THE ARGUMENTATION OF THE PROPERTIES OF BOREHOLE SYSTEM AT LINXIA STATION, CHINA

2 years of four-component borehole strain data and well water level data are preprocessed to remove trends and outliers at Linxia Station from 2013 to 2014. The sum of observation strain values of two orthogonal measurement lines is equivalent to surface strain of borehole, and two thirds of near-ground surface strain is equal to volume strain. The volume strain can be obtained from four-component borehole strain observation, and the properties of the borehole system can be demonstrated from time and frequencies domain based on the comparison between volume strain and underground well water level observation. Results show that the volume strain and well water level is highly negatively correlated in time domain. The sensitivity of borehole system is −0.1620 mm/10⁻⁹. Enlarging the proportion of two coordinates of volume strain and well water level observation curves of selected two months in the two year period, we found that the peak and valley of water level curve corresponds to the valley and peak of volume strain one by one, and the phase lag is very small. In the frequency domain, the monthly tidal factor and the phase lag of diurnal and semidiurnal wave groups of volume strain and well water level were obtained with the Venedikov tidal harmonic analysis method, and the sensitivity and phase lag of the borehole strain system were calculated out subsequently. Results show that the sensitivities of most wave groups are not only close to each other, but also close to the sensitivity of annually periodic change obtained from the time domain, while the errors of phase lag are large. Considering that the phase lag obtained by arctangent is easily influenced by calculation error, the phase lag close to zero from time domain should be better in reliability. The analysis in time and frequency domain showed that the response of well water level to volume strain is substantially linear time-invariant. The borehole system of Linxia station basically meets the superposition, homogeneity and time invariance. Therefore, the borehole system at Linxia station is a linear time-invariant system basically.