Wavelet decomposition is used to analyze barometric fluctuation and earth tidal response in borehole water level changes. We apply wavelet analysis method to the decomposition of barometric fluctuation and earth tidal response into several temporal series in different frequency ranges. Barometric and tidal coefficients in different frequency ranges are computed with least squares method to remove barometric and tidal response. Comparing this method with general linear regression analysis method, we find wavelet analysis method can efficiently remove barometric and earth tidal response in borehole water level. Wavelet analysis method is based on wave theory and vibration theories. It not only considers the frequency characteristic of the observed data but also the temporal characteristic, and it can get barometric and tidal coefficients in different frequency ranges. This method has definite physical meaning.
Based on isotropic linear poroelastic theory and under the undrained condition,we summarize three equations connecting the Skempton’s coefficient B with the groundwater level.After analysis,we propose a method to calculate the Skempton’s coefficient B according to the relationship between water level and tidal strain.With this method we can get the value of B without the earthquake occurrence,which can provide the high frequency waves for research.Besides,we can also get the in-suit Skempton’s coefficient B without the experiment of rock physics.In addition,we analyze the observed data of Changping station recorded in groundwater monitoring network(abv.,GMN)before and after the Wenchuan MS8.0 with this method,and find out there’s a slight change of the value of B after the seismic waves passed by,which implies that the propagation of seismic waves may have brought some variations to the poroelastic medium of the well.
The Kunlunshan Mountain Ms8.1 earthquake, occurred in Nov.14, 2001, is the first event with magnitude more than 8 in the China earthquake monitoring history, specifically at the beginning of digital techniques in precursor monitoring networks. Any investigation of recorded data on this earthquake is very important for testing the operation of the digital monitoring networks and understanding the preparation, occurrence, and adjustment of stress/strain of strong continental earthquakes. In this paper we investigated the coseismic response changes of well water level of groundwater and volume strain meter of bore hole in digital earthquake monitoring network of Capital area and its vicinity, due to the Nov.14, 2001 Ms8.1 Kunlun Mountain earthquake. The responding time, shapes or manners, amplitudes, and lasting time of well water level and strain-meters to seismic wave are studied in comparison. Then we discussed the possibility that the response changes of groundwater to strong distant earthquakes can be understood as one kind of observing evidence of stress/strain changes induced by distant earthquake.
