Bucking coil used in unexploded ordnance detection with transient electromagnetic system

Abstract

Off-time response is usually measured by transient electromagnetic (TEM) system to detect subsurface targets. Compared with off-time response, on-time response of TEM system is more sensitive to high-conductivity targets, such as UXOs. However, it is difficult to obtain on-time response directly due to the strong primary field response. The key problem for on-time measurement is to eliminate the primary field response. Bucking coil is adopted here to compensation the primary field of transmitter coil at receiving coil for on-time response measuring. Firstly, the structure of the sensor with transmitting coil, bucking coil, and receiving coil is presented. Then, the primary field of transmitting coil and bucking coil at the position of receiving coil is calculated theoretically. With the side length of transmitting coil, bucking coil and the receiving coil fixed, the number of turns for transmitting coil and bucking coil are optimized for the best compensation. Calculated results show that the best compensation effect can reach 0.53%. Finally, the sensor with a transmitting coil (constructed with a side length and number of turns of 96 cm and 30, respectively), a bucking coil (constructed with a side length and number of turns of 26 cm and 8, respectively), and a receiving coil (constructed with a side length of 6 cm) was built. Measured results show that the primary field response after compensation is no more than 1% of the original response, and the maximum amplitude is only 0.4V. The on-time response of targets can be accurately recorded.