Development Of Magnetic Core Coil Sensor For Semi-airborne Frequency Domain Electromagnetic Detection

In recent years,my country’s economy has developed rapidly,and the demand for mineral resources has increased year by year.The resources in the easy-to-exploration areas have been basically fully developed,and it is urgent to develop the inaccessible areas on the ground and deep underground minerals.The semi-airborne frequency domain electromagnetic method is an exploration method that uses an unmanned aerial vehicle equipped with a magnetic induction sensor to detect the target area.Compared with the traditional exploration method,it has the advantages of non-contact,high resolution,and wide depth range.Due to the limited load of the semi-airborne,the small size and few turns of the commercial air-core coil sensor lead to low sensitivity and insufficient ability to sense low-frequency signals in deep formations,which affects the effect of deep detection;the commercial core coil sensor has a slender structure and low sensitivity.If it is too high,the magnetic saturation problem will easily occur in mobile detection in the air,and the size is too long and the weight is too large.The magnetic core sensor also cannot meet the detection requirements of the semi-airborne electromagnetic method.Therefore,it is necessary to study a magnetic core sensor for electromagnetic detection in the semi-airborne frequency domain,which can not only meet the light load requirements of the unmanned aerial vehicle platform,but also have sufficient sensitivity in the low frequency band to realize deep underground detection.Aiming at the problem of insufficient low-frequency response of air-core coils,this paper studies light-weight high-sensitivity sensing technology,and designs a sensor and receiving equipment for electromagnetic detection in the semi-airborne frequency domain.The design content and research results of the system are as follows:（1）The principle of the electromagnetic method in the semi-airborne frequency domain and the principle of magnetic measurement of the coil sensor are studied.Based on the analysis of the equivalent second-order response circuit of the coil probe,the induced electromotive force,sensitivity,amplitude-frequency characteristics of the coil probe and the coil itself are established.The mathematical model of the parameters;the parameters that affect the inductance of the coil sensor are studied,and the quantitative relationship between the number of turns and the size of the magnetic core and the inductance of the coil is obtained;the research route of the semi-airborne miniaturized magnetic induction sensor is determined.（2）Starting from the real magnetic field environment of electromagnetic detection in the semi-airborne frequency domain,three-dimensional modeling is carried out on the stratum and air domains,and the emission effects of different excitation source models are simulated;and the distribution of magnetic induction intensity in the air under the excitation of low-frequency emission is simulated,the receiving direction of the magnetic core sensor is determined;by simulating the Bz field value at an altitude of 60 m on the vertical line of the emission source,it provides a theoretical basis for the parameter design of the magnetic induction coil sensor in the semi-airborne frequency domain.（3）In view of the problem that the magnetic core coil sensor is easy to introduce motion noise to saturate the signal during the motion detection process,a lightweight high-sensitivity magnetic core coil sensor is designed.First of all,the magnetic properties of soft magnetic materials are studied,and the circular-wall laminated magnetic core structure is designed.Compared with the traditional magnetic rod with the same aspect ratio,the magnetic core structure has the advantages of low loss and high permeability,and the weight is reduced by 30%;Secondly,the segmented design of the coil bobbin reduces the parasitic capacitance and improves the bandwidth of the coil sensor;the number of segmented coil turns is designed according to the axial magnetic permeability distribution of the magnetic core to make full use of the magnetic gathering ability of the magnetic core;A high input impedance and low noise preamplifier circuit is designed to complete the overall design of the sensor;the developed magnetic core coil sensor has a sensitivity of 28.2m V/n T@100Hz and an equivalent magnetic field noise of 0.07 p T/（Hz）^1/2@100Hz.（4）According to the characteristics of electromagnetic detection signals in the semi-airborne frequency domain and the high impedance characteristics of the magnetic core sensor,a high-precision and high-resolution semi-airborne frequency domain electromagnetic receiver is developed.In view of the large amount of high-frequency noise and power frequency interference in the detection environment,a high-frequency suppression circuit and a power frequency notch circuit are designed to improve the signal-to-noise ratio;the signal acquisition rate and resolution requirements of electromagnetic detection in the semi-airborne frequency domain are analyzed,and a High-sampling rate and high-precision acquisition circuit;designed power distribution network,low-ripple regulated power supply for different modules to ensure stable operation of the system;written host computer program to control the operation of the receiving system and display the acquisition signal waveform,improving the performance of the receiving system practicality.Finally,the joint debugging test of the magnetic core sensor and receiver developed in this paper is carried out,and the actual flight measurement is carried out in the field by using the receiving system carried by the drone.The system can realize the detection target in the 10 Hz-10 k Hz frequency band,which verifies the practicability of the system,provide effective technical means for the detection of deep underground targets,and lay the foundation for the development of deep mineral resources.