Side‐scan sonographs of steep slopes in the Wushieh Reservoir

Abstract

Operators tow side-scan sonar transducers at the optimum height from the bottom to produce better quality side-scan sonographs. This can be done only if the bottom is moderately flat. Sonographs with maximum tonal intensity are easily produced on steep slopes e.g. valley banks. Strong signatures are recorded over all ranges due to the poor geometry of sonographs produced without acoustic shadows. In order to create clear scanned images of a steep slope using a side-scan sonar, rotation of the towed fish or tilting the transducer face to the dipping slope surface is suggested. Cases conducted on the shore of the Wushieh Reservoir, Taiwan, in which the bank slopes varied from 10u to 45u are discussed in this work. In general, the angle of the scanned slope determines the degree of towed fish rotation angle. The rotation angle is set at a smaller angle than the slope, giving stronger returns from the lower slope. In contrast, if the rotation angle is set at a larger angle a null return outside the insonified area in the lower slope is produced. A sub-aqueous landslide was observed in the side-scan sonographs using this strategy. This study also gives results that affirm the value of a side-scan sonar as a powerful tool for underwater inspection.

Automated feature extraction and spatial organization of seafloor pockmarks, Belfast Bay, Maine, USA

“…In the absence of high-resolution seafloor bathymetry data, previous characterizations of entire pockmark fields relied upon visual interpretation of acoustic backscatter data for pockmark delineation, size statistics and spatial distribution (Fader, 1991;Kelley et al, 1994;Gontz et al, 2002;Rogers et al, 2006). Although acoustic backscatter data were the best available in the cited studies, interpreting size dimensions of concave features, such as pockmarks from these data is often ambiguous (Song, 2007). High-resolution bathymetry data collected by multibeam echosounder and swath sonar technologies enable the study of seafloor morphology to reach scales and resolutions similar to studies in subaerial geomorphology based on digital elevation models (DEMs) (Hughes Clarke et al, 1996).…”