| With the aggravation of human activities,the number of geological disasters is also increasing.From 2010 to 2018,there were 115062 geological disasters(landslides,collapses,debris flows,ground collapses,ground fissures,land subsidences)in China,which caused serious casualties and economic losses,and restricted social development and economic construction.Among them,the occurrence of landslide,collapse and debris flow accounts for 96.8% of the total number of geological disasters,which is the key content of geological disaster prevention and control.The monitoring and early warning before and after the occurrence of geological disasters and the accurate detection of geological conditions(geological structure,stratum thickness,damage location and scope,etc.)are the basis for the mechanism research and prevention of geological disasters.By exploring and studying the geological conditions of the areas prone to geological disasters,we can master the actual stratigraphic distribution and geological structure distribution of the area,and provide the most authentic data for the analysis and evaluation of the stability of the study area?Because the occurrence of geological disasters mainly affects the stratigraphic structure and distribution of the shallow underground.In the actual exploration process,it is necessary to use the geophysical method with high resolution in the shallow layer for exploration research.However,in the area where geological disasters occur,the geological environment is relatively complex,and there are certain requirements for the detection depth and accuracy,resulting in the vast majority of applications of geophysical methods can not be implemented(such as electrical method and seismic method),even if it can be implemented,the work efficiency is not high.In this case,a fast and efficient geophysical exploration method is needed.Although the traditional airborne transient electromagnetic method is less affected by the terrain conditions and has higher working efficiency,it has become the choice of mineral exploration,hydrological environment investigation,geotechnical engineering exploration and geological mapping.But because of its high cost,high speed,high altitude and low resolution,it can not meet the requirements of small area operation and shallow high resolution.In order to solve the above problems,this paper focuses on the research of UAV airborne transient electromagnetic shallow underground detection method.The UAV is used as the carrier platform to complete the mission of trasmitting and receiving in the air,so as to achieve high-resolution shallow underground detection.In this paper,the time domain Maxwell equations were used to discretize the differential operators on the tensor grid according to the three-dimensional finite volume method,and the divergence,curl,and node functions were discretely processed.Based on the weak form of Maxwell's equations,the discrete inner product of the staggered cells was approximately discretized,and the cell's center,face,and edge variables were discretized into three different weak forms.The boundary conditions of the ideal electrical conductor and the initial field method were determined.Without considering the displacement current,the backward Euler discretization method was adopted.The influence coefficients of forward parameters are derived to analyze and evaluate which parameters change will significantly change the solution of differential equations,and which parameters change the solution of differential equations insignificantly,and provide a reference for the subsequent electromagnetic response research.In order to study the electromagnetic response characteristics of different geological hazards,this paper constructed different three-dimensional geological hazard geoelectric models according to the actual situation.Firstly,the electromagnetic response characteristics of the three-dimensional underground uniform half-space geoelectric model were studied,and the electromagnetic response characteristics of the model were determined,which provided a background reference for the subsequent analysis of geological disaster geoelectric models.Aiming at the geoelectric model of landslides,the variation of induced current density and magnetic flux density of sliding surfaces at different positions in slices was studied.A non-horizontal layered three-dimensional debris flow geoelectric model was constructed,and the changes in induced current density and magnetic flux density at different thickness locations were studied.A high-conductivity three-dimensional irregular ground collapse cavity geoelectric model was constructed,and its induced current density and magnetic flux density were greatly affected by the shape of the high-conductivity cavity.In addition,in order to study the influence of different geological hazards on flight altitude,the time and induced voltage data at the same measurement point location and the voltage data of the entire survey line were analyzed to determine different flight altitude ranges.Noise reduction processing is an important part of unmanned airborne transient electromagnetic data processing.Based on the theoretical research and practical application,in view of the data jitter and drift caused by UAV's own and external environmental factors,sparse baseline estimation method can be used for noise reduction;percentile combined with DFT and STFT can be used for noise reduction In other words,the random noise or uncertain noise with concentrated energy distribution can be reduced;for some environmental noise or sharp pulse interference,the method of median filter can be used to reduce.Based on Python and Java language,a complete program of unmanned airborne transient electromagnetic data processing was developed.By designing the logic flow and operation interface of data processing at the bottom of the program,it can meet the requirements of flight data processing with large amount of data and achieve better human-computer interaction.By optimizing the two-dimensional and three-dimensional display algorithm,the two-dimensional and three-dimensional data can be edited quickly and efficiently.In order to verify the stability and field performance of the entire unmanned airborne transient electromagnetic system,tests were conducted in the field.The three-dimensional distribution characteristics of the shallow landslide surface were detected to evaluate the feasibility and accuracy of the method in shallow detection applications.The related processing of the data,and the comparative analysis of the data before and after the noise reduction,showed that the noise suppression data by the method in this paper can better reflect the actual situation.Through the comparative analysis of the flight acquisition data and the drilling data,the deep resistivity profile can truly reflect the formation situation.Debris flow geological hazards had also been practically applied.In the area of the debris flow source area,the depth of the impact of the debris flow and the formation conditions have been investigated and compared with the results of high-density electrical methods.The results of the method were in similar,which proved that the system can meet the actual requirements in terms of shallow detection depth and resolution. |
PDF Full Text Request