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Nakatani, Katsuya*; Matsuki, Koji*; Arai, Takashi*; Omura, Kazuo*; Takeuchi, Shinji; Arai, Yasushi; Horimoto, Seiki*
Journal of MMIJ, 123(1), p.17 - 25, 2007/01
Revising the method proposed by Vasco et al., we developed a new method for more accurately estimating groundwater flow by an inverse analysis of tilt data on the surface. The features of this method are that (1) a region (V) where groundwater flow occurs is divided into elements in which the volume change in groundwater per unit volume of rock (dv) and the Skempton coefficient B are assumed to vary in a linear or quadratic manner with the coordinates, that (2) the values of dv are set to zero at the boundaries of the region V and that (3) as constraining conditions which are weighed and added to a squared error in tilt, the sum of squared second derivatives of dv are used. We call the method using linear interpolation Inversion-1 and that using quadratic interpolation Inversion-2. First, analyses by these methods were conducted for two flow models of water injection to know the applicability of the methods. It was shown that both Inversion-1 and Inversion-2 can evaluate the volume change in groundwater much more accurately than the method by Vasco et al. when dv varies relatively gently with the distance from the injection point (Model 1). However, when dv varies steeply with the distance from the injection point (Model 2), either Inversion-1 or Inversion-2 cannot produce good results. This was considered to be caused by the fact that the measured data are much fewer than dv to be determined. Furthermore, the effect of the size of the region V on the estimation of dv was analyzed since it is usually difficult to accurately estimate the size of a region where water flow occurs. The results showed that the effect of the size of V is relatively small for both Inversion-1 and Inversion-2 unless the size of V is much smaller than the real one. Thus, it can be said that it is better to perform an analysis by Inversion-1 or Inversion-2 with a large region of V in the beginning.
Nakatani, Katsuya*; Matsuki, Koji*; Arai, Takashi*; Omura, Kazuo*; Takeuchi, Shinji; Arai, Yasushi; Horimoto, Seiki*
Journal of MMIJ, 123(1), p.26 - 32, 2007/01
Groundwater flow at the site of MIU (Mizunami Underground Research Laboratory) in the Tono district, Japan, was evaluated by a new inverse method using tilt data measured by four tiltmeters during excavation of shafts. The features of the method are that the fluid volume change per unit volume of rock (dv) is assumed to vary in a quadratic manner with the coordinates in elements within a region of groundwater flow, that the values of dv are set to zero at the boundaries of the region and that the sum of squared second derivatives of dv are adopted as constraining conditions which are weighed and added to a squared error in the tilt. An inverse analysis was performed by assuming three sizes for a whole region of groundwater flow. It was shown that there are three regions where the volume of groundwater decreases and two regions where the volume of groundwater increases regardless of the size of the whole region of groundwater flow and that groundwater came mainly from the sedimentary rocks lying between two faults. The latter is consistent with the previous prediction that these faults have a low permeability and may act as a flow barrier. Furthermore, the results of the inverse analysis predict that there may be other hydrogeological structures which prohibit groundwater flow. Thus, the inverse method proposed by the authors for evaluating groundwater flow from tilt data has proved to be reliable in spite of the assumption that the rock mass is a homogeneous and semi-infinite body.
