不同充填介质下的溶洞跨孔电阻率CT探查数值模拟

Study on numerical simulation of karst cross-hole resistivity CT exploration at cave with different filling media

城市地铁修建过程中常常会遇到溶洞等不良地质条件,为更加准确探查地下溶洞的位置和大小,采用有限元法和最小二乘法正反演数值模拟手段,利用孔距、电极距和与钻孔距离三个变量构建了地下溶洞的地电模型,分析并总结了跨孔电阻率CT法对充气、充水和部分充水溶洞的电阻率响应特征及规律。结果表明:部分充水溶洞的水、气分界面明显,低阻区域和高阻区域与溶洞充水、充气部分位置大小一致,且随着溶洞充水量的增加,溶洞低电阻率响应增强,其异常范围向溶洞顶部扩大;跨孔电阻率CT法能够有效识别溶洞充填性状。

Unfavorable geological conditions such as karst cave often occur in the construction process of city subway and other railways, which seriously affect the construction progress and the staff’ssafety. Therefore, in order to guaran tee the normal construction, it is necessary to accurately detect the development features such as the location, size and filling properties of underground hidden karst caves. In this paper, the methods of forward and inversion numerical sim ulation on the basis of both finite element code and least square were used to construct the geo-electric model, by using three variables of underground caves, such as pitch, electrode spacing and relative hole spacing. Meanwhile, it shows the simulation results in a way of combining two-dimensional section with one-dimensional sounding curve, analyzes and summarizes the response characteristics and cross-hole resistivity CT for gas filled, water filled and partially water filled karst caves. The results indicate that the interface between water and gas in some water-filled karst caves is obvi ous, and the positions of the low-resistivity area and the high-resistivity area are the same as those of the water-filled and gas-filled parts of the karst caves. In addition, with the increase of filling water volume in the karst caves, the low- resistivity response of the karst caves increases, and the range of low-resistivity anomalies expands towards the top of the karst caves. The resistivity response characteristics of karst caves gradually decrease with the increase of electrode spacing and hole spacing, and the response attenuation of the hole spacing from 6 m to 16 m decreases more obviously than that from 16 m to 25 m. The response to electrode distance from 1 m to 2 m decreases less obviously than that from 0.5 m to 1 m. Meanwhile, the response of partially water filled karst caves decreases more obviously than that of single filled karst caves. When the electrode spacing and hole spacing increase to a certain range, the detection meth od can’teffectively identify complex karst caves and single filled karst caves;In the process that the karst cave model gradually deviates from the central position of the measured section, the response of the abnormal area weakens when it deviates up and down in the depth direction, and increases when it deviates towards left and right in the transverse direction. Various numerical models indicate that, when the ratio of hole spacing to hole depth and electrode spacing is appropriate, the cross-hole resistivity CT method can effectively identify the filling characteristics of the karst caves, and accurately determine the location and size of the karst caves, which provides a theoretical basis for the cross-hole exploration site and the interpretation of results.