| In today’s rapidly developing society,the demand for mineral resources in the industry is increasing.The geophysical method represented by frequency domain electromagnetic detection is widely used in mineral exploration after many years of development.Detection in the field environment is complex.With multiple layers of media,and the need to use multiple frequency signals for emission detection.Detection devices mostly use single-axis emission coils that can not meet the requirements of detection in closed space.For the above detection requirements and exploration problems,this paper adopts a multi-frequency sequence as the transmitting signal,designs a three-axis transmitting coil,and builds a multi-frequency electromagnetic transmitter device.The research content is as follows.1.The study of frequency domain electromagnetic detection process,electromagnetic field detection propagation characteristics,so that according to the frequency domain detection of different underground materials in the secondary field changes,the target electrical properties in the underground space environment,discuss the working principle of underground detection,according to the finite element method and COMSOL software for frequency domain detection simulation,to verify the design of the emission frequency for underground space detection,for multi-frequency electromagnetic emission device for Theoretical basis.2.The common electromagnetic emission signal characteristics and application problems are analyzed,and the multi-frequency signal is selected according to the emission conditions and detection mode.This paper conducts a comparative study on the signals of electromagnetic emission,analyzes the nature of conventional emission and different pseudo-random signals,selects five-frequency pseudo-random sequences to transmit through high-frequency carrier fusion according to the detection mode of equipment and underground exploration application requirements,uses Labview simulation transmitter to transmit experiments,and also calculates the transmitting depth to prove the feasibility of multi-frequency emission signals in underground exploration applications.3.Based on the demand for triaxial emission in closed space electromagnetic detection,a triaxial square Helmholtz emission coil with a side length of 0.5m is designed as the emission coil of the transmitter to cope with complex environmental detection.The magnetic field strengths of the toroidal and rectangular coils are compared according to the electromagnetic theory,and the magnetic induction strengths of the three-axis square Helmholtz coil are calculated.The simulations of the single-axis square,toroidal,and three-axis toroidal and square coils under the same launch conditions are also analyzed using Maxwell software to verify the good launch performance of the designed launch coil.4.The software and hardware of the multi-frequency point magnetic source transmitter are designed and tested out for transmitter performance.A dual-core microcontroller and FPGA master control module were used according to the functional requirements of the transmitter.The FPGA performs 2~n five-frequency pseudo-random sequences at 512 Hz and 204.8 kHz carrier timing generation,and the driver bridge circuit transmits waveforms through the coil.The microcontroller works through the FMC communication connection and the GPS control,and the power supply transmits through the inverter bridge circuit under the control of the drive circuit.The power supply circuit provides power to the master and GPS synchronously through level conversion of external lithium batteries,thus realizing a complete multi-frequency electromagnetic transmitter.To verify the reliability of the design,the drive and carrier signals of the transmitter are tested in the laboratory environment,which is consistent with the design requirements,and the transmitting test is conducted in the field environment to verify the practicality and reliability of the design. |
