| Traditional controlled-source audio-frequency magnetotellurics(CSAMT)uses a single grounded horizontal dipole antenna as an artificial field source to radiate electromagnetic waves with weak directional coefficients.Then,the orthogonal electric and magnetic field components generated by this artificial field source in the far-field region are recorded to calculate the apparent resistivity and impedance phase of Cagniard.The premise of applying Cagniard’s apparent resistivity formula is that the electromagnetic field in the far-field region is a quasi-plane wave state that enters the ground almost vertically.In theory,as the distance from the field source increases,the plane wave effect of the field becomes more pronounced,but energy attenuation of electromagnetic waves becomes more severe.Due to omnidirectional radiation of dipole antennas and energy loss over long distances,only a small portion of radiation energy covering this area can be effectively utilized during observation in far-field region.Most of other directions’ energies cannot be effectively used.When exploring areas with strong noise or complexity,signal-to-noise ratio(SNR)of raw data obtained often fail to meet exploration needs.In order to improve the SNR in the far-field region,conventional methods usually rely on increasing the transmitter power or shortening the transmission distance.However,the maximum power of the transmitter is limited by the development of power electronics technology,and as power increases,volume and weight also increase correspondingly,which is not friendly for field construction.Shortening the transmission distance will bring many problems such as reducing exploration depth and affecting CSAMT exploration effect due to source effects.In view of the shortcomings of conventional methods in improving SNR in farfield region,this thesis mainly studies array antenna artificial sources that can perform pattern synthesis and adaptive beamforming to enhance energy utilization efficiency of CSAMT artificial sources.In this thesis,based on the interference superposition principle and the pattern multiplication of electromagnetic field,with the help of the idea of phased array,the array antenna type artificial field sources are used to replace the field source of a single grounded horizontal dipole antenna,and various array antennas and methods are used to achieve pattern synthesis and adaptive beam control.In the CSAMT exploration process,it can reduce the sidelobe level,reduce multipath interference on the receiving point,achieve adaptive directional radiation for specific areas,and improve the SNR of the area,effectively enhancing exploration resolution and accuracy.Based on analyzing lateral wave propagation mechanism and low loop impedance characteristics of a single dipole source on a uniform half-space surface,various configuration array artificial sources’ pattern synthesis and adaptive beamforming methods are deeply studied along with their applications in CSAMT including linear array artificial sources,L-shaped tensor array artificial sources and planar tensor array artificial sources etc.,detailed simulation results,implementation methods and pilot scale test verification are provided.The main research contents of this thesis are as follows:1.Based on the analysis of electromagnetic radiation mechanism of grounded horizontal dipole antenna,the concept of "shallow crossing" propagation is put forward for the propagation path of electromagnetic wave.By combining with the radiation characteristics of dipole antennas,the electromagnetic radiation mechanism of a grounded long wire artificial field source is analyzed,and the lateral wave transmission form of electromagnetic waves leaving the grounded horizontal dipole antenna artificial field source is studied.Then,the formation mechanism of far-field plane wave effects based on phased array theory is revealed.On this basis,the concept of "shallow crossing" propagation path is proposed and its formation mechanism is studied for the first time,and the research results have a good guiding significance for improving the accuracy of CSAMT inversion,correcting the field source effect and confirming the far-field region.In the meantime,a low circuit impedance design for the artificial field source of long grounded wires is proposed.By analyzing the characteristics of discontinuous impedance,thermal effect and wire reactance,a new design of electrolytic ion grounding electrode is proposed,which can reduce the contact resistance between the grounding electrode and the earth to a great extent.By using the transmission mode of parallel double wires,the impedance of long wires at high frequency can be significantly reduced.Finally,the far field radiation characteristics of the grounded long wire artificial source are analyzed,and the application of the artificial field source of grounded long wire in CSAMT is numerically simulated.Under the premise of the same transmitting voltage,the low circuit impedance design,especially in the case of high frequency,can greatly improve the current injected into the ground.2.In order to improve the energy density of the main lobe and reduce the multipath interference of the side lobes on the receiving point,a method for low sidelobe level pattern synthesis of CSAMT based on linear array artificial field source is proposed.Based on the analysis of array antenna principle and combined with the linear array model,the design of coaxial oscillator array antenna and parallel oscillator array antenna artificial field sources for CSAMT are completed.Dolph-Chebyshev synthesis method and Taylor synthesis method are applied to synthesize the directional diagram of CSAMT with low sidelobes,which reduces the sidelobe level to a minimum under a certain main lobe width.Simulation results,numerical simulations,and field experiments demonstrate that using linear array artificial field sources in combination with this method of pattern synthesis can reduce sidelobe levels,improve energy density in the main lobe,and reduce interference from sidelobes at receiving points.3.In order to improve the anti-jamming ability of exploration,a CSAMT adaptive beamforming method based on linear array artificial field source is proposed.Based on the analysis of the principles of beam scanning and adaptive beamforming methods,the design of CSAMT field source adaptive beamforming method based on linear array is completed.By processing the electric field data at the receiving end,the parameters of the array transmitting antenna can be locked adaptively according to the maximum amplitude criterion,and the purpose of receiving the maximum SNR can be achieved.Finally,several models are set up to simulate the adaptive beamforming method numerically,and experiments are carried out.The results of numerical simulation and experiment show that the method has advantages in improving the anti-jamming ability.4.Based on the research of CSAMT low sidelobe level pattern synthesis method and adaptive beamforming method of linear array artificial field source,aiming at the problems of uncontrolled radiation energy,limited exploration range and serious energy attenuation of traditional vertical separated double oscillator tensor CSAMT,a tensor CSAMT pattern synthesis and adaptive beamforming method based on L-shaped array artificial field source is proposed.Through pattern synthesis and beam scanning processing of array signals in each direction,the beam can be gathered to a specific area and the beam scanning in the exploration area can be completed,thus realizing the large range and strong amplitude detection in the exploration area.5.In order to reduce the loss of radiation energy along the path,a tensor CSAMT circular polarization pattern synthesis method based on a planar array antenna artificial field source is proposed.Based on the analysis of the principle of generating circular polarized electromagnetic waves with sequential rotation excitation,the design of a multi-dipole planar antenna array with sequential rotation excitation is completed.By establishing several simulation models,the numerical simulation of the generation of circular polarized electromagnetic waves in the whole air domain and half space are completed.The feasibility of the generation method of circular polarized electromagnetic waves is verified by combining the field test data.When the circular polarized electromagnetic wave is used in CSAMT exploration,the path loss can be reduced and the amplitude of each tensor received by the receiver can be more balanced. |
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