Sound Velocity Profiling Enhances Survey Productivity
In 2013, TerraSond (Alaska, USA) completed a major hydrographic project for NOAA in the Chukchi Sea in northwestern Alaska for the purpose of nautical chart updating for safety of navigation. The surveyors planned this shallow water survey within their long-term NOAA contract, realising that the troublesome sound velocity (SV) conditions in the survey area were a key factor to consider. Stopping the survey vessel to conduct regular SV dips was also ruled out as they would result in survey downtime. For the most recent survey, it was decided that the Oceanscience UnderwaySV profiler would be used instead of a larger winch.
Approximately 4,000 nautical miles of multibeam and sidescan sonar data were collected to survey 180 square nautical miles of seafloor in a remote, poorly charted region of the Arctic experiencing increases in freighter traffic during the limited ice free season.
Familiar with the SV challenges in these waters, TerraSond previously installed large profiling winches on their survey vessels when sound velocity variability could be a substantial source of uncertainty.
The Valeport-designed RapidSV probe used in the UnderwaySV system is capable of achieving profile depths of over 400m at typical hydrographic survey speed, courtesy of the slippery probe's 5m/s drop rate. However, in the shallow water of the Chukchi Sea, the probe assembly was augmented with the "slow-fall" buoyancy module to retard the drop speed to a more manageable 2m/s. With a repeatable drop speed, surveyors were able to hit their target depth for each profile, and easily keep the SV probe and the sidescan towfish away from each other.
Casts were conducted every two hours on average during 24/7 production in depths from 10 to 30 metres; over 400 sound velocity profiles were generated during the survey. On each dip, the Valeport wireless telemetry module automatically uploaded the profile from the probe via a Bluetooth radio link to the acquisition PC. In most cases, the data was available on the hard drive of their PC before the probe was recovered over the rail.
TerraSond Charting Program manager Andrew Orthmann was pleased that they selected a simple yet effective option. For similar work in the past they had used various profiler systems designed for moving vessels; by the nature of these large and complex systems, there was a proportionate amount of mechanical issues and breakdowns that frequently resulted in downtime for troubleshooting and repairs. However, TerraSond now found the simplicity of the UnderwaySV system to be an advantage as it proved to be easily serviceable without specialised knowledge, as well as reliable. He reports significant time savings since there was no need to stop or even slow the ship to collect profiles as well as not having to service and repair the system, or wait on parts. Importantly, since the system was easy to use we also collected more profiles then we may have been able to otherwise, positively affecting the final data quality of our multibeam bathymetry.
