Research And Application Of AMT In Geological Prospecting Of Mountain Tunnels

Due to the complex topography and geological structure of the mountain tunnel,it will cause serious distortion to the distribution of the geophysical field,forming a false anomaly,and it is difficult to extract the effective anomaly of the probe.At the same time,the target body is buried deep,so the geophysical survey of mountain tunnels has always been a difficult problem in the engineering field.There is no relatively complete set of theories and methods for geophysical surveys of mountain tunnels.Compared with the traditional survey methods,the audio magnetotelluric sounding method(AMT)has the advantages of large depth of detection,many observation methods,high work efficiency and adaptability to the harsh mountainous environment,and has gradually become the main method for geophysical survey of mountain tunnels.This paper is aimed at common geological problems such as special terrain,stratum lithology and concealed structure of mountain tunnels.From the basic theory of magnetotelluric,forward numerical simulation,and inversion interpretation of measured data,AMT is studied in mountain tunnel exploration.Firstly,starting from the basic theory of audio earth electromagnetic field,the relationship between the wave equation of one-dimensional and two-dimensional earth electromagnetic field and the resistivity and impedance phase of the medium is systematically expounded.Based on the differential equations,boundary value problems and variational problems of the acoustic earth electromagnetic field,the finite element calculation algorithm for the two-dimensional forward modeling of the audio magnetotelluric method is derived.The two-dimensional forward modeling of the magnetotelluric is implemented by the FORTRAN90 language.Secondly,a forward model is established for the common geological problems of mountainous tunnel special terrain,stratum lithology,concealed structure and underground karst.The forward program is used to carry out numerical simulation,and the forward response of four types of geological problems is studied and analyzed,which provides a theoretical basis for the processing and inversion interpretation of measured data.Finally,an example of the investigation of the audio magnetotelluric sounding method of the WLP tunnel and the MP tunnel of the Jiumian Expressway is combined.The technical flow of the method of audio magnetotelluric method in tunnel survey is studied,and a set of method technology and interpretation technology for the audio geodetic survey of mountain tunnels is formed.It is popularized and applied in other tunnels and has achieved good application results.Through theoretical research,model forward modeling and the application of the actual work area,it is shown that the audio magnetotelluric method has the characteristics of economy,high efficiency,large depth of detection and no influence of near-field effect in mountain tunnel exploration,which can provide reference for mountain tunnel exploration.Based on the application experience,it has important theoretical and practical significance.