Development progress of long-term seafloor heat flow monitoring system

The shallow seas and subduction trenches are not only the main potential areas for mineral and hydrocarbon resource, but also areas of frequent tectonic earthquakes. The shallow heat flow and deep temperature distribution are crucial for understanding the process of plate subduction and magma activity. In these areas, the shallow temperature and heat flow fields are strongly disturbed by the bottom water temperature variation (BTV). Thus, its background heat flow needs to be obtained by long-term observation. After a comprehensive analysis of the technical characteristics of the existing long-term seafloor heat flow observation techniques, we proposed a scheme for tethered long-term seafloor heat flow monitoring system (TLHF), and since 2013, carried out a series of pre-developments and tests in the South China Sea, Xingycou Lake, Huguangyan Maar Lake, and Kangding shallow borehole in the Xianshuihe fault zone. The results show: (1) the self-developed long-period and low-power miniature temperature loggers can continuously work for one year in an environment of 2 similar to 36 degrees C. The tethering-type launch and recovery scheme is still feasible even under the conditions of steep terrain, 1.5 knots of velocity and without dynamic positioning. (2) In northern South China Sea, the BTV generally increases as the water depth becomes shallower, whose disturbance to the shallow temperature field cannot be ignored in shallow area. For example, the BTV is only 0.025 similar to 0.053 degrees C during 17 days in Dongsha waters with a depth of 2600 similar to 3200 m but up to 0.182 similar to 0.417 degrees C within 2 days in Xisha waters with a depth of 850 similar to 1200 m. In summer, the seafloor heat flow on the northern slope of the Taixinan Basin (with a water depth of 763 m) from 0.69 W . m(-2) at the shallow surface to -0.25 similar to-0.05 W . m(-2) at a depth of 0.83 m. (3) In Xingycou Lake and Huguangyan Maar Lake, the BTV amplitude gradually decreases and the phase lags during the process of conduction to the deep. That causes the intensity and direction of the heat flow to vary with change of seasons. In Zhonggu Village of Kangding City, the shallow ground temperature is high to 35 similar to 36 degrees C in winter but low to 28 similar to 32 degrees C in summer due to heavy rainfall in summer, and fluctuates synchronously at different depths. The surface heat flow is 0.504 W . m(-2) at the depth of m, and rises to 0.901 W . m(-2) at the depth of m. That indicates the upwelling of thermal fluid from the deep part of the Xianshuihe fault zone. These preliminary work has laid a solid foundation for the development and application of the TLHF system.