3/18AM (General session) Rock Engineering / Mining techn

[2K0101-08-04] Application of Fibre Optic Distributed Temperature Sensing to Detect Vertical Groundwater Flow in a Borehole

○Shuai Feng^{1 [Doctoral course]}, Weiren Lin¹, Susumu Shibutani² (1. Kyoto University, 2. Chi-ken Sogo Consultants Co., Ltd.)

Chairperson: Akihiro Hamanaka (Kyushu University)

Keywords:Fibre Optic Distributed Temperature Sensing, Borehole, Active Heating Test, Groundwater Flow

Fibre optic distributed temperature sensing (FO-DTS) gained rapid adoption in various applications following its development due to its distinctive capability to consistently gather temperature data along the fibre in both spatial and temporal dimensions. FO-DTS system functions by generating a laser pulse through an optical fibre and the fibre temperature is determined using the ratio of temperature-independent Raman backscatter (Stokes) to temperature-dependent backscatter (anti-Stokes) of the incident laser pulse. In this research, FO-DTS system was combined with line source electrical resistance heating to conduct active heating test to detect vertical groundwater flow in a borehole named FDB which is hypothesised to be the cause of the peculiar temperature distribution in this borehole. The temperature was almost constant between 310 m and 430 m depth (approximately 120 m long) in borehole FDB with thermal gradient of about 1℃/km, which is much lower than the average geothermal gradient outside this depth interval (approximately 50℃/km). FO-DTS system was placed from wellhead to a depth of 540 m, while line source electrical resistance heating was distributed from 240 m to 540 m depth. After eliminating the interference of the wire on the temperature, it can be found that in the depth interval from 310m to 430m, it is more difficult to be heated than other depths during the heating process. And after the heating stops, temperature of this depth interval quickly returns to the pre-heating level, while the other depth intervals take much longer time. These different patterns of temperature change during heating and cooling prove that there is a vertical groundwater flow in the depth interval from 310 m to 430 m, and it is the continuous flow of new water through this interval that causes the temperature in this zone to rise slowly during heating and to fall rapidly after the heating is stopped.