Self-Decoupling Structure in Small-Loop Transient Electromagnetic System for Urban Underground Space Exploration

With the advancement of urbanization leading to surface space saturation, underground space development has become a global focus. The small-loop transient electromagnetic (TEM) method has garnered attention for its unique advantages in urban underground space exploration. However, the small distance between the transmitting and receiving coils causes the early secondary field to be drowned out by the primary field, creating a shallow blind area. Existing compensation technology aims to eliminate primary field coupling but faces two significant challenges: weakened transmitting energy and limited receiving area. To address these challenges, we proposed a small-loop TEM system utilizing the self-decoupling structure. This innovative design effectively suppresses primary field coupling through internal and external cancellation without compromising transmitting energy. It also overcomes the limitations of the receiving area, significantly improving the capture of the secondary field. Optimal parameters for the structure were determined, and an analysis of the positional deviation demonstrated that the new design offers enhanced tolerance to such errors in practical applications. Additionally, the unique transition process induced by the multi-coil series was analyzed to ensure optimal signal reception. To further validate the effectiveness of the proposed system, a new small-loop TEM system was implemented, and combined with the ground-penetrating radar (GPR) method for a field experiment. The experimental results confirm that the new system can be an effective tool for urban underground space exploration.