| Summary: | 博士 === 國立臺灣大學 === 海洋研究所 === 95 === To discriminate seabed property has been limited by merely differentiating sonar strength of sidescan sonar. In order to achieve higher accuracy in probing seabed property, forming the image of lineation with higher sonar strength becomes a basic rule for judgment.
Disregarding systematic errors, acoustic intensity of sidescan sonar image shall be compared relatively. With different incident angles, frequency and propagation length of sound wave approaching to the sea floor, different reflected scattering intensity can be shown upon the sonar image.
Micro-topography, including texture, lineation, shadow zones and discriminative blocks shown on the seafloor can be displayed with reasonable sonar scan geometry. Therefore, whether the seabed has been covered by mud, sands, pebbles, rocks or reef can be differentiated.
To detect wave forms lineated on the seafloor, tow fish of sidescan sonar shall be towed in the direction parallel to their strike, and the image of hinge line of sand waves shall be located in the half way of the slant range being set.
When towed fish is towed in the direction perpendicular with the lineation of hinge line of sand waves, the wave length and dipping angles, can be determined from the sonar image at nadir. Under the circumstance, whether the shape of the sand wave is symmetric or not can be shown and the main direction of sediment transportation can be pointed out. In addition, those sand ribbons and megaripples developed on flank of stoss side of sand waves can be easily identified. For those smaller wave forms, a comparatively short range with the fish towed parallel to the direction of hinge line will enhance the contrast shown on the image.
Side scan images of sand waves developed offshore of western Taiwan show their hinge line intersecting at 30-45 degrees with the direction of current flow. Trochoidal sand waves and cat back sand waves has been developed offshore of Wuchi. Trochoidal sand waves are about 200-600 meters in the wave length, and cat back sand waves are 400-800 meters. Cat back sand waves show with the wider cross section that is approximately perpendicular to the direction of the current flow. The denser of sand waves distributed outside the Wuchi River mouth is shown, and their appearing becomes less further to the south or north, which implies sediment supplement and transport force are larger at this region.
The attitude of pipelines lying on the sea floor can be obtained by knowing their geometrical arrangement of sonar images. In addition, the estimation can be more accurate by using multi-beam bathymetrical data in the region. Regarding to the estimation of the elevated together with suspended heights of pipeline above the sea floor, they can be calculated with the Frontward and Backward models derived in this dissertation. Models are tested, and indicated the calculations have been effective.
The deployment of towed fish is designed to rotate its body to scan the slope instead of the flat bottom under the water. The purpose is to observe bottom details of the bank area. In the processes, gradient of the slope can be obtained from the digital bathymetrical data or estimated from the sonar image, so that they can be projected and mapped in a space with the Cartesian coordinates.
The best image to a slope area can be achieved by rotating tow fish with suitable fish heights at an angle identical with the gradient angle of the slope. Without the help of bathymetrical data, we can observe the slope by using different rotated angles, and do the mosaicking and fusion to those sonar images under the proper projection process. It can carve more detailed aspects of the slope image.
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