Under shallow water conditions the propagation of sound is strongly influenced by the multiple reflection at the bottom and the water surface. If a vertical gradient of sound speed exists due to temperature or salinity effects, sound propagation can be described in terms of refracted and multiple reflected rays. The refracted waves, calculated by use of an adapted head wave model, describe the precursor lead times of the received sound pulses and the multiple reflected rays, described as eigenray-propagation are used to calculate time structure of the reverberation. The ray tracing models allow to calculate the observed averaged vertical gradient of sound speed on the basis of the recorded sound pulses. This fact is confirmed by the results of an experiment conducted at the Darss Sill in the Baltic Sea at a water depth of about 20 m, for which the distance between the transmitter and the receiver was 2.3 km. The method allows continuous monitoring of the sound speed stratification. The achieved data are representative for a cross-section of the site of monitoring which is bounded by the sound source and the receiver. [References: 25]
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