With the gradual depletion of non-renewable energy sources such as coal and oil on the earth, natural gas hydrates, which are abundant in marine sediments, are considered to be a clean strategic replacement energy in the future. In this paper, a set of in-situ high-precision terrain deformation data acquisition system for seabed is designed, and the system can also be used for monitoring and early warning of geological disasters in active areas of seabed geological plates. The system can effectively monitor seabed terrain deformation in gas hydrate mining area, and provide data support for seabed safety assessment in gas hydrate mining process.
Natural gas hydrate, also known as combustible ice, is a solid substance formed by methane gas and water dissolved in sediment pores at a certain low temperature and high pressure. Its reserves on the earth are considerable, which is about twice as large as those of oil, natural gas and coal. Especially in the ocean, it has been detected and found in more than 80 oceans around the world so far (Kvenvolden, 2001). However, the exploitation of natural gas hydrate in the ocean is faced with severe security problems, because the decomposition of natural gas hydrate will greatly disturb the structure of sediments containing natural gas and water, and greatly reduce the strength of sediments, which is extremely likely to cause large-scale landslides, earthquakes and serious instability and destruction of marine structures on the seabed (Locat, 2002; Sultan, 2004).
At present, the seabed terrain subsidence monitoring technology includes satellite remote sensing monitoring, multi-beam sonar, side-scan sonar, and the detection method combining global positioning and navigation system detection system. Satellite remote sensing monitoring, multi-beam sonar and side scan sonar have the advantages of all-weather, large monitoring range and easy processing of image data. They can be used for real-time, continuous quantitative and fixed-point monitoring. The disadvantages are low resolution and high cost of long-term observation, which are not suitable for underwater terrain monitoring. The detection method combining global positioning and navigation system detection system uses the sensor array to send a wide range of sound waves to the seabed, and then uses the receiving sensor array to receive narrow beam sound waves. The advantage is that the operation is relatively simple and can accurately describe the three-dimensional characteristics of seabed terrain (Jreng, 2012), but it needs to be carried by scientific research vessels, ROVs or AUVs, which is difficult to achieve real-time, long-term and in-situ monitoring. In view of the topographic survey in the test area of gas hydrate in the South China Sea, some scholars have proposed topographic subsidence measurement schemes, including gravity acceleration measurement system and water pressure measurement system (Yokoyama.2012; Woolsey, 2006). The gravity acceleration measurement system is used to monitor the seabed settlement before and after the hydrate test. The measured acceleration value is used for the secondary integration to obtain the seabed settlement. In the water pressure detection system, the pressure sensor is installed on the seabed surface, and the measured pressure directly reflects the depth of the water body. Both gravity acceleration measurement system and water pressure measurement system are only single point measurement. At present, there is still a lack of an in-situ and long-term surface morphology monitoring system that can well monitor the topographic subsidence of the hydrate test mining area.
