| Water gushing disaster,as a kind of geological disaster often encountered in tunnel construction,seriously restricts the construction progress of the tunnel.Especially in the karst areas in China where the geological conditions are complex and varied,the groundwater is abundant,and the caves are very developed.The existence of water-bearing structures seriously threatens the safe production of tunnel construction projects.Therefore,it is of great engineering significance to carry out the detection and prediction of water-bearing structures in tunnels.As a water measurement method capable of direct and quantitative,the magnetic resonance sounding(MRS)method has improved rapidly in the international and domestic markets for more than ten years,and its research direction has gradually expanded from the ground to the tunnel.In this paper,the distribution of the excited electromagnetic field generated by the tunnel MRS method under various working conditions is calculated by establishing a three-dimensional tunnel solid model,and the factors affecting the excitation electromagnetic field distribution and the MRS response signal are analyzed.Taking Dayong tunnel which located in Anshun-Liupanshui line of Guizhou Province as an example,the feasibility of the application of MRS in tunnels is discussed,and the MRS method is verified by the engineering construction results.Comsol Multiphysics 5.2 software is used to create a three-dimensional tunnel model including surrounding rock,tunnel cavity,water-bearing structures and other elements.At the same time,a metre-level excitation coil perpendicular to the ground is laid on the face of the three-dimensional tunnel model.And the excitation coil is excited by the current at the Larmor frequency to simulate the excitation in the actual tunnel,so as to simulate the excitation situation in the tunnel.The excitation field of MRS is numerically simulated.In this numerical simulation,the influence of coil equivalent area,coil shape,thickness,location and water content of water-bearing structures on the distribution of excitation electromagnetic field is discussed,and the influence range,peak value and profile shape of excitation electromagnetic field under different working conditions are also discussed.The results demonstrate that the micro-distribution of the excitation electromagnetic field is greatly affected by the equivalent area and coil shape,while the thickness,location and water content of the water-bearing structures have relatively small influence on the distribution of the excitation electromagnetic field.At the same time,based on the excitation field in the tunnel,the MRS signal response and noise interference are further discussed.The field experiment of the Anshun-Liupanshui line Dayong tunnel section in Guizhou Province is located in the karst peak cluster geomorphology area.The geological conditions are extremely complicated.There are underground dark rivers near the tunnel,and water inrush disasters occur frequently.It is necessary to find out the water-bearing geology in the tunnel.In this paper,Dayong tunnel is used as the research object of engineering application.The engineering construction obstruction of the tunnel and the geological conditions of regional hydrology and structure are introduced in detail.The relationship between Dayong tunnel and the dark river is discussed.The tunnel is described in detail.The actual situation of coil layout,signal response and signal inversion during the magnetic resonance sounding geological prediction process was carried out,and the magnetic resonance sounding method was verified by the engineering construction results.The results show that MRS detection has satisfactory reliability in tunnel applications.In this paper,the excited electromagnetic field of MRS in tunnel is simulated and analyzed,it provides a practical reference for the application of MRS in tunnels. |
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