Environmental causes of variations of the sound velocity structure in a shallow bay

Measurements of sound velocity, temperature, and salinity were made in St. Andrew Bay, Florida. These measurements were made tostudy one-way sound transmission in shallow water. Three effects concerning the interaction of the environment with the sound velocity structure are described: first, the salinity structure in a bay may play a more important role than the temperature in determining the sound velocity gradients; second, the medium displayed a layered structure rather than being a heterogeneous mixture containing patches of differing sound velocity; and third, tidal currents seem to be responsible for either partially mixing thermal layers or vertically displacing the thermal gradients. Thus, in a shallow bay there are large fluctuations and changes in temperature and salinity resulting in correspondingly large changes in the sound velocity structure. The usually present salinity gradient adds to the complexity of the interaction between the seasonal temperature changes and tidal currents. Therefore, predicting sound velocity gradients in a shallow bay is generally more formidable than for the open sea.