Research On Key Technologies Of The Recording System In Towed Frequency-domain Electromagnetic System

In recent years,with the economic development and the improvement of the urbanization process,the development of urban underground space has become an important development direction for modern urban construction.Accurate exploration of the underground structures in urban areas is an effective way to reduce the incidence of the engineering accidents and the improve engineering efficiency.The towed frequency-domain electromagnetic method adopts small loop transmitting and receiving devices to obtain underground resistivity information by collecting the total field,which has the advantages of high work efficiency,abnormal sensitivity to low resistance,high resolution,and no need to damage the surface,which is very suitable for the rapid exploration of the shallow underground structures in small urban spaces.However,limited by the application environment and the traction of the mobile platform,the diameter of the transmitting coil is often less than 10 m,the transmitting magnetic moment is small,and the secondary field is weak.In order to improve the amplitude of the measurement signal,it is necessary to increase the effective area of the receiving coil,but with the increase of the number of turns of the receiving coil,the measurement bandwidth is significantly reduced,resulting in a blind spot in shallow exploration.In addition,the frequency-domain electromagnetic method of the transceiver separation device is used to measure the total field signal,the amplitdue of the secondary field is much smaller than the primary field,so that only the amplitude of the total field is difficult to identify the target anomaly,and it is very easy to produce false information due to the change of the primary field by changing of the transceiver distance.Based on the analysis of the conductive loop model,measuring the phase of the total field relative to the primary field can effectively avoid the interference of the primary field,which has the advantage of strong anomaly recognition ability.Therefore,it is of great significance and application value to develop of low-noise,wide-band coil sensors,and phase measurement technology of separation devices.At present,in view of the shallow blind spot problem caused by the insufficient measurement bandwidth of the receiving coil,the traditional way is to correct the system response by obtaining the static calibration file of the receiving coil.However,the receiving coil parameters are affected by the environment,and there is a signal distortion problem in correctting the response of the system using a static calibration file.For the phase measurement problem of the separation device,the traditional method is to obtain a field reference information by means of a reference coil,which is coplanar with the transmitting coil horizontally,and underground geological information is obtained by a vertical direct coil.However,the vertically placed receiving coil is less coupled to the earth,which reduces the exploration effect.Aiming at the above problems,the dual-air-core coil sensor is proposed to solve the problems of the contradiction of the measuring bandwidth and the effective area and static calibration file ignoring environmental factors.The phase measurement method of the electromagnetic detection signal of the separation device is proposed to solve the problem of the abnormal target is difficult to identify due to the near-field effect.On this basis,a hybrid sampling rate and large dynamic range recording system were developed and verified by field experiments.The research work is briefly described below:(1)In order to solve the problems of the contradiction between the effective area and the measurement bandwidth of the receiving coil sensor,and the shallow blind zone caused by the insufficient measurement bandwidth of the receiving coil,the dual-air-core coil sensor is proposed based on low noise and wideband sensing technology.The sensor consists of a high-frequency coil and a low-frequency coil.The interaction model between the two receiving coils is established to eliminate the effects of crosstalk by combining the coil characteristics.The amplitude frequency response of the low frequency coil is obtained based on the linear reference of the high frequency coil by precisely adjusting the damping coefficient.The coil parameters are obtained to realize the correction of the frequency response by fitting method.The measurement bandwidth of the receiving coil is greatly improved with the advantage of a large effective area.Experiments show that the bandwidth of-3 d B is 200 k Hz,the phase measurement bandwidth(±5°error)is 220 k Hz,the effective area is 59.5 m²,and the equivalent input magnetic field noise is 18 f T/Hz^1/2@1 k Hz.(2)In order to solve the problem of the correction error by using static calibration file to correct the system response,ignoring the influence of environmental factors,the system response correction technology based on the dual-air-core coil sensor is proposed.The sensor consists of two receiving coils with different damping coefficients and the same winding method.The parameters of the receiving coil are obtained to correct of the system response by fitting the amplitude ratio or phase difference.This method does not require current measurement,and can dynamically obtain the parameters of the receiving coil with the advantage of high real-time performance.Through the outdoor abnormal loop experiment,the method described in this article is more consistent with the trend of the theoretical results than the static calibration file.(3)In order to solve the problem of the abnormal target is difficult to identify due to the secondary field is submerged within the strong primary field,the phase measurement method of the electromagnetic detection signal of the separation device is proposed.Taking the Beidou or GPS synchronization signal as the reference,the time delay of the acquired signal and the reference signal is obtained by increasing the timestamp from the hardware time stream,so as to generate the reference signal.The phase between the acquired signal and the reference signal is obtained by the cross-power spectrum method,and the phase measurement of the electromagnetic detection signal between any two systems is realized.The experiments show that the synchronization accuracy is 48 ns and the phase measurement error is less than 0.173°@10 k Hz.(4)For the separation electromagnetic detection device,there is a contradiction between the sampling rate and the dynamic range.The mixing rate,dual-range large dynamic range acquisition method is adopted,and the corresponding recording system is developed.The experimental shows that the sampling rate of high-speed channels is 5 Msps and the sampling rate of low-speed channels is 144 ksps,and the dynamic range is increased from140 d B to 165 d B.Finally,the recording system of the towed frequency-domain electromagnetic method in this paper was used to conduct detection experiments in Changchun City.The experimental results show that the phase of the total field relative to the primary field can clearly display the position of the shallow anomalous target,which is more obvious than the amplitude of the total field.The method described in this paper has certain application prospects,and provides certain support for the upgrading of the towed frequency-domain electromagnetic detection method.