A wave-theoretic method for estimating the effects of internal tides on acoustic wave transmission

Abstract

The effect of the first mode semidiurnal internal tide on a ducttype sound speed profile was approximated by varying the minimum velocity of the parabolic profile in a sinusoidal fashion with a period of 12.42 hours. The corresponding effect on the acoustic field was determined by using the normal mode solutions to the parabolic profile to compute the signal level and phase at a fixed receiver due to a fixed cw source. A systematic computer study was performed to determine the dependence of signal level and phase upon the source frequency, receiver range and tidal amplitudes. Numerical results showed that the acoustic phase variation over a tidal cycle was very nearly a sinusoid with 12.42 hour period and a peak-to-peak amplitude which was approximately a linearly increasing function of source frequency, receiver range and tidal amplitude. A simple analytical expression was developed which predicts these linear dependencies and closely approximates the phase variations computed numerically. Thefluctuations in signal level during a tidal cycle were found to depend upon the detailed structure of the acoustic field in the immediate vicinity of the receiver. In general, however, the fluctuations in signal level increased in frequency with increasing source frequency, tidal amplitude and receiver range.