Determination of sound velocity profile errors using multibeam data

A sounding on the vertical beam will be in error by precisely the proportional error in the mean sound velocity of the water column. However, a beam launched at /spl plusmn/45/spl deg/ will not have such an error. The time required for this beam to reach a specific depth has very low sensitivity to sound velocity profile (SVP) errors. Although such errors do induce a small perturbation of the athwart ship location of such a sounding point, the computed depth will be correct for that sounding point. This results in survey tracks at approximately /spl plusmn/45/spl deg/ with correct depth data to serve as reference data for the nearly vertical beams of crossing swaths. These crossing swaths may come from checklines used in most hydrographic surveys. The sounding data of multibeam echo sounder (MBES) systems are a result of processing information from many sources. These include ship's heading and navigation data, vertical reference data and sound velocity data in addition to the basic MBES itself. Data from each of these sources is subject to individualized errors contributing to the overall data quality-or the lack thereof. To limit these errors, system planners often have established error budgets for various components of the system. With the advent of new technologies, the last remaining obstacles to absolute depth precision may soon be SVP errors and roll bias. (Roll bias as used herein means that uncompensated portion of the difference between the receive array and the vertical reference unit.) This paper describes a post processing solution that can significantly reduce errors related to both SVP and roll bias. This process is specific for swaths that include beams at /spl plusmn/45/spl deg/ and is suitable for all such multibeam data derived from a Mills Cross type of MBES.