Abstract:

Geohazards such as submarine landslides and turbidity currents are an urgent safety issue to be solved in marine engineering during national deep sea development. To avoid such risks and achieve geohazard monitory and early warning, we develop an in-situ monitory system for complex deepwater engineering geology (SEEGeo), using resistivity, acoustic and pore pressure monitoring to monitor the physical and mechanical changes in deep seabed sediments. This monitoring system mainly includes the seabed carrying platform, monitoring system, communication control system, and power supply system. The monitoring system performs long-term in-situ monitoring of resistivity, acoustic and pore pressure changes in seabed sediments; the communication control system is responsible for communication and data transmission from seabed to sea surface then to land; and the power supply system provides power for one year of long-term operation on the seabed via an unique seawater battery process. So far, SEEGeo has completed the offshore test and carried out multiple sea trials in the South China Sea on board scientific research ships (e.g., “Marine Geology No. 6”, “Dongfanghong No. 3”, “Zhang Jian”), and collected large amounts of survey data. For example, using Wenner method, resistivity monitoring obtained an average resistivity of 0.207 Ω·m at the water-soil interface location. Super excess pore pressure monitoring, using open-structured optical fiber-based differential pressure sensing, observed four landmark stages: 1) pressure accumulation during penetration (up to 34.942 kPa over 0.182 h); 2) pressure decay after penetration (down to 9.973 kPa over 0.810 h); 3) real-time response to environmental stress (pressure change between 8.327-14.384 kPa); and 4) residual pressure (average 11.150 kPa). And acoustic monitoring, adopting two sets of one transmit-three receive setup, obtained a seawater average sound velocity of 1 533 m/s, as well as average sound velocities of 1 586, 1 587, 1 784, 1 735 and 1 831 m/s in seafloor sediments from the top to the bottom layers. The successful development of SEEGeo will significantly improve the current technical capabilities for long-term in-situ marine engineering geology monitoring, complex engineering geology evaluation and geohazard monitoring and early warning.

Key words: natural submarine geohazards, physical and mechanical properties, in-situ monitoring, sound speed, resistivity, pore pressure