含碳结构对龙门山断层带电导率影响的实验探索

Experimental studies on the relationship between carbonaceous structure and electrical conductivity of the Longmenshan fault zone

碳是影响岩石电导率大小的一个重要因素,可能是造成龙门山断层带电导率异常的重要原因之一.为了研究不同的碳含量、矿物颗粒粒径与碳晶体结构对断层带电导率的影响,在干燥、常温、0.2~300 MPa的压力条件下实验研究了人工模拟断层泥样品(石英粉末与含碳粉末混合的样品,简称模拟样品)和采自映秀-北川断层八角庙剖面的天然断层岩样品(简称天然样品)的电导率.实验结果显示,当模拟样品中的含碳粉末连通时,电导率与碳体积率的关系符合逾渗理论模型;而含碳粉末未连通时,电导率随总孔隙度降低而指数性升高.同时模拟样品的电导率也随石英颗粒粒径的变化而发生改变.相比于模拟样品中的含碳粉末主要分布于石英颗粒支撑的孔隙中,天然样品中的碳则主要以碳膜的形式赋存在颗粒边缘,导致碳体积率相同的条件下,模拟样品的电导率小于天然样品.此外,天然样品的电导率(<9×10^(-4)S·m^(-1))也要小于野外大地电磁探测的结果(0.03~0.1 S·m^(-1)).在今后的实验中还需要考虑在动态摩擦条件下对含有完整含碳结构的天然样品进行电导率的实验研究.

Carbon is a crucial factor influencing rock electrical conductivity and its enrichment in the fault rocks might be one of the key mechanisms responsible for the anomalously-high electrical conductivity observed in the Longmenshan fault zone. To investigate the effects of the content, grain size and crystal structure of the carbon present in natural fault zones, in this study, electrical conductivity measurements have been performed on simulated fault gouges that were prepared from both synthetic （mixture of carbon and quartz） and natural fault rocks at room temperature and 0.2~300 MPa lithostatic pressure condition. The synthetic samples show a sharp increase in electrical conductivity when the volumetric fraction of carbon （φc） reaches a critical value. This observation is consistent with the prediction from the percolation theory. Our results also show that the grain size of less conductive component （quartz） can affect the electrical conductivity, but in the manners that are different between connected and unconnected samples. Microstructural analysis further revealed the presence of carbon films at the grain boundaries in natural samples. Furthermore, the natural samples have lower electrical conductivities （〈9×10-4 S·m-1） than the synthetic samples that have similar φc-values. The measured values are also lower than those determined from the magnetotelluric survey in the study area （0.03~0.1 S·m-1）. This discrepancy suggests the necessity to measure the natural samples under in-situ, dynamic conditions.