Thee speed of sound in water rises and falls with temperature, current speed, salinity, and pressure. Precisely measured underwater acoustic travel times can be used to calculate these parameters for a body of water. If the salinity and pressure along the acoustic path is known, a system of two underwater hydrophones transmitting bi-directionally can indirectly measure both temperature and water velocity. Summing the bi-directional travel times isolates the temperature component, while subtracting the travel times isolates the component of current speed.
There has been much work in using this method for current point detectors as well as networks covering very large distances (> 100km). The feasibility of using acoustic travel times for measurements in the 100m - 3km range has been less explored. Accurate measurements at this range may be valuable in iceberg tracking for the offshore oil and gas industry, as well as in building high resolution maps for environmental studies. This range presents some new challenges, as a distributed network must remain highly synchronized to obtain suffciently accurate travel times. Multipath travel is also highly problematic in this range.
A system has been developed and used to measure tidal currents in the North Atlantic Ocean. The design was built from the ground up. The hardware, software, and data processing are built to work with non-specialized low-cost equipment in a variety of harsh marine deployment conditions. Field testing showed that the system was capable of measuring currents to within ±4cm/s across a 130m range. Quantitative error analysis is included which shows that much higher accuracies could be achieved at the 1km range.
The full report can be found here.
Adapted from https://research.library.mun.ca/14455/