"How is sound used to measure currents in the ocean?" (continued)
RAFOS Floats (SOFAR spelled backward) are floating instruments designed to move with a current and track the current's movements.
 |
| Courtesy of The RAFOS group at URI |
The RAFOS Float keeps track of its own position by listening for the signal from sound sources in the water near the study area. The RAFOS Float uses the time of travel and the phase of the sound to determine its position. Because the RAFOS Float moves with the current, the float's position tracks the path of the current. The RAFOS float can be designed to float at different depths, allowing the full structure of the current to be studied.
Reciprocal Transmission
Just as a boat going downstream with the current in a river travels faster than a boat going upstream against the current, a sound pulse moving in the same direction as a current travels faster than one moving against the current. Sound pulses transmitted in opposite directions at the same time (called "reciprocal transmissions") will therefore have different travel times. The pulse traveling with the current will have a shorter travel time than the pulse traveling against the current. The difference between the two travel times can be used to compute the current.
 Two sources and two hydrophones are necessary to measure current velocity by transmitting sound in opposite directions through the current. High precision measurements are required because the difference in the travel times of oppositely traveling pulses is tiny. Sound travels at about 1500 meters per second in the ocean, while ocean currents typically have speeds of only 0.1 meters per second. Sound traveling with such a current will travel at about 1500.1 meters per second, while sound traveling against the current will travel at about 1499.9 meters per second. (Even strong currents such as the Gulf Stream have speeds of only about 1 meter per second.)
Acoustic Current Meters (ACM) apply this basic principle to measure ocean currents without using propellors or any other moving parts. See for example: http://www.falmouth.com/products/3d_acm/3D-ACM.htm. The same basic principle has been applied to measure the average current in the Strait of Gibraltar using acoustic sources and receivers (transceivers) separated by about 20 kilometers (http://gibraltar.ucsd.edu/). It has also been used to make precise measurements of ocean tidal currents using acoustic sources and receivers separated by up to about 1000 kilometers.
| 
