20KHz acoustic fluctuations due to thermal finestructure in the upper ocean.

Abstract

The Acoustic Variability Experiment (AVEX) described here measured acoustic pulses which travel a wholly refracted (400m) path between a source and receiver in the upper ocean. The 20 KHz pulse travel times had rms variations between 1 and 60 microseconds, calculated over each half hour sample period, and corresponding pulse amplitude fluctuations between 3% and 14%. The temporal and spatial structure of the temperature field were measured simultaneously with the acoustic transmission. A spectrum of a 15-day temperature record showed the presence of inertial and tidal motions, and internal waves at higher frequencies. Microstructure was intermittent and appeared to be associated with internal waves. The temperature integral scales calculated from the spatial autocorrelation functions ranged from 1.2 to 14m. (est) in the vertical and from 3.4m to greater than 50m (est) in the horizontal. The temperature structure was anistropic with the average horizontal to vertical scale ratio equal to about four. A comparison of the measured acoustic amplitude variance with temperature variance and scale lengths measured at the receiver station showed poor correlation using the theoretical models of Chernov and Debye.