Research On The Full Space Effect Of Transient Electromagnetic Method And Iterative Inversion Method In Tunnels

Unpredicted underground water is one of the most important threat to the efficiency and safety of tunnel excavation.Transient electromagnetic method(TEM)is well known as one of the most effective techniques for underground water detection among the existing advanced geological prediction techniques.However,there are still some problems to be improved to apply TEM technique in tunnel advance detection.For example,the transmitting and receiving loops are mainly surrounded by surrounding rock media in tunnel,which is a full space condition,completely different from the half-space condition of ground TEM detection.In contrast to ground TEM detection,where an almost infinite space is available to arrange the survey points from different angles relative to the low-conductivity anomalous body,the available measurement space when employing the TEM near a tunnel face is very limited(only a few to ten meters).Therefore,it is not feasible to infer the scale of a conductive anomaly by deploying multiple measurement points in different positions.In order to solve the above problems,this thesis compared the transient electromagnetic diffusion field in full space and half space conditions,transient electromagnetic response signals with and without anomalous body conditons through the finite element software ANSYS,which proved that the whole space problem is not the simple overlay of half space but two different types of electromagnetic field diffusion:the half space electromagnetic diffusion is like the shape of a"ring",and the full space electromagnetic diffusion is like the shape of a"spinning cone".Based on the assumption of the layer model of transient electromagnetic,the mathematical analytical formula of the proportional coefficientK_full/halfof the anomalous transient electromagnetic response signals is proposed in the first time.It is discovered that the proportion coefficientK_full/halfis not a fixed constant,but is a function over factors such as times,electrical conductivities of surrounding rock,anomalous body radius and so on.The numerical simulation of 3D TEM model is established by software Comsol to verify the relations between the proportional coefficientK_full/halfand various influencing factors.Similar physical-scale model tests are used to verify the relations.A graphite cylinder with a conductivity of 125000 S/m is used to simulate the water-filled cave,and Na Cl solution with a concentration of 20%and a conductivity of 18.56 S/m is used to simulate the surrounding rock.The transmitting and receiving loops are improved and designed to better accommodate the physical similarity tests.The results of mathematical analysis,numerical simulation and similar simulation test show a consistent trend,indicating that the proportional coefficientK_full/halfis a function varying with a variety of factors between 1.0and 2.5.Iterative inversion method is proposed based on 3D FDTD forward calculations and a direction algorithm.A total of 125 groups of 3D FDTD forward calculation results are analyzed to identify the correlations between the response data and the geometric conditions of the water-filled anamolous body.A direction algorithm is established based on the correlations,thereby increasing the iterative inversion convergence speed.Employing the proposed iterative inversion method,the location and volume of the water-filled anomalous body in front of the tunnel face are successfully inverted.Through an iterative program,the inversion results of simulations involving the detection of water-filled caves under tunnel conditions are accurately analyzed,and the relative error is less than 10%.The application of the iterative inversion method to the Mingyue Mountain Tunnel project suggests that this method is capable of interpreting the size of water-filled caves and is valid for a narrow tunnel face with only a single available measurement point.The proposed iterative inversion method can be used alone or in combination with other detection techniques,thereby providing engineers with a better early warning system for detecting water-filled caves in tunnels.