Effects of Meteorological and Hydrological Changes on Ground-Strain Measurements

Abstract

Using continuous records obtained from four extensometers with laser interferometer systems at the Amagase Observatory, effects of meteorological and hydrological changes on ground-strain measurements have been investigated. Seasonal variations with a strain amplitude of the order of 10^-6 have been observed with horizontal and vertical componetns in directions normal to the axis of the observation tunnel over a period of thirteen months. On the other hand, seasonal variations observed with horizontal components along the tunnel are smaller than 1×10^-7. As a possible source of these variations, we consider the hydrological perturbation due to the seasonal variation of the groundwater pressure around the observation tunnel. After removing seasonal variations from observed data of ground-strains, there remain transient variations of the order of 10^-7 in components normal to the tunnel. While, variations of this sort are not found in components along the tunnel. These transient variations cannot be explained by the elastic strains caused by the negative surface-load due to the passage of the typhoon or due to the fluctuation of the water level in a nearby reservoir, but can be explained by the hydrological effect of groundwater pressures in pores of a porous medium under the rapid draw-up or draw-down of the water level of the reservoir.