海底热流长期观测系统研制进展

浅海和俯冲海沟等海域，不仅是矿产和油气资源主潜力区，也是构造地震频发区，其浅表热流和深部温度信息对于了解板块俯冲和岩浆活动等过程至关重要.这些区域浅层地温场和热流场受到底水温度波动（BTV）强烈扰动，其背景热流需由长期观测来获取.在全面分析了国内外海底热流长期观测技术特点后，我们提出了系缆式海底热流长期观测方案，2013年起陆续开展了部分核心技术的预研究及一系列海底、湖底及浅孔试验.结果表明：（1）自主研制的长周期低功耗微型测温单元，在2~36℃的环境下可连续观测1年；系缆式投放与回收方案即使在地形陡峭、1.5 kn流速及无动力定位等条件下仍然可行.（2）南海北部BTV总体随水深变浅而增强，在浅水区对浅层地温场扰动不可忽略.例如，在水深2600~3200 m和850~1200 m海域分别为0.025~0.053℃（17天内）、0.182~0.417℃（2天内），而台西南盆地北坡（水深763 m）夏季的海底热流由浅表的0.69 W·m^-2转变为0.83 m以深的-0.25~-0.05 W·m^-2.（3）兴伊措和湖光岩玛珥湖BTV向深部传导过程中其幅度逐渐减弱、相位滞后，进而导致热流方向与强度随季节发生变化.而康定中谷浅层（7 m内）地温在不同深度处同步波动，且冬高（35~36℃）夏低（28~32℃）.推测为夏季大量降雨所致；其热流浅部低（0.504 W·m^-2）深部高（0.901 W·m^-2），指示着鲜水河断裂带深部热流体上涌.这些预研究工作为后续系缆式海底热流长期观测系统的正式研制与应用奠定了扎实基础.

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 2~36℃. 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 0.025~0.053℃ during 17 days in Dongsha waters with a depth of 2600~3200 m but up to 0.182~0.417℃ within 2 days in Xisha waters with a depth of 850~1200 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~-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 35~36℃ in winter but low to 28~32℃ 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 3~5 m, and rises to 0.901 W·m-2 at the depth of 5~7 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.