| Detection is an essential prerequisite for urban underground space development or field resource exploration.At present,underground space detection and sensing technology has become an important research direction.However,due to the complex surface conditions,the underground space and the abnormal body positioning will be restricted by various factors such as geology,background interference and space.Therefore,it is very important to design an instrument that can obtain underground information more effectively than existing instruments and achieve high-precision detailed imaging.In order to guide the design of the instrument system,the response of the magnetic dipole source under the layered geodetic model is calculated based on Maxwell's equations,and a simple linear relationship is obtained between the ratio of the secondary magnetic field and the primary magnetic field and the apparent conductivity of the earth.Considering the skin effect of electromagnetic field and the different detection depth at different frequencies,a wide-band scanning emission method is designed.The apparent conductivity of the earth is calculated point by point from the secondary magnetic field response from high frequency to low frequency,and then imaging in the depth direction.However,the current large magnetic moment transmission system can only achieve large magnetic moment at a series of limited frequency points.The inductance characteristics of the transmitting coils with different configuration are investigated by using the method of formula calculation and finite element simulation,investigates the ability of the broadband magnetic moments transmitted by a nonresonant matched transmitter coil which is drove by a broadband power amplifier.It is concluded that choosing a single-turn coil with a large area can obtain the largest transmitted magnetic moment in high-frequency band.At the same time,the receiving coils are designed for different frequency bands,which avoids low coil sensitivity at low frequencies and selfresonance at high frequencies.According to the theoretical calculation,the secondary magnetic field signal is several orders of magnitude smaller than the primary magnetic field signal,so a primary magnetic field elimination model is constructed.After a customized compensation coil is configured at an appropriate position,the result shows that the primary magnetic field signal attenuation can reach-40.32 d B.A weak signal detection device is designed to obtain the weak secondary magnetic field signal.Level 3 adjustable programmable amplification is suitable for different coil sensitivities,signal processing such as oversampling and decimation filtering can improve the SNR of the signal.And the quadrature vector demodulation method is used to obtain the amplitude and phase information of the weak secondary magnetic field signal,so as to realize the detection of V-level signals at different frequencies,meet the needs of theoretical calculations.Frequency domain electromagnetic method detection system unavoidably exist error,the self-calibration method is studied without relying on the external environment.Analyze the phase shift caused by the power amplifier and the transmitting and receiving coils.Real-time monitoring of the current amplitude and phase in the transmitting coil,according to the coordinate rotation formula to correct the secondary magnetic field signal through the principle of digital phase correction.The field experiment results show that the self-calibrated instrument can image the apparent conductivity in the depth direction,and better locate the left and right boundaries and cavity structures of the underground space. |
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