Geoacoustic Inversion Using Vertical Line Array Data

Abstract

The long-term goal of this work is to develop a method to extract depth-dependent normal modes and to invert for the acoustic parameters of the ocean and ocean bottom using acoustic data measured on a vertical line array. Funding from this grant was also used to improve, maintain, and distribute a stateof-the-art acoustic normal mode model. OBJECTIVES The objectives of the FY00 work were to apply the mode extraction and geoacoustic inversion techniques developed in previous years under ONR funding to measured data and evaluate the strengths and limitations of the methods. APPROACH The technical approach for the geoacoustic inversion method being investigated is (1) to use measured data on a vertical line array (VLA) to extract the depth-dependent mode functions of the environment, and (2) to invert for the environmental parameters by using a non-linear least squares technique for finding the best match between extracted and modeled mode functions. The approach for evaluating the usefulness of the method is to apply the method to the ACT-II data measured in the Hudson Canyon area. This work was carried out by Tracianne Neilsen as her Ph.D. dissertation topic in the Physics Department at the University of Texas at Austin, under the supervision of Evan Westwood. A more detailed description of the technical approach is given below. The required experimental setup for the mode extraction method consists of a source of opportunity moving in the vicinity of a VLA. The time-dependent, single-frequency pressure field measured on the VLA may be viewed as a matrix of pressures versus receiver depth and source-receiver range. A singular value decomposition (SVD) is performed on the pressure matrix. Under certain conditions, it may be shown, using the standard normal mode expression for the pressure field, that the resulting eigenvectors correspond to the depth-dependent normal mode functions of the waveguide. We refer to this procedure as mode extraction.