Significance of Pore Pressure Monitoring in Monitoring of Crustal Stress and Strain

Abstract

Pore pressure of the rock mass is successfully monitored at higher frequencies (0.1 Hz ~ 2 Hz) using borehole. The frequency response of monitoring of pore pressure is significantly improved by closing the well, which reduces a high-cut response caused by wellbore storage effect in open wells. The response of pore pressure for stress or strain change is represented by linear poroelastic theory that describe mechanical coupling between rock mass and pore fluid. We revealed that even in crystalline rock the pore pressure change induced by deformation such as barometric pressure change, earth tides, and seismic waves is in agreement with theoretical model derived from the linear poroelastic theory. Pore pressure can be a proxy of stress and/or strain as studies on such as crustal deformation that have been made. It should be noted, however, that the applicable range of frequency is usually limited and absolute value of stress is difficult to be determined from pore pressure monitoring. Since amplitude and phase of pore pressure change induced by deformation depends on frequency, the sensitivity of pore pressure as a proxy of stress differs with frequency. The sensitivity may vary with time during a cycle of earthquake. The response to seismic waves follows prediction from poroelastic theory for both large and intermediate earthquakes. The sensitivity is robust at least for the amplitude of deformation induced by seismic waves. The response of pore pressure to tidal strain varies before and after large earthquake is observed, which may be attributed to apparent change of the sensitivity caused by change in diffusivity.