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New Bottom Roughness Calculation from Multibeam Echo Sounders for Mine Warfare

Abstract : Bottom roughness has a significant effect on acoustic backscattering on the ocean bottom. Sonar systems rely on backscattering and shadows to detect objects lying on the seafloor. The seafloor is rather complex, and it can include craters, gullies, seaweed, rocks, sand ridges, tall obstructions, deep holes, and sloping regions. Underwater mines can be hidden near these seafloor features, making detection more difficult. High-resolution (1 m x 1 m) seafloor data collected by the Navy using a multibeam echo sounder (EM710) off the western coast of Saipan was processed by MB Systems. The advanced least-squares method was used to establish a new bottom reference level from the EM710 data. After removing the reference level, the high-resolution bathymetry data converts into bottom roughness percentage using a threshold. The calculated bottom roughness percentage is ready to be incorporated into current Navy doctrine. Two new methods, depth gradient and mathematical morphology, have been developed in this thesis to calculate bottom roughness without the reference level. Statistical analysis was conducted to illustrate the added value of the new bottom roughness calculation.