A PROBLEM AND EXPLANATION FOR BOREHOLE STRAIN METER RECORDS OF CO-SEISMIC STRAIN STEPS
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摘要: 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。Abstract: Observation data from 4-compnoent borehole strain meters can be self-checked by ΔεⅠ+ΔεⅢ=ΔεⅡ+ΔεⅣ. However, when strain meters record the co-seismic strain steps, the ΔεⅠ+ΔεⅢ is not often equal to the ΔεⅡ+ΔεⅣ, 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 ΔεⅠ+ΔεⅢ and ΔεⅡ+ΔεⅣ, 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.
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Key words:
- finite element method /
- wounded element /
- kill element /
- co-seismic strain steps
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图 2 不同台站不同频率范围记录的应变观测曲线
Figure 2. Observation curve at different station and different frequency domain
图 4 0.5 MPa围压无扰动状态下的应力场
Figure 4. Stress field in 0.5 MPa confining pressure with no disturbance
图 5 不同数量的单元滑动引起的1+3和2+4的误差曲线
(a)到(r)依次是：无滑动、4.11%滑动、8.56%、11.44%、20.44%、24.33%、26.33%、29.11%、30.78%、43.00%、47.56%、51.11%、64.44%、70.89%、78.56%、82.89%、100%。角度定义从模型中间轴线右侧开始为0°，逆时针旋转
Figure 5. Difference curve between1+3 and 2+4 caused by different amount elements' shear slip
图 6 0.5 MPa围压加载0.2 MPa扰动后的应力场
Figure 6. Stress field in 0.5 MPa confining pressure with 0.2 MPa disturbance
图 7 不同扰动下发生滑动的区域(红色部分为滑动区)
Figure 7. Different slip area resulted from different disturbance
图 8 0.1 MPa围压加载0.2 MPa扰动下的应力场
Figure 8. Stress field in 0.1 MPa confining pressure with 0.2 MPa disturbance
图 9 0.1 MPa围压下加载不同扰动时剪切滑动和张裂的区域
Figure 9. Shear slip and tension fractured area caused by different disturbance in 0.1MPa confining pressure
图 10 不同扰动下的1+3和2+4应变曲线
Figure 10. Strain curve of 1+3 and 2+4 resulted from different disturbance
表 1 各介质材料参数
Table 1. Material parameters
材料 弹性模量/Pa 泊松比 弹性金属筒 2.1×1011 0.31 水泥 2.0×1010 0.30 围岩 3.0×1010 0.25 
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表 2 不同扰动值造成不同大小的滑动区域
Table 2. Different slip area resulted from different disturbance
扰动/MPa 滑动区域 ±0.100 无 ±0.130 无 ±0.132 无 ±0.133 4.11% ±0.134 8.56% ±0.135 11.44% ±0.140 20.44% ±0.143 24.33% ±0.145 26.33% ±0.148 29.11% ±0.150 30.78% ±0.170 43.00% ±0.180 47.56% ±0.190 51.11% ±0.200 53.78% ±0.250 64.44% ±0.300 70.89% ±0.350 75.33% ±0.400 78.56% ±0.500 82.89%
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