Broadband geoacoustic inversion using mode traveltime dispersion

Abstract

Inversion of sediment compressional sound speeds using the data from SUS charge explosions acquired during the Shelf Break Primer Experiment in 1996 is presented. A hybrid approach combining a Genetic Algorithm and Levenberg-Marquardt nonlinear least squares method is used for the inversion. This inversion is based on the time-frequency dispersion of the acoustic signal calculated using a wavelet approach. Group speeds for modes 1–7 and for frequencies ranging from 10–200 Hz were obtained from the wavelet scalograms of the acoustic signal. The ocean waveguide depth was approximately 90 m. The minimized objective function is weighted to incorporate the quality of the data at various frequencies and modes. Forward modeling is done using adiabatic normal mode theory assuming a mildly range-dependent environment. Resolution and error of the inversions are expressed in terms of the Hessians and a posteriori model standard deviations. Inversion results are compared with compressional speeds based on the data from the nearby AMCOR-6012 site using Biot–Stoll theory. In addition, gravity cores taken at selected locations in the experimental site provide some corroboration of the sediment compressional speeds in the top 4 to 5 m of the sediment. [Work supported by ONR.]