Design of the three-component coil sensor for semi-airborne frequency domain electromagnetic detection

Semi-airborne frequency domain electromagnetic method (SAFEM) usually uses a horizontal coil to observe the signal of the magnetic field in the vertical component. However, in the actual detection process, the signal not only lacks the information of horizontal component magnetic field, but also it is difficult to correct the signal amplitude in the distortion area of the magnetic field, which results in low accuracy and large error of apparent resistivity interpretation. In this paper, a three-component coil sensor for SAFEM detection is researched and designed, which consists of three groups of mutually perpendicular receiving coils to receive three-component magnetic field signals. Firstly, in order to meet the load requirements of the rotorcraft, a small circular coil with the diameter of 0.5 m is designed to receive the signal of the magnetic field in the vertical component, and two square coils with the side length of 0.01 m are designed to receive the signal of the magnetic field in the horizontal component, and the mutual coupling of the three groups of coils is analyzed to optimize the relative position of the coils. Secondly, according to the physical structure characteristics of the coil sensor, the final physical structure parameters are determined by synthesizing the analysis results of the parasitic resistance, inductance, and capacitance of the coil sensor. Further, by analyzing the frequency response characteristics of sensors and preamplifiers under different damping states, the optimal damping coefficient adjustment range is determined. On this basis, a low-noise differential preamplifier circuit with damping state adjustment is designed as the SAFEM signal conditioning module. Finally, the reliability of the three-component coil sensor is verified by conducting the test in shielding room and field detection experiments. The results show that the small size of the three-component coil sensor can meet the detection requirements, and can provide sensitivity and measurement accuracy comparable to commercial sensors. In the field experiment, the three-component magnetic field response signals in the experimental site are obtained by using the sensor proposed in this paper, which further proves the effectiveness of the three-component sensor.