Reduction of refraction effects due to inadequate sound velocity profile measurements in multibeam echosounder systems

Abstract

The single most important acoustical variable in the water is its speed. The average speed of sound in the ocean is about 1500 m/s, but its precise value in a location is strongly depends on temperature, pressure and salinity of that particular location. These factors change rapidly in time and space due to various reasons. In data acquisition, the collection of these denser sound speed data becomes critical. These inadequate sound speed measurements cause unknown propagation through the water column that adds a major uncertainty to the multibeam echosounder measurements (MBES). There are two types of sound speed measurements made in the flat array multibeam sonars. Surface Sound Speed (SSS) is measured at the face of the transducer and Sound Velocity Profile (SVP) is measured through the water column. SSS is used to determine the beam pointing angle (beam steering) and SVP is used to determine the depth and position (ray-tracing) of each beam. From these, the SSS is measured almost at each ping vies and SVP may be once or twice a day depending on the situation. When it comes to ray tracing, one has the options of using either the SSS or the surface value of SVP (SSVP). Some multibeam software manufacturers use the SSS in Snell’s refraction constant determination while others use the surface value of the last performed SVP. In this study, both methods of refraction constant determination are evaluated. The results clearly showed that the use of SSS for Snell’s refraction constant determination gives about 25% to 30% better results in multibeam bathymetry against refraction than the use of SSVP. A combined solution of SSS and SSVP provide a better, simpler and cost effective method of reduction of refraction effects in MBES. The results also demonstrated that the effects of inadequate sound speed measurements in each phase of bathymetric calculations would result in both depth and positional errors.