Analyses on the consistency between well water level and bulk strain in Fujian Province

The observation data of the bulk strain and underground well water level at Fuzhou and Yong'an seismic stations were investigated in both time domain and frequency domain for the consistency analysis. Besides, the mechanical mechanism of the borehole system was explored so as to demonstrate the physical meaning of the phenomenon. The results indicate that the bulk strain is highly correlated with water level in the time domain. The correlation coefficient between the bulk strain and well water level at Fuzhou station is 0.7939, and the sensitivity of borehole system is 0.0558 mm/10^-9. The well water level shows consistent tendency with bulk strain at Fuzhou station. The correlation coefficient between the bulk strain and well water level at Yong'an station is -0.7543, and the sensitivity of borehole system is -0.0005 mm/10^-9. The peak and valley of water level change correspond uniformly to the peak and valley of bulk strain at Fuzhou station respectively; whereas variation of the water level has negative correlation with variation of the bulk strain at Yong'an station. The results imply that the water level responds synchronously to the bulk strain at the two stations and the phase lag is approximately zero. In the frequency domain, three months observation data were utilized to obtain the monthly group factor and phase lag of the semidiurnal and diurnal tidal waves by the Venedikov harmonic analysis method. From the sensitivity and phase lag of the borehole system, it is clear to see that the sensitivity of most wave groups are not only close to each other, but also close to the sensitivity of annually periodic change obtained in the time domain, whereas with large phase lag variances. Considering the uncertainties of the phase lag obtained by arctangent, the phase lag value close to zero in the time domain should be more reliable. All in all, the results from both time and frequency domains demonstrate that the observation of well water level and bulk strain are consistent at Fuzhou and Yong'an stations. The main reason lies in the confined aquifer borehole observation system of the two stations which in principle meets the physical requirements of the superposition, homogeneity and time invariance. Therefore, it is concluded that the borehole system at Fuzhou and Yong'an stations is linearly invariant in time.