Effects of Sound Speed Fluctuations Due to Internal Waves in Shallow Water on Horizontal Wavenumber Estimation

Abstract

There is considerable current interest in the influence of water-column variability on acoustic propagation and its effects on geoacoustic inversion. In general, the effect of sound speed fluctuations due to internal waves in the water column is to promote the coupling of energy between propagating acoustic modes. The effects of mode coupling include fluctuations in individual modal amplitudes and arrival times along with time spreading of the original pulses. In contrast to the broadband case, little research has been conducted on the effects of internal waves on cw modal-based inversion methods. These techniques require estimates of the propagating modal eigenvalues for a cw point source field as input data to the inversion algorithm. In much of the literature, wavenumber estimation is performed with the assumption that pressure is given by an adiabatic mode sum. Changes in modal content as a function of range are then attributed to local changes in the waveguide boundaries, specifically, the bottom. For a shallow-water waveguide including internal waves, the adiabatic assumption is violated and estimates of local wavenumber content is affected. This paper addresses the nature of the these affects on the wavenumber estimation problem. In particular, numerical studies of internal wave effects are conducted with respect to identification and bias of individual modes along with the ability to resolve closely spaced eigenvalues. Preliminary results for a weak internal wave field show that mode coupling leads to an enhancement of the wavenumber spectral estimates due to the energizing of weak modes that were previously not excited.