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All depth data, or bathymetric data, acquired must be corrected for the velocity of sound in the water. Sound travels through saltwater at an average rate of 1500 meters/second (freshwater 1435 meters/second). As sound travels through the water, it is affected by differences in salinity, temperature, and pressure. Sound is deflected into different directions with these changing factors through the different layers of water. These effects need to be taken into account to accurately portray the seafloor.
To determine sound velocity through the water column, Rainier collects conductivity, temperature, and pressure data using a CTD sensor called a SEACAT. This instrument is deployed into the water at least every four hours during multibeam acquisition. Casts are taken more often if the location or conditions change. In glacial areas or near river deposits, casts are taken frequently as the salinity in the water column tends to have significant variations.
To take a sound velocity cast, the ship or launch is maneuvered into a position such that the cast line will not end up in the propeller. The SEACAT is secured to a winch wire or line. The sensor is exposed and the instrument is turned on.
The SEACAT is placed just below the water's surface for two minutes to allow the sensor to obtain its initial readings. The SEACAT is lowered one meter per second through the water column until it reaches the seafloor. Then it is hoisted back to the surface. As the instrument runs through the water column, the sensor obtains conductivity, temperature, and pressure data.
Once the SEACAT is aboard, it is connected to a computer. The sensor data is downloaded using a special program. A survey technician or junior officer uses the program to analyze the data. If the data looks reasonable, the launch or ship will begin or continue to acquire soundings.
Rainier recently acquired a new sound velocity profiling device, the Odim Moving Vessel Profiler (MVP). The MVP is used on NOAA’s other hydrographic survey vessels, Fairweather and Thomas Jefferson. Rainier’s MVP is a smaller version than the ones used on the other ships, and it is carried on one of her survey launches. The MVP measures sound speed directly by operating like sonar. It sends a short sound pulse through the water to a plate on the unit with a known distance. The MVP listens for how long it takes the sound reflection to return from the plate and calculates the sound speed based on distance and time.
The MVP’s sensor is towed from the A-frame of a survey launch. When the launch is surveying, the MVP’s ‘flies’ at a depth just below the surface until it is triggered, and sent to take a cast. A winch on the launch lets out enough cable for the ‘tow fish’ to dive near the bottom. Once it has gone close enough to the bottom, it is reeled in to the point of where it is again ‘flying’ below the surface. This device is useful because it eliminates the time spent stopping the launch to take CTD casts. The survey crew can cast as often as necessary to capture the sound velocity profile of a dynamic environment.
