10:45 AM - 12:15 PM
[HRE12-P04] Verification of the existence of vertical groundwater flow in a borehole using a heating flow meter
Keywords:Futagawa fault drilling, Vertical groudwater flow, Heating flow meter
Repeated measurements and long-term observation of temperature distribution and groundwater level in a borehole (FDB borehole) that penetrated the Futagawa fault, which triggered the mainshock of the 2016 Kumamoto earthquake, were carried out continuously. Temperature profiles with high reproductivity were obtained at depths of approximately 100 to 650 m. The temperature distribution was almost constant (geothermal gradient of about 1℃/km) between 310 m and 430 m depth (approximately 120 m long), which is much lower than the average geothermal gradient outside this depth interval (approximately 50℃/km) in this borehole. This extremely low geothermal gradient of 1℃/km in the 120 m interval is unprecedented and unique worldwide. Furthermore, thermal conductivity measurements of rock core samples collected from this borehole showed that the temperature distribution could not be explained as a phenomenon due to thermal conduction. Then it was inferred that this unique temperature distribution was related to the vertical groundwater flow in the borehole.
A heating flow meter was invented to test this conjecture, and experiments were carried out in this borehole. The heating flow meter consists of a heater and several thermometers above and below it fixed to a rope. After this flow meter is lowered to a specific depth, the heater heats a column of water and causes temperature increase, and thermometers distributed above and below the heater measure the vertical temperature profile with time.
Experiments in the depth range where the extremely low geothermal gradient was observed showed that the heating caused temperature increase recorded by thermometers below the heater, which varied regularly with the distance from the heater. Furthermore, the duration of the temperature rise is almost the same as the heating time. In contrast, the thermometers above the heater recorded no significant temperature variations. This proved the existence of the downward vertical groundwater flow in this depth interval. Experiments conducted at other depth intervals showed the opposite results. Only thermometers above the heater recorded regular increases in temperature with the distance from the heater, while the lower thermometer recorded no significant temperature changes. In addition, the duration of the temperature rise was longer than the heating time, which is considered heat transfer or diffusion. This demonstrated that there is no significant vertical groundwater flow in these depths.
This heating flow meter showed utterly different patterns of temperature rise between the depths considered to have vertical groundwater flow and those without. This verified the existence of downward vertical groundwater flow in the depths where the extremely low geothermal gradient was discovered. It is also planned to study this groundwater flow quantitatively.
A heating flow meter was invented to test this conjecture, and experiments were carried out in this borehole. The heating flow meter consists of a heater and several thermometers above and below it fixed to a rope. After this flow meter is lowered to a specific depth, the heater heats a column of water and causes temperature increase, and thermometers distributed above and below the heater measure the vertical temperature profile with time.
Experiments in the depth range where the extremely low geothermal gradient was observed showed that the heating caused temperature increase recorded by thermometers below the heater, which varied regularly with the distance from the heater. Furthermore, the duration of the temperature rise is almost the same as the heating time. In contrast, the thermometers above the heater recorded no significant temperature variations. This proved the existence of the downward vertical groundwater flow in this depth interval. Experiments conducted at other depth intervals showed the opposite results. Only thermometers above the heater recorded regular increases in temperature with the distance from the heater, while the lower thermometer recorded no significant temperature changes. In addition, the duration of the temperature rise was longer than the heating time, which is considered heat transfer or diffusion. This demonstrated that there is no significant vertical groundwater flow in these depths.
This heating flow meter showed utterly different patterns of temperature rise between the depths considered to have vertical groundwater flow and those without. This verified the existence of downward vertical groundwater flow in the depths where the extremely low geothermal gradient was discovered. It is also planned to study this groundwater flow quantitatively.