An advanced detection method for unfavorable geological boulder based on electrical source in drill holes under shield machine

Recent years, due to the increasingly complex terrain, geological conditions, and working environments in engineering, the accuracy requirements in explorations have been continuously growing. In this study, with the goal of high-resolution prospecting for unfavorable geological boulder under complex detection environment (such as subway shield machine tunnel). A tunnel model with unfavorable geological boulder was constructed, and time domain finite element method (TDFEM) was adopted for the 3D transient electromagnetic forward modelling. In order to realize high resolution in the positioning of the small-scaled boulders, a detection method in complex environments was explored. A hole was drilled from the central point of the tunnel face toward the tunnel construction direction. Then, an electrical source was placed into the hole and array data was collected on the tunnel face. Electromagnetic sounding was achieved through the movement of the source, and the plane positions of the geological bodies were determined through the electromagnetic field distribution characteristics on the tunnel face. It was observed that the Ex and Ey (horizontal component of electric field) had higher resolution for high resistance geological boulder. Therefore, the results indicated that in complex environments, collecting the horizontal components of the electric fields on the tunnel face excited by the electrical source within the drill hole could provide a feasible method for the fine explorations of small-scale high resistance boulder.