Matched-field processing gain degradation caused by tidal flow over continental shelf bathymetry

Abstract

Temporally variable, range dependent sound-speed profiles measured during ebb flow and estimated for slack flow are used to quantify the variability of matched-field signal-processing gain degradation in shallow water propagation channels controlled by tidally driven stratified flow over variable bathymetry. Calculations along a 9.3 km range establish phase changes in the acoustic signal as the primary cause of a 3-9 dB degradation in the coherent matched-field processing output of a full water column vertical array. The work indicates that over a tidal cycle acoustic signal properties and matched-field processing gain can be expected to change continuously in a shallow water stratified channel that has bathymetry variability. Acoustic signals propagating in such tidal flow-controlled environments may be expected to display repeatable (over successive tidal cycles) and predictable changes in their phase coherent properties. These results suggest that matched-field processor replica fields used in the shelf/slope propagation environment will have to be updated regularly during a tidal cycle to maintain maximum processor gain.

Acoustic propagation under tidally driven, stratified flow

“…For example, measurements of array gain degradation for data acquired on the New Jersey shelf are consistent with the prediction that phase contributions dominate the degradation. 29 Results illustrated in Figs. 3-6 are consistent with phase dominance, a conclusion drawn from a direct comparison of frequency-time SGD plots for the individual amplitude and phase contributions obtained through Eq.…”

“…͑5͒, separating the total degradation associated with the full complex field into the product of its individual amplitude and phase contributions, respectively. 29 Signal gain degradation, in the form 10 log ⌫, is discussed below for the important case of propagation under tidally driven stratified flow. We note here that in the next subsection a different interpretation is given for the vectors e and f, and Eq.…”

Matched-Phase Weighting Beamformer to Improve the Gain of a Long Linear Array in the Range-Dependent Ocean Waveguide

“…Hence, the weighting amplitude can be uniform, and a suitable weighting phase should be designed to compensate for the phase of the channel transfer function. According to (12), AG will be close to the ideal value if = i i   or 0 i   for all elements, i.e., the weighting phases are matched with those of the channel transfer functions. However, for a passive sonar system, i  is difficult to estimate by the received signal since the signals have been modulated by the acoustic channels (as in (11)) which have spatial and temporal fluctuations.…”

“…Hence, these processes require the hydrological parameters of the ocean waveguide and an accurate model of sound field to accurately calculate the replica vector. For the range-dependent waveguide, however, the replica fields have to be updated regularly to maintain a high gain [12]. The accuracy and speed of the acoustic field calculated are often not guaranteed due to the complex changes of hydrological and seabed parameters.…”