| Summary: | This study investigated the effect of sound refraction in the water column on the accuracy of depths calculated from oblique swath angles of Multibeam Echosounders. This was accomplished by analysing data collected from the Tamar Estuary, Plymouth Sound and the Northeast Atlantic Ocean. Sound speed was computed from this data using the Chen & Millero, MacKenzie and Medwin formulae for the estimation of sound speed in sea water. Resulting values were graphed as sound velocity profiles and barographs thus visually illustrating discrepancies in sound speed estimation between the three sound speed formulae, leading to the conclusion that the sound speed formula used for the calculation of sound speed in the water column is a source of error related to the horizontal position in a charted depth. The last phase of analysis utilised ray tracing to compare the horizontal distance a sound ray refracted in the water column from sound pulses emitted at near vertical and oblique angles. In all analysis, the extent of refraction increased as the swath angle increased as did the error between the estimated sound speeds from the Chen & Millero, MacKenzie and Medwin formulae. This study establishes the sound velocity formula used to compute sound speed in the water column and refraction at large oblique angles of Multibearn sonars contribute to errors associated with outer swaths of Multibeam sonars, thus contributing error to the horizontal position of chart depths.
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