Correcting Multibeam Echosounder Bathymetric Measurements for Errors Induced by Inaccurate Water Column Sound Speeds

Tannaz H Mohammadloo,Jonathan Beaudoin,Weston Renoud,Mirjam Snellen,D G Simons

doi:10.1109/access.2019.2936170

Abstract

In this contribution a method for correcting bathymetric measurements affected by inaccurate water column sound speed profiles (SSPs) is presented. The method exploits the redundancy in the multibeam echosounder measurements obtained from the overlap of adjacent swaths by minimizing the difference between depths along overlapping swaths. Two optimization methods are used, i.e., Differential Evolution (DE) and Gauss-Newton (GN). While DE inverts for the sound speed by minimizing the depth variation, GN inverts for both bathymetry and sound speed by minimizing the squared sum of the differences between the modeled and measured travel times. The inversion method assumes a constant SSP in the water column. Applying the method to a salt wedge survey area with large variations in the water column sound speed indicates a good agreement between the original depth measurements and those derived after the inversion with the mean and standard deviation of the depth differences equaling 0.009m and 0.024m, respectively. This indicates that even with a simple parametrization of the sound speed in the water column, the correct bathymetry can be derived from the inversion. The SSP inversion method is also applied to an area with existing refraction artefacts. It corrects the bathymetry and reduces the mean and standard deviation of the depth standard deviation by a factor of around 2.75 compared to the case where the measured SSPs were used. Furthermore, the SSP inversion method neither manipulates the existing morphology nor introduces artificial bathymetric features in the areas where such refraction artefacts are not present. Considering constant SSPs, both DE and GN give almost identical results with GN being faster. However, GN is less flexible with regards to varying sound speed parameterizations.

Highlights

Multibeam echosounder (MBES) systems are widely used for conducting bathymetric surveys
A sailed track within the survey area is divided into subsets consisting of 20 consecutive pings
The method considers the part of the seafloor where the depth measurements from these pings in the track under consideration and its adjacent ones overlap

Summary

INTRODUCTION

Multibeam echosounder (MBES) systems are widely used for conducting bathymetric surveys. Reference [19] adopted a relatively similar approach by using the measured depth and considering this as the true depth for the outer parts of the swath This true depth, in combination with other parameters such as beam angle and two-way travel time, was used to invert for the constant gradient SSP. For modern well-calibrated MBES systems, these differences are in general due to the use of an erroneous sound speed profile stemming from a lack of and/or incorrect sound speed information In this contribution, we propose a method for estimating the sound speed and depth that fully employs the redundancy of the overlapping MBES swaths. Data taken in a complex environment with varying water column sound speeds with depth (salt wedge estuary) is used and the performance of the method with the assumption of constant SSP is assessed. The actual steering angle, θm, differs from the direction aimed for, θ

THE OPTIMIZATION METHODS CONSIDERED

RESULTS

ILLUSTRATION OF USE OF THE PROPOSED METHOD IN PRACTICE

SUMMARY AND CONCLUSION