Refraction Correction of the Acoustic Wave in Multibeam Systems

Abstract

Dynamic surveys of the sea bottom that rely on determination of reflection points of the acoustic wave transmitted by the echosounder use the wave field theory for determination of the trajectories of the acoustic ray. Complex multibeam echosounder systems that use diagonal beams to determine the coordinates of reflecting points on the sea bed, must take into consideration the refraction phenomenon. Beam inflexion depends on the variability of sound’s velocity in water. It is increasingly more important for beams most inflected from the plumb line. It is a determinant of the angle zone of echosounder’s beam and swath width. The efficiency of bathymetric surveys relies on the swath width. It is 3-5 times the water depth for a multibeam echosounder and up to 12 times water depth for an interferometric one. High value of swath width achieved for high beams inflexion decreases the reliability of the Digital Sea Bottom Model built by data recorded during the bathymetric surveys. This article presents results of correction of the phenomenon of non-linear propagation of the acoustic wave in the multibeam echosounder calibration process in the area where the depth does not exceed 30m. A calibration test was utilised on the perpendicular profiles to determine the variance of depths. Depths measured by the diagonal beams were verified using a vertical beam during surveys on perpendicular profile. Popular models for determining the trajectory of an acoustic wave were used – one based on Snell’s law and another, a new one developed by TU Delft. The geospatial data was sourced using R2Sonic multibeam echosounder during the preliminary stage of bathymetric sounding.