Abstract
Underwater structures such as bridge abutments are damaged by a flood and longtime erosion. So far we do not have an effective way to check the safety for this kind of such underwater structures. Therefore, it is considered that there are a lot of potential needs of precise geomorphology and geometry observation. Recently an imaging sonar system C3D, integrated bathymetry and side scan sonar, can provide a high resolution, wide swath and high accuracy in bathymetry, and has been expected to be used for underwater floor mapping in shallow water. However, in the area under a bridge, even the C3D is used, high quality geomorphologic results cannot be obtained because GPS signals will be blocked out and the positioning precision deteriorates tremendously. Though total station (TS) integrated laser ranging and transit has been expected to take the place of GPS in its outage area, it shows difficult to maintain the positional precision as the sampling interval precision is low. Therefore, we developed a new TS with short delay and small dispersion in its data output in cooperation with Topcon, which is synchronized with GPS time. The new positioning system can obtain enough positioning accuracy even in case of GPS outages. With this we constructed a high accuracy geomorphologic measuring system utilizing the C3D imaging sonar which has three times swath width of traditional multi-beam sonar and two thousand measured point data each ping. In order to validate the new system, a survey was conducted under two bridges in the upper and lower reaches of the Tone River respectively. The results showed that the proposed system can be used to measure underwater structures with adequate accuracy.
