Frequency response of the pore pressure wells - from tidal to seismic frequency -

Poroelastic theory states that when a porous aquifer is under complete undrained conditions, i.e. a porous medium is compressed or extended without allowing fluid to flow, the applied stress is always borne by pore fluid and skeletal framework of the rock. Thus the pore pressure must be a direct sensor of the crustal stress change. However, this has never been clearly observed, because the pore pressure of aquifer would often measure through the water level in an open well under the assumption that it is in equilibrium with the pore pressure. In open wells, changes in pore pressure must be accompanied with flow into or out of the wall, which definitely prevents the well from being a gage of pore pressure in an aquifer. Limited hydraulic communication between well and aquifer causes the delay and attenuation in frequency response. And the dimension and geometry of well also greatly affect the performance. We could overcome these shortcomings by stopping up a flowing well bore and directly measuring the fluid (pore) pressure, p within it, and we found that the fluid pressure of such closed wells show the first-order response from tidal to seismic frequencies. A site-investigation tunnel was excavated at 350 m depth from the gallery of the Kamioka mine across the Mozumi fault, a part of the Atotsugawa fault system in central Japan. We have monitored the fluid pressure using pressures gage (full scale is 2.069 MPa, resolution is 16 bit, and sampling rate is 20 Hz) at two well bores on both sides of the fault; C-well, the diameter is 140 mm, the length is 600 m, the inclination is -70°, and the flowing rate is 375 l/min; A-well, the diameter is 76 mm, the length is 15 m, the inclination is -5°, and the flowing rate is 25 l/min. We have also observed the barometric pressure, b in the tunnel. On stopping up a well bore, the pore pressure have gradually built up and attained the stable state in a half year (C-well; 1.43MPa, A-well; 0.99MPa). Tiny but clear fluctuations of pore pressure due to the earth tides were observed immediately after stopping up the well bore. Their amplitudes are C_O1= 52 Pa, C_M2=105 Pa, A_O1=24 Pa, A_M2=59 Pa at the equilibrium. There was no phase delay between two well-bores. The barometric efficiency, η ( = Δp / Δb ) could be properly determined; 0.46 for C-well, and 0.37 for A-well. Two hydrograms of the April 26, 2002 Mariana earthquake (M_W=7.1; the epicentral distance is 24.0°) were indistinguishable from each other, except for their amplitude(Δp_C/Δp_A=1.27). Besides the sameness of two waveforms, they are also significantly similar to the radial recording of STS-1 seismometer (velocity of ground motion) including shear and surface waves. These observations are confirmed that when the seismic (plane) waves are transmitted through the porous medium saturated with water, there is no relative movement between pore fluid and skeletal framework of the rock, and that the applied stress is always shared with fluid and skeletal framework of the rock at a constant ratio.