Abstract: According to observation specifications of geothermal wells, the water temperature sensors should be placed as deep as possible. Generally, the depth is more than 100 m and less than 200 m. However, it is observed that the placement depth of water temperature sensors has a significant effect on the dynamic characteristics of water temperature observation. In most observation wells, the dynamic characteristics between water temperature and water level in the same well, such as time dynamics, tidal effects, co-seismic responses and post-earthquake step-changes, show high correlations or consistencies, which is usually relevant to the placement depth of water temperature sensors. In this paper, the dynamic characteristics of water temperature affected by sensor placement depth is discussed, when the water temperature sensor is placed in the observation aquifer, it is affected by the flow movement between the well and the aquifer, which will cause the water temperature changes with the change of the well water level; when placed under the observation aquifer, the water temperature is not affected by the change of well water level, and most of the dynamic is relatively stable. This is mainly because the formation of water temperature dynamics is controlled by two different mechanisms, hydrothermal dynamics and geothermal dynamics, and the conditions of the well-aquifer system also play a role.

Key words: Water temperature, Dynamic characteristics, Sensor placement depth, Effect mechanism