Research And Application On Multi-source Ground-airborne Transient Electromagnetic Method With Galvanic Sources

The ground-airborne transient electromagnetic method,which can achieve a balance between detection depth and working efficiency,has the advantages of both ground transient electromagnetic method and airborne transient electromagnetic method.Moreover,the method can be well adapted to detailed geological survey in areas with complex topographic conditions.However,although many companies and individuals have developed many ground-airborne systems in the past decades,and have successfully applied them to geothermal,volcanic structure surveys,deep and concealed deposit surveys,etc.,the current theoretical methods of ground-airborne transient electromagnetic method are still in the research stage.Besides,the current existing technologies are basically based on single radiation source ground-airborne transient electromagnetic to provide forward and inverse algorithms and approximate interpretation methods.Only a few articles have analyzed the influence of the field source form on the transient electromagnetic field.In addition,because the current method is basically excited by a single electrical source,the electromagnetic field excited by it can only be coupled with the geological body from a single angle.Therefore,it is difficult to obtain a holographic image of the geological body,which greatly restricts application and development of this method.Only by adopting the excitation method of multi-radiation field sources can this problem be solved fundamentally.In this context,this paper has carried out the following researches.In this paper,the forward theory of three-dimensional vector finite element method for transient electromagnetic field of ground air transient electromagnetic method with multiple radiation sources is briefly explained from four aspects: boundary value problem,vector variation equation,vector FEM and source loading.Through the first and second equations of Maxwell's equations in the time domain,the boundary value problems satisfied by the electric and magnetic fields are derived.According to the vector analysis identity,Gauss formula and boundary conditions,the weighted residual equation of the electric field governing equation is rewritten as the vector variation equation of the vector finite element method.In addition,the hexahedral element is used to divide,and the vector variation equation of the vector finite element method is discretized by the backward difference method,and the relational expression of the derivative of the magnetic induction intensity with respect to time is obtained.Finally,the loading method of multi-radiation sources is briefly explained.Then this paper has used the three-dimensional vector finite element forward simulation program to simulate and measure a single geological body first,and analyzed it from the perspective of numerical simulation.Through the comparison of the attenuation voltage multitrace map of a single geological body,it can be seen that when two radiation sources emit currents in opposite directions,the excited energy has a certain superimposition effect in space,which can effectively increase the intensity of the ground detection signal,thereby increasing Detection depth.The analysis of the field distribution map of the geoelectric model with two bulk targets shows that multi-source excitation can greatly improve the resolution ability of the ground-airborne transient electromagnetic method for the two bulk targets,especially when the electrical difference between one of the targets and the surrounding rock is small,the resolution is improved more obviously.Therefore,the multi-radiation source excitation can excite the electromagnetic field coupled with the target in multiple directions through the distributed line sources,and then the coupling information of the underground geological body in multiple directions and different heights can be obtained.At the same time,the multi-radiation field source can enhance the radiation intensity of the source electromagnetic field,suppress random noise,improve the signal-to-noise ratio of the electromagnetic response,and reduce the volume effect of the electrical source.Next,in this paper,eight complex geoelectric models of block targets are established for further numerical simulation,and the theoretical data obtained are processed for apparent resistivity.The processing results when a single field source is excited are compared with those when multiple field sources are excited together.Studies have shown that multi-radiation source excitation can be coupled with the target from multiple angles,thereby reducing the influence of volume effects,improving the resolution of complex targets,and providing theoretical reference and technical support for the multi-radiation source ground-airborne electromagnetic method for deep fine detection.Finally,this paper gives a detailed description of the actual exploration method and data processing flow of the goaf in Weijiadi Coal Mine,Gansu Province,and makes apparent resistivity processing for the measured data,forming five sections.Comparing the apparent resistivity profile drawn by single-source excitation and multi-source excitation,it can be clearly found that multi-source excitation is helpful to improve the detection and recognition ability of coal goaf,which lays a solid foundation for the application of multi-radiation source ground-airborne transient electromagnetic method to practical exploration.