Frequency characteristics of the response of water pressure in closed well to volumetric strain in high frequency domain

The expression for the response of water pressure in closed well to crustal strain is newly developed. It is shown that the response of water pressure in closed well is not the same as the undrained response of poroelastic rock and has characteristics different from the response of water level of open well. The oscillations of water pressures and crustal strains due to the seismic wave of the 2010 Chile earthquake were observed. The oscillations of water pressures were basically negative-proportional to the oscillations of volumetric strains and the signals of water pressures existed in the frequency range from 0.002 to 0.1 Hz. In comparison with the volumetric strains, it is found that the responses of water pressures in closed wells are frequency-dependent. The new expression explains the frequency characteristics of the responses. The response is useful for the estimation of rock property. In addition, it is expected that the expression is applied to the study of phenomena caused by the pore water pressure in deep stratum which is difficult to observe directly.