Tropical Geography ›› 2020, Vol. 40 ›› Issue (2): 278-288.

### Reconstruction of Shipwreck and Surrounding Relief at a Coral Reef in the South China Sea Based on Side Scan Sonar Image

Liu Xiaoju1,5, Shi Qi1,2,4(), Yang Hongqiang1,2,3,4, Zhou Shengnan1,5

1. 1.Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology; Chinese Academy of Sciences, Guangzhou 510301, China
2.Innovation Academy of South China Sea Ecology and Environmental Engineering, Chinese Academy of Sciences, Guangzhou 510301, China
3.Nansha Marine Ecological and Environmental Research Station, Chinese Academy of Sciences, Sansha 573199, China
4.Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), Guangzhou 511458, China
5.University of Chinese Academy of Sciences, Beijing 100049, China
• Received:2019-11-14 Revised:2020-02-21 Online:2020-03-10 Published:2020-05-15
• Contact: Shi Qi E-mail:shiqi@scsio.ac.cn

Abstract:

As sonic imaging instrument, side scan sonar is widely used in the qualitative detection and recognition of underwater landforms and objects. Some researchers have attempted to quantitatively reconstruct underwater landforms using the Shape From Shading (SFS) method, which is based on the principle of computer vision. At present, it is sometimes used in the investigation of coral reefs in China. In the present study, a quantitative reconstruction of shipwreck and its surrounding relief is carried out at a coral reef in the South China Sea based on single side scan sonar image. After the pre-processing steps of data format conversion, image generation, water removal, image gain and correction, and coordinate conversion, the side scan sonar image of the shipwreck area was used to retrieve the underwater topographic data. The study involved using an SFS algorithm based on minimization approach with the measured bathymetric data as the initial topographic condition to reconstruct the three-dimensional relief of the shipwreck and surrounding area. The mean absolute error was 0.77 m and the mean relative error between the retrieved and measured water depths was 4.5%, revealing the topographic and geomorphic features of the shipwreck area. The results show that the shipwreck area is located on the western side of the coral reef and has the typical geomorphologic form of a reef slope with water depth increasing from -1.8 m to -165.4 m from east to west. It can be divided into two relatively planar terraces and two steep slopes according to the water depth and slope gradient. The first terrace is approximately 46 m wide, with a mean depth of approximately -4.1 m and a mean gradient of approximately 1.4°; the first slope is approximately 35 m wide, with a mean depth of approximately -16.1 m and a mean gradient of approximately 28.8°. The second terrace is approximately 87 m wide, with a mean depth of approximately -26.7 m and a mean gradient of approximately 5.7°; the second slope is approximately 90 m wide, with a mean depth of approximately -63.3 m and a mean gradient of approximately 39.0°. The shipwreck is located on the second terrace of the reef slope. The shipwreck lies in EES-WWN orientation and is approximately 45 m long, 9 m wide, 4 m high, with a tonnage of approximately 500-600 tons. However, the shipwreck is no longer intact. The quantitative inversion and reconstruction of underwater landforms using side scan sonar images is an effective complement to the qualitative analysis of the images and can reveal the topographic and geomorphic characteristics of the coral reef in more comprehensive and detailed ways. In the future, the quantitative analysis method of side scan sonar imaging will have widespread applications in the investigation of topographic reconstructions, substrate types and coral community distributions, human activity footprint monitoring, and identification of special objects such as submerged reefs and shipwrecks.

CLC Number:

• P229.1