Fluctuations of high-frequency acoustic pulses in three shallow-water experiments

High-frequency acoustic propagation and scattering experiments were conducted near Panama City, Florida, in August of 1991 and 1993, and in Eckernforde Bay, Germany in May 1993. Environmental measurements were made in conjunction with acoustic measurements. The water depth at all sites was approximately 30 m. Sources and receiver arrays were mounted 6 to 8 m from the bottom and were separated by about 80 m. Data were obtained from 20 to 180 kHz. Means, standard deviations, and coefficients of variation of 100 to 150 direct path pulses for each frequency characterize two scales of temporal variability in the data. The short term variability with a period of several seconds was attributed to wind waves while the larger scale changes with a period of several minutes were related to internal waves. The amplitude of the fluctuations depended on a combination of factors including the depth and slope of the thermocline, turbulence from internal waves, wind waves, tides, and current interactions, and whether or not multipath arrivals interacted at the receiver array. Spatial variability was noted among several closely spaced hydrophones. Differences among frequencies depended on environmental factors that were changing with time of day when data were collected, as well as wavelength and beam pattern effects.