Fine detection of large-scale complex geological structures based on geophysical prospecting techniques

摘要

Deciphering the internal composition of large-scale fault zone is a prerequisite for constructing conceptual models of fault zone architecture and is essential for a proper understanding of the seismological, mechanical and hydrological properties of fault zones. To this end, we conducted fine detection and interpretation of a large-scale fault (F31 fault) in the first Chinese underground research laboratory (URL) construction site for high-level radioactive waste geological disposal repository to elucidate the fault zone architecture. The fault, located in the west of the URL site, is an approximately 5.5-km scale steeply dipping normal fault extending continuously in a straight line. In this study, geophysical prospecting techniques including high-density electrical resistivity tomography (ERT) and reflection seismic tomography (RST) were employed to investigate and interpret the subsurface architecture of the fault zone along the strike of fault core and perpendicular to fault core, respectively. In addition, a 4-m long three-dimensional prospecting trench was excavated manually to observe the geological structure characteristics of the fault zone. Geophysical exploration results suggested that the shallow surface within the fault zone, which is mainly characterized by low conductivity and low wave velocity transmission medium, shows complex geological characteristics due to severe weathering, while the relatively complete bedrock lies beneath the overburden layer. Along the fault strike, the bedrock is buried shallowly at the south of the fault zone. In contrast, at the north, it is mainly characterized by a thick weathering layer overlying the bedrock. For the detection results of vertical fault core, both geophysical methods have deciphered the location of the fault core and the spatial distribution characteristics of F31 fault zone. However, compared with RST results, ERT also clearly diagnosed that there is another secondary fault zone with a certain scale at the hanging wall of F31 fault. In terms of the F31 fault core, ERT suggested that the fault dips about 70° NW and the width of the fault core is 2-5 m, which is consistent with the results of the 280° ∠ 75° discontinuities and the approximately 2.6-m long fault core revealed by the prospecting trench. These results provide an important basis for further understanding the internal structure of faults in this site and the avoidance design of the fault-affected range for the underground engineering layout.