SACLANT Undersea Res. Ctr., I-19138 La Spezia, Italy
Geoacoustic inversion using single-frequency data measured on a vertical array has become a popular method for estimating sub-bottom parameters. Inclusion of multiple frequencies in the inversions allows exploitation of both the waveguide dispersion and the different penetration into the sub-bottom at different frequencies. This is achieved, however, at the expense of greater required computing time, and more complicated processing schemes. Several approaches exist for defining a broadband object function, i.e., coherent or incoherent addition across frequencies and hydrophones. Each approach has specific requirements with regard to the recording geometry and the source spectrum. For coherent addition across frequencies not only the source spectrum but also the absolute propagation time must be estimated. When a useful object function has been defined it is optimized using genetic algorithms [P. Gerstoft, J. Acoust. Soc. Am. 95, 770--782 (1994)] for both the sediment and the geometric parameters. Different approaches are illustrated on broadband data from the Mediterranean North Elba site with center frequencies of 170 and 330 Hz. The water depth was 130 m and the source range was 5.5 km. The normal-mode code snap was used as the forward model.