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

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 similar to 300 MPa lithostatic pressure condition. The synthetic samples show a sharp increase in electrical conductivity when the volumetric fraction of carbon (phi(c)) 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 (<9x10(-4) S.m(-1)) than the synthetic samples that have similar phi(c)-values. The measured values are also lower than those determined from the magnetotelluric survey in the study area (0.03 similar to 0.1 S.m(-1)). This discrepancy suggests the necessity to measure the natural samples under in-situ, dynamic conditions.