Research On The Application Of CSEM 3D Inversion In Tunnel Survey In Ankang Area

With the rapid development of my country's national economy,in order to improve the transport capacity and running speed of railways,a railway route with a gentle slope and a large radius is a better choice.When choosing a route,considering the new railway's occupation of land resources and interference with human life,it usually crosses the mountainous area in the form of a tunnel.In recent years,the proportion of tunnels in railway routes has increased year by year,and tunnel survey is also a major and difficult point in the entire railway survey and design.The mountainous terrain with complex structure and diverse geological conditions make it difficult to effectively understand the shape and distribution of underground anomalies using traditional geological mapping.Drilling methods can effectively respond to geological information,but due to the constraints of work cost and terrain conditions,it is difficult to use in large areas.Therefore,geophysical exploration methods are widely used in tunnel surveys.Taking the geological survey of a railway tunnel in Ankang as an example,the thesis uses the controlled source electromagnetic sounding method to investigate the main line of the tunnel and the surrounding area to determine the possible dangerous areas around the tunnel route.The control source data collection work area is located in Shangluo City,Shaanxi Province,in the hinterland of the Qinling Mountains.The mountainous conditions are extremely complex.In order to meet the requirements of the exploration depth,the transmission source is set at 9.5km from the rural farmland.The length of the source is designed to be 978 m,and the wires between the power supply electrodes have a certain angle and cannot be straight.To solve this problem,this paper firstly calculates the change of the electromagnetic field value in the measurement area under the condition of irregular emission source arrangement,and then discusses the measurement data when the shape of the wire is irregular and the electromagnetic response of the horizontal electric dipole in the ideal state Whether the deviation between them has a significant effect on the observation results.On this basis,the area-controllable source electric field data collected in the work area is further processed and calculated.Thanks to the outstanding contributions of many scholars in the controllable source 3D inversion research,the controllable source 3D inversion technology is now relatively mature.The use of three-dimensional inversion can effectively avoid false anomalies caused by one-dimensional and two-dimensional inversion,and it is more conducive to ascertain the spatial distribution status of underground geological bodies.Therefore,this paper expects to apply the controllable source 3D inversion to the tunnel survey in Ankang area under the existing technology and calculation conditions,combined with the geological conditions of the work area and the exploration and drilling records to study its actual application effect.The three-dimensional forward modeling theory used in this paper refers to the ideas of Commer et al.It decomposes the total field excited by a finite-length wire source into two parts: a primary field generated by a layered medium;and a secondary field generated by a three-dimensional anomaly.For calculation,the staggered grid finite difference method and the virtual interface method are used for direct integration to perform a field numerical calculation.The inversion method uses a kind of conjugate gradient method,nonlinear conjugate gradient method(NLCG),the advantage of NLCG is that the algorithm is relatively simple,the calculation memory requirements are relatively small,the convergence speed is fast,and it is a large-scale unconstrained optimization Common method of problem solving.Finally,this paper draws on previous scholars' research results on grid size,frequency selection,error threshold and other parameters in the inversion process,selects appropriate inversion parameters,and inverts the measured data.And the accuracy of the 3D inversion results was verified based on the regional geological profile,surface exposed rock mass and borehole core data;the stratigraphic sequence and tectonic model of the work area were established;the measurement of different strata from the Sinian to Devonian in the work area was summarized Lithology.The three inversion results were compared with the collated drilling data and geological map of the construction area.By analyzing the 3D resistivity model obtained by inversion and cross-section slices along the survey line,this paper provides a comprehensive geological interpretation of the three-dimensional inversion results of the controllable source electromagnetic method in each work area,dividing strata,faults,and identifying electrical anomalies Zone and risk zone.In this paper,through the calculation of the influence of the shape of the wire on the electromagnetic observation of the controllable source,the three-dimensional inversion study of the measured data of the controllable source draws the following conclusions:(1)The degree of change in the shape of the source wire is within 1/4 times the distance from the power supply electrode,and the influence caused by the shape of the wire can be ignored;(2)Combined with the geological data and drilling data of the work area,the three-dimensional inversion resistivity model of controllable source can better divide the lithology interface and the location of the fault,and accurately reveal the underground structure;(3)According to the three-dimensional inversion results of the controllable source,with reference to the borehole data,geological data of the work area,and the results of the survey,the risk identification and the danger zone are divided along the tunnel.