Acquired sounding depths need to be corrected for sound velocity. Sound travels through saltwater at an average rate of 1500 meters/second (freshwater 1435 meters/second). As sound travels through the water, it can be affected by differences in salinity, temperature, and pressure. These effects need to be taken into account to accurately portray the seafloor.

To determine sound velocity through the water column, the 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 manuevered 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.

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The CTD is used to collect data on Conductivity, Temperature, and Depth of water for sound velocity calculations.

Sound velocity casts are performed off the back of the launches every 4 hours during acquisition.

The A frame is used to lower and retrieve the CTD from the ship.