Multi-physical distributed fiber optic observation in a 3211-m-deep scientific borehole at Jiajika lithium mine, western Sichuan

As the first scientific borehole of lithium deposits in the world and the deepest one on the Tibetan Plateau, the scientific borehole at the Jiajika lithium mine in western Sichuan Province is located near the Xianshuihe fault and the Sichuan-Tibet railway in the eastern margin of the Tibetan Plateau. The drilling project started on June 3, 2020. and was completed on January 22, 2021, with a final borehole depth of 3211.21 m. Observation of the deep physical fields and shallow environmental changes in this deep borehole is of great significance for investigation of the crustal activity in the Tibetan Plateau. the site stability evaluation during construction of the Sichuan-Tibet railway, and ecological environment protection. Based on the distributed fiber optic sensing (DFOS) technique, we independently developed special sensing fiber optic cables with high-strength and high-temperature resistance, which were successfully installed in the 3211-mdeep scientific borehole. Hence the Jiajika distributed fiber optic observation borehole has the highest altitude and the largest depth in the world. This borehole enables the distributed in-situ observation of multi-physical parameters, such as strain_ temperatures. ground vibration and moisture. This paper presents the preliminary results in the first observation stage, including in-situ measurement of thermal conductivity of rocks, distributed acoustic sensing (DAS) of the borehole backfilling process, earthquake activity and seismic imaging. The distributed heating and temperature sensing system reveals that the thermal conductivity of rock mass varies greatly with depth due to the existence of fissure water and pegmatite dikes. The DAS signals can detect the friction vibration between sands and the sensing optical cable during the borehole backfilling, which enabled the evaluation of the backfill depth and quality. In addition, the DAS signals can effectively distinguish natural earthquakes from artificial blasting, providing a new method for remote monitoring of seismic risk of active fault zones and rock bursts. Our in-situ observation data also provide important depth constraints on high-resolution seismic imaging for mining exploitation. This study verified the feasibility of DFOS technique to the full-section multi-physical parameter observation in deep boreholes. This technique can be applied under high temperature and high pressure conditions in a deep borehole, showing its unique advantages of long-distance. real-time, continuous and distributed observation. Integrating the distributed strain sensing, distributed temperature sensing, and distributed acoustic sensing (3Ds) fibers into one fiber optic cable ensures the installation quality of fiber optic cables in deep boreholes. This design allows wide application of the DFOS technique to multi-physical field monitoring in geophysics and geological engineering.