Determination Of Hydraulic Properties From Pore Pressure Fluctuation Measured At Multiple Depth Intervals

Fluctuation of water level induced by atmospheric loading has been observed in many wells and has been used to estimate hydraulic properties of the subsurface material. However, the methods of estimating hydraulic properties developed in previous researches have some problems concerned with the attenuation of atmospheric pressure in unsaturated zone and the too many unknown parameters relative to data available from the conventional water level measurement in a well. In this study, we introduce the concept of measuring pore pressure at multiple depth intervals in a well, to overcome these problems. This approach increases the number of the data available and enables us to estimate the specific storage and the hydraulic diffusivity separately from the data. Furthermore, we developed the theory based on the poroelasticity to estimate the hydraulic diffusivity in saturated zone unaffected by the attenuation in an unsaturated zone and the method of estimation by the careful arrangement of the instruments. Our method developed in this study has advantages over the previous methods with respect to the accuracy of estimation, because we can expand the data window for dimensionless depth and can avoid the attenuation in the unsaturated zone. We developed a new device for measuring pore pressure at multiple depth intervals in a single borehole to apply this method to an actual field. Using data obtained from this device, we demonstrate that the specific storage and the vertical hydraulic conductivity in Tertiary sedimentary rocks can be estimated both in fully saturated zone and saturated zone underlying an unsaturated layer. The results showed that the specific storage and the vertical hydraulic conductivity can be estimated within an order of magnitude error, which was well within the acceptable range, considering otherwise both are not easy to measure in situ. The method presented here seems to be useful to estimate hydraulic properties in site investigation especially at an early stage, because hydraulic properties can be estimated from only passive monitoring pore pressure, which is becoming popular in site investigation.