Radiometric Beam Pattern Extraction and Relative Calibration Method for Backscatter Strength Data from Motion-Stabilized Multibeam Systems

Multibeam sonar backscatter strength (BS) data are increasingly being utilized for seafloor substrate classification, which is a vital component of fisheries, habitat mapping, and many offshore engineering applications. The artifacts in the BS data must be minimized before classification is attempted. This paper looks at the artifacts in BS data due to the radiometric effects of the transmit and receive array. This is particularly confounded for the multisector geometry, where the across-track transmit lobes of individual sectors are much narrower compared to a single sector system. These multisector systems have significant variations in the source level within a sector (up to 8 dB) and between sectors (up to 5 dB). This article also addresses the additional factors affecting the measured intensities due to the roll, pitch, and yaw motion stabilization. To address these complications, a new method is developed to unambiguously compute individual sector-specific radiometric beam pattern (RBP) residuals for the combined effect of the transmit and receiver arrays. For a given frequency and azimuth, the BS of a particular seabed type depends only on the grazing angle, any apparent change in the BS at a given grazing angle due to ship's motion is caused by radiometric properties of the sonar. This is the basic principle used in the development of the new method. The developed method separates the received intensities first based on grazing angles and then further by various sonar-relative angles in along- and across-track. The intensity variations within a given grazing angle for different sonar relative angles are used to estimate variations in the combined transmitter and receiver radiometric effects. This article describes the developed RBP extraction method using real data collected with Kongsberg EM 710 multisector multibeam system. It also demonstrates corrected BS data with minimized transmit and receive radiometric effects. It is estimated that the presented method reduced backscatter uncertainty due to motion artifacts by up to 2 dB, enabling improved seafloor characterization based on quantitative analysis of BS data.