Abstract:

The effects of internal waves on the coherence of propagating modes and the validity of the adiabatic assumption are very important issues in both matched-field processing and acoustic tomography. While much theoretical research has been done on long-range propagation of modes in deep water, there have been few opportunities to compare theoretical predictions with experimental measurements. Deployment of mode-resolving vertical arrays as a part of the Acoustic Thermometry of Ocean Climate (ATOC) experiment has provided data sets, which may offer insights about mode coherence at long ranges. This study examines the modal composition of ATOC receptions using the short-time Fourier techniques described in earlier work [Wage et al., J. Acoust. Soc. Am. 101, 3113(A) (1997)]. Initial results reveal a frequency selectivity in the arrivals for the lowest ten modes. Since frequency-selective effects can be modeled by a random multipath channel, the problem of estimating channel characteristics from pulse-compressed M-sequence receptions is discussed. In particular, this study investigates various techniques of averaging across receptions to obtain the mean arrival time and spread for each mode. [Work supported by ONR.]