Hydraulic fracturing (HF) has been widely used in the coal and rock dynamic disasters prevention and control and coalbed methane production in underground coal mines. However, how to effectively and accurately monitor and evaluate the hydraulic cracks propagation process and impact range of HF has not been resolved. Therefore, we established a true triaxial HF direct current (DC) monitoring experimental system. And we conduct the first-ever HF-DC monitoring experiments under true triaxial loading conditions in the world. The response characteristics of the emission current, apparent resistivity, and apparent resistivity difference contour map during the whole experimental process are studied. The results show that the DC monitoring data contains a wealth of effective information on hydraulic cracks propagation. The emission current and apparent resistivity reflect the overall electrical conductivity of the rock. The contour map of the apparent resistivity difference between two measuring lines can accurately characterize the propagation process and spatial distribution of hydraulic cracks. The evolution process of difference contour map shows that hydraulic cracks are more likely to spread to areas with poor homogeneity. The change range and degree of apparent resistivity are mainly affected by the spatial distribution, water-bearing state of hydraulic cracks and wet area surrounding the hydraulic cracks. As the hydraulic cracks network expands, a strong conductive channel will be formed inside the rock, which will change the conductive properties and patterns of the rock. By comprehensively using the temporal and spatial evolution of the apparent resistivity difference contour map and the regional apparent resistivity change curve, the DC monitoring method can accurately characterize the dynamic expansion and spatial shape of hydraulic cracks of rock.

hydraulic fracturing, true triaxial loading conditions, direct current monitoring, apparent resistivity, cloud map evolution, hydraulic crack propagation