The effect of sound velocity on wide swath multibeam system data

The French Hydrographic Service (SHOM) have been operating a wide swath deep water multibeam system for three years now and a shallow system for longer. As an hydrographic service, SHOM is very anxious to control the quality of gathered data. So the latest studies have been focused on these issues, one of which dealt with the sound velocity effect to depth accuracy. Multibeam echo-sounders (MBES) need sound velocity for different purposes, depending on the equipment, the depth surveyed, the swath width... This paper describes and quantifies, for different equipment types, how sound velocity mishandling or variations can decrease the accuracy of the depth measurements. This is illustrated with real data, and sound velocity variations observed on some mid Atlantic surveys. Sound velocity is an important issue to the user, because it may change significantly in time or position, and causes large errors on wide swath system outer beam bathymetry. To limit these errors, it is possible to decrease the swath width and increase the vessel speed accordingly, or to make numerous sound velocity profile measurements, thus spending either a lot of money or time. For some equipment, the user should know about this issue to choose the best parameters in configuring his or her system. The conclusion of othe authors' study surprised them, as it was somehow different from what they initially expected.