Adjustable Compensation Technique for Primary Field Elimination in the Transient Electromagnetic System

Elimination of primary field effect is crucial to the success in the transient electromagnetic (TEM) system. Instrument and algorithm solutions have been developed to compensate for the unwanted primary field signals during current turn-off. These solutions include bucking coil, opposing coil, and crossing-loop coil. They are mainly structural designs in the sense that the size and turns of the compensation (CX) coils are prespecified and compensation currents are fixed. This article proposes an adjustable compensation system for the elimination or removal of primary field effects. The proposed system differs from the above structure-oriented coil designs in that the compensation currents are adjusted electronically based on the current operating conditions. Compensation can also be easily readjusted to cope with the situation that the coil frame is moved from the set location. The experimental results are presented to verify the effectiveness of the proposed compensation system and to demonstrate that the system can work for the small-loop configuration.