Observations of matched-field autocorrelation time in the South China Sea

Matched-field processors can suffer degradation in dynamic environments due to mismatch between data and replica vectors. One measure of such degradation is the autocorrelation time scale of the data vectors. Matched-field autocorrelation times for 300- and 500-Hz narrow-band signals have been extracted from data acquired during the 2001 Asian Seas International Acoustics Experiment (ASIAEX). The acoustic signals were received on a vertical hydrophone array located near the South China Sea (SCS) shelf break at a range of 18.9 km from the acoustic sources. The water depth along the propagation path ranged from 100 to 125 m. For acoustic signal propagation through a diurnal internal tide that takes the form of a uniform depression of the thermocline, the matched-field correlation time at 300 Hz is approximately τ₁2/=15 min or, nondimensionally, n₁2/=200 000 cycles. For acoustic signal propagation through packets of nonlinear internal waves, the correlation time for the 300-Hz signal is approximately τ₁2/=2 min or n₁2/=30 000 cycles. A nonlinear internal wave packet entering the acoustic propagation path at 11:40 UTC on May 7 was observed to be unambiguously associated with a drop in the signal correlation time. A two-dimensional advective, frozen-ocean acoustic propagation model produces matched-field correlation times that are qualitatively similar to these observations. It is concluded that, in this environment, nonlinear internal wave packets propagating in the acoustic path shorten the lifetime of replica vectors and cause matched-field processor degradation.