基于Gassmann方程的流体替换方法在珊溪水库地震研究中的应用

引入基于Gassmann方程的流体替换方法,在分析地震波P波速度、波速比与岩石孔隙度和饱和度关系的基础上,应用于珊溪水库地震波速比和P波速度变化特征研究,得到:(1)珊溪水库震中区岩石始终处于接近水饱和的饱水状态,波速比和P波速度"下降-回升"的变化实质上反映了震中区岩石"孔隙度增大(饱和度减小)-饱和度增大"的变化,每一丛地震的波速比由极小值逐渐增大为极大值是由于岩石从不饱和状态变化到饱和状态;(2)根据每一丛地震波速比的变化,计算得到珊溪水库流体扩散率αs=1.06×104 cm2 s-1,该数值与美国南卡罗莱纳水库、巴西Acu水库、广东新丰江水库的流体扩散率基本一致;(3)震源区岩石孔隙度上限值为8.7%~2.0%,该数值与华东勘测设计研究院通过室内岩石物理力学性质试验测定的珊溪水库坝址区新鲜流纹斑岩的孔隙度平均值一致。

Application of Gassmann Equation-based Fluid Substitution Method to the Research of Reservoir-induced Earthquakes at Shanxi Reservoir

By analyzing the relationship between seismic P-wave velocity, velocity ratio, and rock porosity and saturation in the Shanxi reservoir, the fluid substitution equation method based on the Gassmann equation was used to estimate the rock porosity and fluid diffusion rate in the reservoir''s earthquake source region. The results show that the rock of the epicentral area is always close to the water-saturated state. The changes in P-wave velocity and the velocity ratio from decreasing to increasing reflect that the rock in the epicentral area changes from increasing porosity (decreasing saturation) to increasing saturation. The velocity ratio gradually increases from the minimum value to the maximum value because of the changes in rock conditions from unsaturated to saturated. Moreover, the fluid diffusivity of the Shanxi reservoir is 1.44×10⁴ cm² s^-1 to 0.84×10⁴ cm² s^-1, which is consistent with that of the South Carolina reservoir in the United States, the Acu reservoir in Brazil, and the Xinfengjiang reservoir in Guangdong, China. Further, the maximum rock porosity in the source area is 8.7%~2.0%, which is consistent with the average value of porosity of new rhyolite porphyries in the Shanxi reservoir dam site measured by the East China Investigation and Design Institute by lab testing of the physical and mechanical properties of the rock.