Fluctuations in sound transmission through internal waves associated with the thermocline: A computer model for acoustic transmission through sound velocity fields calculated from thermistor chain, CTD, XBT, and acoustic backscattering

Abstract

Data sets obtained from towed thermistor strings, instrumented tow bodies, conductivity temperature depth sensors, expendable bathythermographs, sound velocimeters and high-frequency acoustic backscattering systems provide detailed information on the temporal and spatial variability of the sound velocity field in the upper ocean. The data sets show that internal waves and other small scale fluid phenomena such as instabilities can significantly perturb the sound velocity field and consequently sound propagation. Little attention has been focused on the magnitude of short-range, high-frequency acoustic propagation variability caused by internal wave field and/or water mass variability. A ray trace program package has been developed which numerically integrates with a fourth-order Runge–Kutta method the time dependent eikonal equations through a range dependent sound velocity field. The sound-speed profiles, which are calculated from field data, are entered into a matrix with sizes larger than the numerical integration steps. Consequently, the profiles are interpolated between matrix steps with the bicubic interpolation technique of Akima. Calculations for sound-speed fields perturbed by internal waves show that at a fixed position the intensity of the sound intensity field can vary by as much as 20 dB as a function of time.