Development of automatic neutron moisture logging system for measurement of volumetric water content in unsaturated zone
2005
Ishida, S.(National Inst. for Rural Engineering, Tsukuba, Ibaraki(Japan)) | Tsuchihara, T. | Imaizumi, M.
The study of complex behaviors of infiltration water related to the groundwater recharge and contamination requires a new technology for measuring moisture with high precision in the unsaturated zone, where the water table is 5 m or more below the ground surface. The authors developed an automatic neutron moisture logging system using a low-speed winch that showed less error in the volumetric water content even if repeated measurements were taken. They carried out test measurements at two sites; one site in a diluvial upland area in Ibaraki prefecture and the other site for artificial-recharge test in Niigata prefecture where a terrace sand and gravel layer was distributed. The results from the Ibaraki prefecture site showed that the new method was clearly. able to measure the difference of the volumetric water content in the pre- and post-rainfall owing to the decreased error of repeatability for the measurement points compared with a conventional manually operated method. The permeability structure of the unsaturated zone in Niigata prefecture consists of an upper layer with relatively low permeability and a lower layer with a relatively high permeability. The new method was able to visually depict the downward movement of water in the unsaturated zone such as follows 1) The depth where volumetric water content increase between the measurement intervals moved downward with time. 2) The downward movement velocity of the depth where volumetric water content increase between the measurement intervals was related to the permeability structure of the unsaturated zone. The two test measurements proved the effectiveness of the new method.
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