岩石破裂过程中电阻率变化机理的探讨

受压岩石在破裂过程中,视电阻率会出现明显的变化,其异常形态与实验条件有很大的关系. 实验中对磁铁石英岩样品施加了单轴压缩,岩石的破裂经反复加载和卸载实现,并在岩样的裂隙中注入了食盐溶液. 在实验的各阶段,对样品的主剖面重建出一系列内部真电阻率分布的图像,揭示了介质内的微细结构,从而探讨了视电阻率变化的原因. 实验发现,岩石中裂隙的存在及所含液体的饱和状态,是岩石在主破裂前控制电阻率变化的两个最重要的因素;低应力状态属常态导电过程,孔隙度的变化是主要因素;高应力状态属裂隙表面导电机制,随着破裂面在岩体内部出现,水和孔隙有了完全贯通的平面,多种导电机制都得以发挥作用. 此外,体导电结构的变化在宏观上表现为各向异性和图样有序性的增强.

STUDY ON THE MECHANISM OF RESISTIVITY CHANGES DURING ROCK CRACKING

The apparent resistivity of rock usually shows marked variations under the application of pressure. Its anomalous pattern greatly depends on the experimental conditions. The magnetite quartzite rock sample was compressed uniaxially in this study. The cracks in the sample were produced by cyclic compression processes (several loading unloading cycles) and salt solution was injected into cracks of the sample. Resistivity tomography was introduced to the present simulating experiment. A series of true resistivity images on the main prospecting section were reconstructed at every stage of the experiment. These images well reveal the microscopic structure in the medium and explain the reason of the changes of apparent resistivity. The experiment brings to light that the existence of cracks in the rock and the saturation state of the contained liquid are the most important factors controlling the change of resistivity before rock failure. The mechanism of resistivity change is very different at low and high stress the former is a conventional electrical conducting process depending mainly on variation of rock porosity, the later belongs to crack surface electrical conducting mechanism. As the cracks occur in the rock, liquid and pores are connected by the crack surface and so variety of mechanisms can play roles on the electrical conducting. Meanwhile the changes of bulk electrical conducting structure will also express as the enhancement of anisotropy macroscopically.