Noncoplanar Positive Compensation Decoupling Technology in a Small-Loop Transient Electromagnetic System

In the transient electromagnetic method (TEM), the early primary field signals can interfere with the secondary field signals due to the turn-off time and mutual inductance between the transmitting and receiving coils, resulting in a shallow blind area. This primary field coupling interference (PFCI) is particularly pronounced when using a multi-turn, small-loop coil structure, which significantly limits TEM’s effectiveness in near-surface detection. To address this issue, several decoupling techniques have been proposed. Among them, the off-center self-compensation technology is considered effective and has been applied in systems such as airborne TEM. However, this approach has drawbacks because it is susceptible to primary field remanence caused by structural errors, and its eccentric design results in inadequate detection capabilities and lateral resolution. To overcome these limitations, we proposed a new non-coplanar positive compensation decoupling technology. This device enhances structural stability by positioning a positive compensation coil co-axially with the transmitting coil and coplanar with the receiving coil. Additionally, the positive compensation coil effectively enhances the device’s shallow detection capability by both reducing the relative horizontal distance between the transmitting and receiving coils and providing stronger shallow excitation signals. Simulation and field experiment results demonstrate the effectiveness of the new device.