| semi-airborne frequency-domain electromagnetic method is an electromagnetic exploration method widely used in complex terrain areas to rapidly detect deep structures.It uses ground transmitter and excitation wire source to form an emission system to radiate alternating magnetic field(primary field)outward.The primary field and secondary field signals generated by anomalous bodies are received by sensors and data receiving equipment on the flight platform and the analog signals are digitized and stored.Then the formation information is analyzed by analyzing the secondary field variation law.Through the amplitude and phase information of the three-component magnetic field signal under the excitation of the two-field source,the divergence of tipper can be obtained and imaging can be performed.However,the analysis shows that the dipper divergence data is seriously affected by signal amplitude and phase data.Hollow coil sensor is a device that converts magnetic signal into electrical signal.The false sensitivity coefficient will bring system noise.Since the transceiver distance of the detection system is usually large and the transceiver device is far away,the wired synchronization cannot be realized.Therefore,the system usually uses the high-precision global positioning system as the reference timing source to realize synchronization and provide the phase reference.However,the random noise of GPS signal and the clock shift caused by the aging of the system clock inevitably lead to the synchronization error of the distributed system.In view of the above problems,the amplitude and phase calibration method of electromagnetic detection signal is designed.The main work is as follows:1.The working principle and advantages of the ground-to-air frequency electromagnetic detection system are analyzed,then the electromagnetic response of the ground-to-air electromagnetic system is analyzed,and the influence of the amplitude and phase information of magnetic field signals on the ability to identify anomalies by means of tilting divergence is analyzed according to the relationship formula between tilting divergence and three-component magnetic field.2.In order to calibrate coil sensitivity,a calibration method of hollow coil based on magnetic source excitation was designed,and the calibration coefficient of the receiving system was obtained according to magnetic source excitation and coil induced voltage.At the same time,the sources of standard uncertainty related to calibration are analyzed,and the uncertainty is evaluated.3.A high precision system synchronous clock source based on FPGA is designed.First,on the basis of analyzing the characteristics of GPS signal,it realizes the acquisition of high quality GPS second pulse signal and the actual frequency of crystal oscillator by taking advantage of the characteristic that the random error is expected to be 0 under long time statistics.Then the frequency offset average compensation algorithm is designed to calibrate the frequency offset of the constant temperature crystal oscillator due to aging and other reasons.Finally,the system clock after calibration is obtained through waveform synthesis,so that the system clock is close to the nominal frequency of the crystal oscillator.4.The amplitude and phase calibration methods designed were tested experimentally,and the coil calibration method based on magnetic source excitation was analyzed by simulation and experiment,and the sensitivity coefficient of the coil was obtained.In order to compare the calibration results of the coil,Brownbeck coil is used to calibrate the same coil,and the reliability of the coil calibration method studied in this paper is verified by comparing the coil amplitude-frequency response.The feasibility of the system clock calibration method is analyzed by simulation,and the reliability of the synchronous clock source is verified by comparing the crystal frequency before and after calibration,effectively improve the precision of the system clock. |
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