Amplitude and Phase Fluctuations in Shallow Water

Abstract

When an acoustic wave propagates through a turbulent medium, such as the ocean, the amplitude and phase of the incident wave experience fluctuations due to the variations of the acoustic index of refraction of the medium. Time series of experimental data of both the amplitude and phase fluctuations, the index of refraction, and the horizontal and vertical current components were obtained for a nearshore environment off Southern California. The acoustic signal output from a fixed narrow-beam ultrasonic source radiating in the CW pulse mode at 80, 90, and 100 Hz was received by a fixed directional receiver 25 m away. The vertical displacement between the two transducers was 17 m, and a sampling interval of 0.25 sec was used. This exercise was conducted in the presence of a strong seasonal thermocline separating water masses of different acoustical properties. The thermocline undergoes time- and space-dependent vertical motions related to internal waves. The projector was situated above and the receiver below the thermocline. Herein the line-of-sight propagation problem is considered and attention is given to the correlation functions and variances of the amplitude and phase fluctuations of the acoustic wave and their relation to the variation of the index of refraction along the path of wave propagation. It is shown that the turbulence near the transmitter has more effect on the fluctuation than the turbulence near the receiver.