Measurement of Vertical Sound Speed Profile Using Doppler Shift in Water Tank with Temperature Gradient

Abstract

Oceanic vertical sound speed profiles are important for acoustic measurement and acoustic communication, as sound propagation depends on variations in sound speed, particularly during long-distance propagation. XBT (expendable bathythermography) is often used to measure vertical temperature profiles because of its advantages of low cost and ease of operation. As a sound speed profile is calculated from this temperature profile and statistical salinity, there is necessarily some discrepancy between it and the real sound profile because of the lack of real salinity data. Although the use of a CTD (conductivity, temperature, and depth) sensor combined with a sound velocity meter provides a more accurate sound speed profile, it entails the time-consuming process of deploying and retrieving a lengthy cable. In this paper, the authors propose a concept of an expendable, low-cost sound velocity meter like XBT using Doppler shift. In the proposed method, sound speed near the probe can be estimated from its descent speed and the Doppler shift received at the sea surface from the source in the probe. A 1D model of this method was performed numerically, and a tank experiment showed good agreement between the sound speed estimation from Doppler shift and the conventional sound speed calculation from CTD.