ABSTRACT:

In-situ measurement of subsurface temperature (temperature prospecting) has been developed as a method of groundwater-vein surveying. However, the usual method is labour intensive and the temporal resolution is poor, so we could never obtain sequential data of the temperature distribution. Continuous observation of the temperature distribution (i.e. distribution of the preferential flow-path) using fibre-optic temperature sensor is reported in this paper. High temperature regions indicate the groundwater pathway. The intensity of its signature was found to show slight fluctuations. In addition to that, it was found that these high temperature regions also have relatively large vertical temperature gradients. These signatures may be associated with the fluctuations of the preferential flow-path of groundwater, indicate the dynamic state of groundwater flow, and provide new parameters for the temperature prospecting method.

INTRODUCTION

The existence of preferential flow-paths for groundwater is an important factor which influences form of groundwater flow. It could be not only an ideal place for the water resource, but also a cause of disaster such as road and river bank failure. Therefore, having an accurate grasp of the preferential flow-path, such as distribution, scale and dynamics of preferential flow-path is important to prevent natural hazards and to develop water resources. In-situ measurement of subsurface temperature (temperature prospecting) has already been used in groundwater surveying (e.g., Cartwright, 1968a; Krcmar and Masin, 1970; Takeuchi and Akibayashi, 1977). This method is based on the concept that the subsurface temperature is affected by flux and temperature of the groundwater flowing through the measurement point. This method has been established both empirically (e.g., Cartwright, 1968b; Yamamoto et aI., 1994; Yasuda and Takeuchi, 1998) and theoretically. However, the conventional method is labour intensive, and has poor temporal resolution. Thus, until now, the method only indicates the static distribution (i.e. a snap-shot) of the preferential flow-path.

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