| The SAEM(semi-airborne Electromagnetic Method),which uses a ground transmitter and air receiver to record magnetic fields,has the advantages of large survey area,strong adaptability to complex terrain,low cost and high efficiency.However,the existing SAEM system is mainly in the time domain,so the survey area and depth is limited.The only frequency-domain system uses magnetic coil as the transmitting source,and its data processing and interpretation methods are limited,which cannot give full play to the exploration potential of ground-air frequency-domain electromagnetic(GAFEM)methods.The GAFEM system is expected to break through the limitations of existing SAEM systems and has the potential to expand the survey area and depth.Aiming at the problem of large motion noise of the system,the generation mechanism and suppression method of system's motion noise were studied,effectively suppressing the influence of motion noise.It is of great significance and practical value to improve the detection capability of the GAFEM system and to popularize the development of detection GAFEM technology.This paper starts with the principle of the GAFEM method,clarifies the system observation scheme and technical index requirements,studies the imaging method,and discusses the problem that the existing system affected by the motion noise does not meet the technical index requirements.The mechanism of noise generation and the suppression method of the motion noises with different sources are studied,they are effectively suppressed by means of system optimization and data processing.Finally,the effectiveness of the optimized system and the proposed data processing technology is demonstrated through field comparison experiments and detection applications.The main research contents are as follows:(1)The paper studied the basic theory of the GAFEM method,and clarified the technical index requirements of the system.Based on the basic theory of electromagnetic field and electromagnetic wave,the 1D forward modeling of the GAFEM method is established.The effects of different model parameters and observation parameters on the response are analyzed,and the observation scheme of the system is defined.The method of calculating the whole area apparent resistivity and apparent depth is studied.Aiming at the detection requirements within the range of 6 km of the whole frequency band,the technical indexes of the system are clarified: the frequency band range is 10 Hz ~ 10 kHz,and the noise level of the system must be lower than 30 pT/?Hz.The noise level of the existing system in the frequency band of 700 Hz~1 kHz and the low frequency band of 2 kHz and that is lower than 50 Hz does not meet the detection requirements of the system.It provides the technical index foundation for studying the motion noise suppression technology and optimizing the receiving system.(2)The mechanism,controling and coupling suppression technology of flight platform noise were studied.The noise comes from the rotating magnetic field generated by the stator of the motor in the rotocraft.The noise is mainly distributed in the control frequency of the motor and its harmonic frequencies.The noise's frequency and intensity is affected by the motor speed.By optimizing the design of assembly parameters including the elastic coefficient of suspension rope,the quality and the suspension distance of receiving coil,the noise will be suppressed.In the conventional electromagnetic noise environment,the measured noise level of the optimized receiving system is lower than 10 pT/?Hz in the frequency band above 50 Hz,which can meet the system's technical requirements.(3)The mechanism and suppression technology of low-frequency motion noise are studied.Low-frequency motion noise comes from the low-frequency motion of the receiver coil in the geomagnetic field,and the noise is distributed in the frequency band lower than 20 Hz,and noise is the strongest at 1 ~ 2 Hz.When the system processes the received data for a short time,the noise will produce serious leakage in the and reduce the signal-to-noise ratio at the effec.Low frequency motion noise suppression based on wavelet transform is studied,using coif5 and decomposing the signal to 10 layers.Then,the low-frequency motion noise baseline in the data can be obtained and removed,which effectively improves the signal-to-noise ratio of 10~50 Hz frequency band.(4)The mechanism and suppression technology of attitude noise at the same detection frequency are studied.The attitude noise comes from the motion of the receiving coil in the three-dimensional magnetic field generated by the transmitting source.The position deviation of the moving receiver coil is less than 4 m,and the height error is less than 2 m.For a homogeneous earth of 100 ?·m,the deviation of the receiver coil introduce a relative error less than 5% in the measured magnetic field,which is within the error tollerance.Therefore,the influence of the deviation of the receiver coil can be ignored,and the attitude noise of the system is mainly affected by the receiver coil's attitude angles.Since the measured magnetic field amplitude after attitude deflection is still a monotone function varying with resistivity,the paper proposed an attitude noise correction algorithm based on the apparent resistivity imaging with the attitudes,and realized the attitude correction.For static and dynamic changes of attitude angle,the paper studied the influcences of the attitude's measurement accuracy and sampling rate,and clarified their requirements to ensure the correction accuracy.Finally,the optimized system,and data processing technology based on the above research content have been applied in the detection experiment in Nanjing,Changchun,and Liaoyuan City.The detection results show that the GAFEM system and the data processing technology can acquire effective detection information and reflect underground electrical anomalies,which verifie the effectiveness and practicability of the motion-noise suppression technologies proposed in this paper.The GAFEM system obtained the maximum source-receiver distance of 6 km and depth of 1 km,in these surveys.Compared with the other ground-based electromagnetic systems,the GAFEM system has higher detection efficiency and has adaptability to detect in the survey area with complex terrain.Compared with the time domain or the existing frequency-domian SAEM systems,the GAFEM system also has a wider detection range,a larger detection depth and a higher detection efficiency.To sum up,the motion moise suppression techonologies proposed in this paper improve the system's technical index,expanded the detection application scope of the method,and laid a foundation for the further development and promotion of the GAFEM method. |
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