Surface wave influence on acoustic propagation in very shallow water

In late spring 1998, SACLANTCEN conducted an extensive experiment off the coast of Italy to study the effects of the environment on direct path propagation at medium to high frequencies. Ten towers with transducers and hydrophones were carefully positioned in a water depth of 910 m. One of the major observations from the data was a strong correlation of sound speed fluctuations and current velocity variability with surface waves. In calm sea conditions, the sound speed fluctuations were small, driven primarily by temperature changes. For medium to rough conditions, the current velocity variability was large and caused by surface wave induced particle motion. Not only were the surface waves the main driving mechanism for the observed oceanographic fluctuations, but the surface wave spectra was directly reflected into the acoustic phase (travel time) fluctuations. In this paper these relationships are explored and a model is developed which calculates the effect of the surface waves on one- and two-way acoustic signal propagation. Excellent agreement is seen between the data and the model. For the two-way propagation, the model also explains the directional behavior observed in the measured phase fluctuations. This indicates a wave dependent change in the average sound speed in addition to effects due to particle motion.