Kevin D. LePage
Bolt Beranek and Newman, Inc., 70 Fawcett St., Cambridge, MA 02138
One method of characterizing the propagation in a shallow-water region is to collect a series of measurements which together form a composite of the range-dependent transmission loss (TL) between a source and receiver at single frequencies. Here, a wave-based method of extracting the bottom impedance matrix is presented. As opposed to traditional approaches to extracting environmental information from these measurements, where the bottom properties are perturbed until the modeled and the measured transmission loss agree, the wave-based method is a two-part method, where the horizontal wave numbers of the waveguide are first determined such that all the features in the measured TL are accounted for, and then the water column properties are used in conjunction with a finite-difference scheme [M. B. Porter and E. L. Reis, J. Acoust. Soc. Am. 77, 1760--1767 (1985)] to determine the bottom impedance at these wave numbers. One attraction to this approach is that most features in the data are modeled, and all the unique information about the bottom is contained in an impedance matrix over horizontal grazing angle and frequency. Since many of the unknown physical processes which affect shallow-water transmission are concentrated in the bottom, modeling issues are reduced by this approach.