Comparison of three multibeam acoustic wave path refraction compensation techniques

Abstract

The main mechanism of bathymetry survey is relied on the measurement of the travel time of the two ways sound wave. Speed of sound wave is dependent on the water density. Density of every water layer is affected by temperature, salinity and pressure. According to Snell’s Law, when sound wave travel through water layers with different densities, the travel speed will change and leads to refraction. Thus, the variation of sound speed will significantly affect the accuracy of the bathymetry result, as the refraction will cause the sound wave to be refracted from their ideal propagation path, and, resulting false depth and position. Compensation of the refraction can be completed by Trigonometry Method, Curvature Method and the Combined Method. The Trigonometry Method is implementing the averaging of the sound speed for a determined layer of depth. Conversely, the Curvature Method assumes the sound wave to travel in an arc of circle of different radius in every different depth layer. Meanwhile, the Combined Method is the combination of the Trigonometry Method and Curvature Method. This study aims to identify which approach provides the better positional result and depth. The sound speed data of the water column is recorded using the sound velocity probe at four different study areas. The outer beam of the particular ping is selected for comparison and analysis. As a result, the average of overall difference in horizontal distance between Trigonometry Method and Combined Method obtained from 30° beam and 60° beam is 0.038m and 0.122m, respectively. From the simulation, as the sound speed increases or decreases continuously, the horizontal difference among Trigonometry Method and Combined Method exceeds the special order of International Hydrographic Organisation Standard (IHO) at the depth level where the sound speed exceed the difference of 10m/s from the transducer.