Path‐averaged ocean measurements in the deep, stratified tidal channel of Hood Canal using acoustical scintillation

Abstract

Path‐averaged current speed, effective refractive index fluctuations, and stratification measurements were made using high‐frequency (67 kHz) acoustical scintillation measurements in the northern entrance to Hood Canal, Puget Sound, Washington. This experiment made use of a four‐transmitter and four‐receiver array configured in a T‐shape; the two‐dimensional feature of this array was designed to measure both along‐channel small‐scale properties as a result of advection and vertical properties as a result of acoustic refraction from temperature/salinity stratification. With long path lengths and stratified conditions, acoustic propagation resulted in multipath arrivals which were separable for most of the measurement period. A maximum likelihood estimation algorithm is developed that tracks both the direct path signal at approximately 25–30 m depth and the upward refracted signal into the near surface and calculates amplitude, phase, and travel time for each. The acoustical signals are then inverted to estimate path‐averaged along‐channel flow properties, turbulent effective refractive index levels, and changes in stratification. Along‐channel flows approach 50 cm s−1, and the acoustic measurement agrees very well with a simple tidal model of the currents and shows some deviations from independent measurements during maximum flood tide. Current velocity contributions to the effective refractive index fluctuations are analyzed, and results indicate that both sound speed and velocity fluctuations contribute to the acoustic scattering. The vertical acoustic arrival angle to first order appears to be a sensitive indicator of small changes in stratification.