钻孔应变观测同震阶变时的一个问题及对策分析

A PROBLEM AND EXPLANATION FOR BOREHOLE STRAIN METER RECORDS OF CO-SEISMIC STRAIN STEPS

针对分量式钻孔仪记录同震应变阶跃时常出现的ΔεⅠ+ΔεⅢ与ΔεⅡ+ΔεⅣ观测值不等,无法自检,且观测到的应变/应力阶跃大于理论计算值等现象,利用有限元方法计算分析可能的原因,建立不同围压和地震波应力扰动条件下的多种模型,运用各向同性"杀死单元"和横向各向同性"杀伤单元"法,模拟分量式钻孔仪的金属外壁和固结水泥环之间产生张裂隙或剪切滑动的情况。计算结果显示,若钻孔内的金属圆筒和水泥环之间发生剪切滑动,基本不会影响到观测;然而两者之间发生张裂会使得ΔεⅠ+ΔεⅢ与ΔεⅡ+ΔεⅣ出现较大差异,严重影响到同震阶变的观测。今后在井下安置探头时应该尝试新的改进方法。

Observation data from 4-compnoent borehole strain meters can be self-checked by Δε^Ⅰ＋Δε^Ⅲ= Δε^Ⅱ＋ Δε^Ⅳ. However,when strain meters record the co-seismic strain steps,the ΔεⅠ＋ΔεⅢis not often equal to the Δε^Ⅱ＋ Δε^Ⅳ,and observed values are greater than the theory solution.In this paper,the possible reasons were analyzed and several models with different initial confining stress and different stress disturbance caused by earthquake wave were established,with the finite element method of ＂kill element ＂and transverse isotropic ＂ wounded element ＂to simulate the tension fracture or shear slip occur between metal cylinder and the cement sheath,and discuss which one is the real reason. The results indicate that tension fracture between the metal cylinder and the cement sheath account for the difference of Δε^Ⅰ＋ Δε^Ⅲand Δε^Ⅱ＋ Δε^Ⅳ,and affect the co-seismic strain steps observation. Therefore, some new improved methods are needed when arranging borehole probe in the drilling-hole in order to reduce the observation error in future.