Double Receiving Coils Eccentricity Self-Compensating Small-Loop Transient Electromagnetic System Based on Robustness Analysis

The ground small-loop transient electromagnetism (TEM) provides a basis for detecting shallow underground space. However, the strong primary field interference from the transmitting coil to the receiving coil, along with the transition process of the receiving coil, can cause serious distortion of the early secondary field signals. This leads to the loss of effective shallow underground information. In this paper, we utilize the eccentric self-compensating structure to weaken the primary field interference. Aiming at the current position sensitivity of the eccentric structure, we propose a statistical method to realize the robustness analysis of the eccentric structure and find the optimal eccentric position where the primary field coupling between the transmitting and receiving coil is approximated to be zero. To address the impact of the coil transition process, a double receiving coils structure is proposed. This ensures that the number of turns, the secondary field flux and the secondary field response strength in the single receiving coil structure remain unchanged. Compared with the conventional eccentric structure of a single receiving coil, the bandwidth of the receiving coil sensor was increased from 103.5 kHz to 218.3 kHz, and the Signal-to-Noise Ratio (SNR) of the measured early secondary field signals improved from 18.5 to 27.9, representing a 50.81% increase in SNR. This study not only reduces primary field interference but also reduces the impact of the coil transition process, thereby capturing more early secondary field signals and enhancing the shallow detection resolution of the ground TEM.