Backscatter Analysis of Multibeam Sonar Data in the Area of the Valdivia Fracture Zone using Geocoder in CARIS HIPS&SIPS and IVS3D Fledermaus

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This thesis deals with a backscatter analysis of multibeam echosounder data acquired during RV Sonne cruise SO213 leg 1 in the area of the Valdivia Fracture Zone in the South-East Pacific. An area of 53 x 25 km² was systematically mapped with profiles in diverse directions using a Simrad EM 120 of Kongsberg (12 kHz). For ground-truthing three sediment samples were recorded. The raw backscatter data was processed with Geocoder in CARIS HIPS and SIPS and FMGeocoder Toolbox (Fledermaus). Geocoder was originally developed by Dr. Luciano Fonseca and is licensed by the University of New Hampshire. It was implemented in both commercial software packages recently. At first, radiometric and geometric corrections for image restoration, enhancement, and geometric rectification were applied to the dataset. Both software packages returned similar results, but it could be observed that the computation of the actual slope for a removal of topographic effects did not work to full extent in either software. Following initial processing, mosaicing of individual data files into one large seabed image was accomplished using a weighted interpolation method to reduce nadir and seam-effects. This method assigns lower weights to backscatter measurements of the very inner and towards the outer beams. An examination of the results revealed that the mosaic obtained by FMGeocoder Toolbox showed less artifacts and therefore a more homogeneous image. Geocoder provides the possibility of an automatic sediment classification employing an Angular Range Analysis (ARA). This was tested for both software products with diverse settings (with and without ground-truthing) for an evaluation of the results. The estimations of sediment type distribution in both Geocoder implementations differed significantly from one another. Even though the removal of topographic effects did not work properly, the sediment classification yielded acceptable estimations for some of the tested settings. Although the seafloor topography influenced the appearance of backscatter strengths in the mosaic, a conclusion on the sediment type distribution of the investigation area was possible based on the angle-invariant backscatter information, the recovered sedimant samples, and echograms of a sub-bottom profiler.

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Dufek, T. (2012): Backscatter Analysis of Multibeam Sonar Data in the Area of the Valdivia Fracture Zone using Geocoder in CARIS HIPS&SIPS and IVS3D Fledermaus , Master thesis, HafenCity Universität Hamburg.

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