that a range-independent fluid seabed model managed to reproduce the TL data with an accuracy of about 3 dB. Range-dependent modeling reduced the mismatch further by 1 dB. The models were of two fundamentally different types: Solid seabed with media parameters consistent with the knowledge of the geology in this area, and a fluid seabed with unrealistic parameters, in particular unrealistically low sound velocities in the sediment. The low sediment velocities seem to have compensated for losses caused by shear wave excitation in the solid model. Both types of models managed to reproduce the data with an accuracy of about 2 dB. An example indicated that, if the absolute signal levels are not available, then matching of the relative decay rates in the TL data can be used as an alternative.