Calculation of borehole coupling coefficient based on borehole volume strain and area strain observation

Obvious borehole coupling effect exists in the borehole strain observation system (rock, expansive cement and strainmeter steel tube).The real strain of rock in the crust can be obtained by determinng the coupling coefficient, so that comparativity of the data measured at the different points can be improved. According to the double bushing theory and elastic mechanics model, this paper establishes the mechanical models of volume strain and area strain observation under the action of three-dimensional stress. Then we deduced the borehole coupling coefficient formula at different observatories, from which we found that the coefficient was closely related to stress in the holes. The different strain signals can be correspondently attributed to different sources. That is to say, the stress ratio (the ratio of axial stress of drilling to plane stress) is different, the coupling coefficient is not the same. The volume strain decreased with the stress ratio increasing, and area strain increased. In addition, the coupling coefficients under plane stress had nothing to do with the additional stress, but only with the observation system as shown by the constant measurements. Finally, the affecting factors were analyzed. The results show that borehole coupling coefficient of volume strain and area strain increased with the rock elastic modulus and Poisson’s coefficient increasing, and the change in the former is larger, effect of borehole coupling material of cement on the two quantities is small.