Joint estimation of water column and sediment acoustic properties from broadband towed array data using modal inverse method

Broadband data, collected on a towed horizontal array of length 2375m during a geophysical survey, are used to perform joint inversion for the water column and sediment acoustic parameters using modal inverse technique [S. D. Rajan, J. F. Lynch, and G. V. Frisk, J. Acoust. Soc. Am. 82, 998–1017 (1987)]. To perform the inversion based on modal wave numbers, we first obtain the pressure field as a function of range at a discrete set of frequencies from the broadband data. The sources of errors and their impact on the estimation of modal wave number from such data are discussed. Due to the possibility of large errors in the estimation of the modal wave numbers of the higher-order modes, the inversion was performed using the modal wave numbers of the dominant lower-order modes. The resolution estimates indicate that the wave speeds in the water column and top three sediment layers have resolution lengths that are approximately 1.5 times the layer thicknesses. With respect to deeper sediment layers, the resolution lengths are large, as only lower-order modes were used in inversion. These results are consistent with a posteriori variance estimates. The model from modal inverse can serve as the initial model for nonlinear methods, such as those based on matched field processing [P. V. Nagesh, S. D. Rajan, and G. V. Anand, Conference on Underwater Acoustics Measurements: Technologies and Results, Heraklion, Crete, Greece (2005)].