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Abstract
Refraction artifacts are often present in shallow water multibeam surveys and can degrade the quality of the final product if they are not adequately addressed. This thesis consists of the implementation of a systematic analysis and correction software package that addresses refraction artifacts in a post-processing context.
The methodology consists of the estimation of the variation in the water sound speed distribution, by using the information given by the multibeam dataset itself. This is done by the evaluation of the appropriately modeled Sound Speed Profiles (SSP), which is applied either in addition to an already existing SSP, or applied directly to the raw data. Refraction errors are most developed in the outer parts of the survey line coverage. The software developed takes advantage of this observation by utilising the nadir data because they are almost unaffected by refraction errors. Two methods of analysis are considered in this study. The first method uses two neighbouring parallel lines to generate corrections. The second uses the crossing check lines.
Both methods are used to evaluate the refraction coefficients of a two-layer SSP model, which, when applied, should bring the outer parts of the survey line as close as possible to the real seafloor (as observed at nadir).
The software developed is tested on an actual multibeam dataset. This data has been acquired off Saint John Harbor (N.B.) with a SIMRAD EM1000 sonar. The application of the new post-processing tool reduces refraction artifacts. The reduction of such artifacts improves the extraction of useful information contained in the multibeam data. The method used allows as well the computation of correction SSPs that provide characteristics of an equivalent water mass.
