title: An Optimized Air-Core Coil Sensor with a Magnetic Flux Compensation Structure Suitable to the Helicopter TEM System.
abstract: The air-core coil sensor (ACS) is widely used as a transducer to measure the variation in magnetic fields of a helicopter transient electromagnetic (TEM) system. A high periodic emitting current induces the magnetic field signal of the underground medium. However, such current also generates a high primary field signal that can affect the received signal of the ACS and even damage the receiver. To increase the dynamic range of the received signal and to protect the receiver when emitting current rises/falls, the combination of ACS with magnetic flux compensation structure (bucking coil) is necessary. Moreover, the optimized ACS, which is composed of an air-core coil and a differential pre-amplifier circuit, must be investigated to meet the requirements of the helicopter TEM system suited to rapid surveying for shallow buried metal mine in rough topography. Accordingly, two ACSs are fabricated in this study, and their performance is verified and compared inside a magnetic shielding room. Using the designed ACSs, field experiments are conducted in Baoqing County. The field experimental data show that the primary field response can be compensated when the bucking coil is placed at an appropriate point in the range of allowed shift distance beyond the center of the transmitting coil and that the damage to the receiver induced by the over-statured signal can be solved. In conclusion, a more suitable ACS is adopted and is shown to have better performance, with a mass of 2.5 kg, resultant effective area of 11.6 m² (i.e., diameter of 0.496 m), 3 dB bandwidth of 66 kHz, signal-to-noise ratio of 4 (i.e., varying magnetic field strength of 0.2 nT/s), and normalized equivalent input noise of 3.62 nV/m².
authors.affiliation: Key Laboratory of Geo-exploration Instruments, Ministry of Education of China, Changchun 130061, China. cchen@jlu.edu.cn.|College of Instrumentation and Electrical Engineering, Jilin University, Changchun 130061, China. cchen@jlu.edu.cn.
authors.forename: Chen
authors.initials: C
authors.lastname: Chen
authors.affiliation: Key Laboratory of Geo-exploration Instruments, Ministry of Education of China, Changchun 130061, China. feiliu14@mails.jlu.edu.cn.|College of Instrumentation and Electrical Engineering, Jilin University, Changchun 130061, China. feiliu14@mails.jlu.edu.cn.
authors.forename: Fei
authors.initials: F
authors.lastname: Liu
authors.affiliation: Key Laboratory of Geo-exploration Instruments, Ministry of Education of China, Changchun 130061, China. lin_jun@jlu.edu.cn.|College of Instrumentation and Electrical Engineering, Jilin University, Changchun 130061, China. lin_jun@jlu.edu.cn.
authors.forename: Jun
authors.initials: J
authors.lastname: Lin
authors.affiliation: Key Laboratory of Geo-exploration Instruments, Ministry of Education of China, Changchun 130061, China. zhukaiguang@jlu.edu.cn.|College of Instrumentation and Electrical Engineering, Jilin University, Changchun 130061, China. zhukaiguang@jlu.edu.cn.
authors.forename: Kaiguang
authors.initials: K
authors.lastname: Zhu
authors.affiliation: Key Laboratory of Geo-exploration Instruments, Ministry of Education of China, Changchun 130061, China. yanzhang@jlu.edu.cn.|College of Instrumentation and Electrical Engineering, Jilin University, Changchun 130061, China. yanzhang@jlu.edu.cn.
authors.forename: Yanzhang
authors.initials: Y
authors.lastname: Wang
keywords: ACS; helicopter TEM system; magnetic flux compensation structure; optimization of the ACS