Research On Ground-airborne Frequency-domain Electromagnetic Methods With Multiple Sources

With the urgent demands of resources exploration,environmental geological surveys and geoscience research,methods and technology for fast survey and deep exploration have become an important research direction in the field of geophysical prospecting.Ground-airborne electromagnetic methods employ the artificial source on the ground and a moving receiver in the air,which unite the advantages both of the ground electromagnetic methods and airborne methods.Thus the ground-airborne electromagnetic methods have the potential of high efficiency and large depth of detection and can be used in complex terrain region where it is difficult to enter on the ground.The ground-airborne electromagnetic methods can worked in time domain and frequency domain mode.Because of sharp attenuation of time-domain electromagnetic signal in time and space,the limited source-receiver distance restricts the efficiency and detection area.Because the ground-airborne frequency-domain electromagnetic(GAFEM)methods collect steady signal,it has the ability to realize detection of area with large depth and distance.However,recent research on ground-airborne frequency domain electromagnetic method is rare and its theoretical basis is still weak.Current detection methods and instrument is the simple transplant of the ground method and system.Theses conventional methods with single source have the disadvantages of low efficiency,low signal-to-noise ratio,low accuracy and heavy weight,which limited the promotion and application of GAFEM.In order to solve the existing problems in the conventional system and the methods,this article study on the methods and technology with multi-source excitation for the ground-airborne frequency domain detecting,realizing the high-efficiency,high-precision and high-SNR(signal-noise ratio)survey and laying the foundation on practical application of the ground-airborne frequency-domain electromagnetic method and instrumentation.In the paper,the mainly completed innovative work and research results are shown as follows:1)The forward modeling is established and implemented based on one-dimensional and three-dimensional structure for GAFEM method.For one dimensional structure,the formulas of electromagnetic response in the air generated by the dipole source and grounded wire source are derived from the basic Maxwell's equations.The program for calculating the vertical component of the magnetic field is also written.Based on the finite element method,with the help of the COMSOL software,the simulation model and calculation method of the three dimensional structure are established by setting the boundary conditions,the solver and the mesh.By comparing with the analytic solution of the homogeneous half space,the correctness of the modeling and solving method is verified.Based on forward modeling,the basic characteristics of airborne magnetic field are analyzed.The observation methods and instruments along with their limitation of GAFEM with single source is subsequently introduced and pointed out.2)The multiple sources electromagnetic exploration methods were studied to get high efficiency,high signal-to-noise ratio and high detection precision.First,to get high efficiency,we analyzed the frequency spectrum characteristic and frequency point setting method of 2n pseudo-random sequence coding waveform,and designed valuable frequency combination for multiple sources.Second,to get high signal-to-noise ratio,we calculated electromagnetic response and secondary field characteristics of three typical source incentive modes and gave the combination source design principle.Finally,the tipper amplitude and real induction vector potential on target recognition under the stimulus of single source and orthogonal sources were compared,and the advantages of multi-sources electromagnetic exploration method were further verified.The study of multiple sources laid the foundation for the electromagnetic exploration method which had high efficiency,high signal-to-noise ratio and high detection precision.3)The mutual influences of multiple sources and the suppression of the observation error are studied and set up.Double source circuit model is established and the main factors of the double source were determined.The impact of mutual inductance and mutual resistance on emission current were studied.The influence of current changes on transmitting system safety and the observation accuracy of multi-source different frequency method were analyzed.The minimum offset distance to parallel emission source was also calculated and evaluated.For the detection method of multi-source with same frequency,the article analyzed observation error in data processing of single equivalent source caused by offset.Combining with the actual parameters of the system,the suitable offset range was designed to satisfy the safety of system and guarantee the detection accuracy4)The implementation and control methods of multi-source excitation system were proposed and studied,and the field experiments of multi-source method were carried out.The structure and parameters of multi-source excitation system are analyzed.In each single portable compact transmitter,the power circuit,the low voltage detection circuit and the control circuit are designed.Based on the system,the workflow and three loop parallel control method were put forward.For multi-source synchronization problems in the work,the synchronization scheme was designed and the synchronization output of multi-source current is achieved.Based on the research and development of multiple source systems,the multi-source field experiments were carried out,and we measured the mutual influence between multiple sources.We also verified the feasibility of signal enhancement with multiple source worked in the same frequency and the efficiency increase while the multiple sources worked in independent frequency.In this paper,the calculated results of homogeneous half-space by method of GAFEM forward modeling are in agreement with the analytical result,which verifies the validity of the calculation method and formula,and lays the theoretical foundation of ground-airborne electromagnetic exploration.By comparing with the conventional single source transmitting method,the advantages of multi-source excitation in signal quality,detection efficiency and target recognition are verified.Through the analysis of the mutual influence of the multiple sources and the research of the observation error suppression method,this paper provides the theoretical guidance for the field implementation and layout of multi-source,and ensures the security of the system.Using the developed transmit system to expand experiment,which verifies the effectiveness of offset adjustment for the observation accuracy.Meanwhile,it also verifies the feasibility of multi-source and multi-frequency emission to improve the efficiency and multi-source with same-frequency emission to improve the quality of the signal.The ground-airborne electromagnetic exploration with multiple sources can make up for the shortcomings of single source emission,and raise the level of existing methods.It can provide the new ideas and technical means for the blind area exploration,high-efficiency,high-precision and large-depth survey,which are still a problem for current electromagnetic methods.The methods also lay the theoretical foundation for the development of new geophysical instruments.