Forward Modeling On The Advanced Detection Of Tunnels By Using The Method Of Aggregated Current Induced Polarization

At present,the proportion of tunnel construction using TBM(shield tunneling machine)is increasing.Compared with the tunnel constructed by the traditional drilling and blasting method,the TBM machine occupies most of the space in the narrow tunnel,which makes the traditional tunnel advanced detection method originally applicable to the drilling and blasting method no longer applicable to the advanced detection work of the TBM method due to the unsatisfactory equipment layout space.However,there are few advanced detection methods for the construction of tunnels by TBM method at present.To solve this problem,this paper makes a detailed study on the forward modeling problem of the concentrated current frequency domain induced polarization method.Starting from the boundary value problem of point power supply,the functional problem is derived by the weighted residual method,and the variational problem is solved by the finite element method.Based on the principle of electric field superposition,the current accumulation effect is realized by combining multiple same source.By using C++ object-oriented programming technology and integrating Intel MKL framework and Eigen open source linear algebra algorithm library,a 2.5d finite element forward modeling program was written.A two-layer geoelectric model with analytic solution was used to verify the correctness of the forward modeling program,and a multi-source model with different electrode distances was designed for multiple groups of ground surfaces and tunnels.The validity of the focusing current was illustrated by comparing and analyzing the electric field distribution of different electrode distances.This paper generalizes the complex and changeable geological structures encountered in the actual tunnel construction,USES the forward simulation results of the generalized model to verify the effectiveness of this method,and explores the variation rules of the percented-frequency effect and normalized potential parameters obtained from the simulated measurement.Gathered in current induced polarization method in the process of the forward modeling of advanced detection in tunnel design,multiple sets of forward model and cavity model from a single fault,found in the forward modeling results of this method in the water tunnel construction distance more close its detection sensitivity,the greater the detection range by abnormal body scale,resistivity,and the impact of factors such as location.From the combination of fault and cavity model found in forward modeling,the abnormal experience of tunnel face close to shield its rear abnormal body of information,for the exist in abnormal side,gather the current method can effectively suppress the interference of the side anomalous bodies and the measurement results can well reflect the abnormal body tunnel in front of the target.The forward analysis above shows that the method of setting multiple electrodes to form current is effective.The method has the advantages of suppressing side interference and improving detection accuracy.In general,the effective detection distance of this method is 20 m in front of the tunnel,which can meet the requirements of the actual advance detection of the tunnel.Forward modeling is the basis of inversion interpretation,and the above contents provide a theoretical basis for further inversion of focusing current frequency domain induced polarization method.