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Sound Velocity Profilers

Schematic of  time-of-flight sound profiler.
Schematic of the sound velocity profiler

Knowledge of the precise sound velocity is critical in processing and interpreting bathymetric data, as water depths are calculated by multiplying the one-way travel time by the sound velocity, with appropriate corrections made for the geometry of the acoustic energy path.Therefore, the USGS Woods Hole Science Center operates two sound velocity profilers in conjunction with our bathymetric data acquisition. Both systems provide “time-of-flight” sound velocity measurements, whereby an acoustic pulse is generated, sent across a known distance to an interface, then is reflected back toward the source. The travel time for the sound to travel that precisely-known fixed distance is recorded and converted to the sound velocity in that environment (T. O’Brien, personal communication). The velocity of sound in water can vary from approximately 1400 to 1575 m/sec, depending on the temperature, salinity, and pressure-depth.

Sound velocity profiler
Schematic of the sound velocity profiler

One system, the velocimeter, is placed on the head on the side-mounted swath-bathymetry system and is run continuously to measure the speed of sound in water at the transducer face; an acoustic pulse is generated every second. The sound velocity information is recorded on the swath-bathymetry acquisition system and on the navigation system. This velocimeter is a small cylinder approximately 124 mm in length and 33 mm. diameter, with the electronics housed in the cylinder and a sensor open to the water with an endcap. The acoustic pulse travels from the cylinder to the endcap and back to the cylinder. This provides a sound velocity measurement at/near the transducer face continuously while surveying is underway.

The sound velocity profiler (SVP) system generates a vertical profile of sound velocity with depth at a single location. It requires the ship to stop and remain in one position while it is hand deployed. It is lowered into the water and allowed to stabilize to the ambient water temperature. Then the system begins to internally log pressure/depth and sound velocity, based on the same time-of-flight principle as described above. Once the profile is complete, the system is retrieved and the data transferred from the sound velocity profiler to a file and stored for use in correcting bathymetric measurements during data processing.

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