Three-Dimensional Transient Electromagnetic Responses Of Water Bearing Structures In Tunnels And Prediction Of Water Inrush Sources

Two parts of researches are discussed in this work on three-dimensional transient electromagnetic (TEM) responses characteristics of water bearing structures in front of a tunnel face and water inrush prediction. The first part presents a modified three dimensional finite difference time domain (FDTD) modeling algorithm for arbitrary complex models in TEM surveys. Modeling for TEM detection in tunnels is also established and responses are simulated and analyzed for typical water bearing sturctures. The second part presents a modified three dimensional TEM survey configuration using multi-component and multi-array receivers in tunnels. A possible way to interpret the acquired data is also discussed. This paper studies some key problems in the two parts.Modeling and inversion in three dimensions of ground and airborne TEM has been applied in industrial production in the United States, Canada, Europe and Australia etc. However, TEM detection in tunnels has a pseudo-homogeneous full space environment containing a tunnel cave which makes the progress of electromagnetic much complex. Current methods cannot be applied directly in three dimensional forward modeling of TEM surveys in tunnels. The key issue is the current introduced excitation source does not work for TEM detection in tunnels. This work introduces the source current dentisy into Maxwell equation according to the Ampere circuital theorem and taking the Maxwell equations in source and source free media into consideration. In fact, the modification makes the models arbitrarily complex in resistivity distribution though the computation time cost increases. The CPU+GPU heterogeneous computing is used to improve the simulation efficiency. In order to demonstrate the algorithm, a homonegeous half space model is used to compare with the analytical solution, result shows less than5%relative error in decay emf and less than3.5%relative error in late time resistivity. Then, four types of three layered earth models (A, H, K and Q) are used to compare with linear digital filter solutions. The model with3D low resistivity bodies in homonegeous half space is also used to compare with Wang&Hohmann’s (1993) original FDTD solution and Newman’s IE solution. Then, several improvements are designed to make the algorithm suitable for TEM modeling in tunnels. A parallel three dimensional modeling algorithm is then established for TEM survey in tunnels. Some typical water bearing struvtures during tunnel construction, such as vertical or tilted water-filled faults, water filled karst caves etc., are simulated and analyzed. Different influences, e.g. distances between the water bearing structures and the tunnel face, sizes of the structures, resistivity differences etc., are considered in simulation. Some responses characteristics are obtained. The responses of tunnel boring machine (TBM) is also simulated and analyzed. This present a numerical try on the use of TEM in tunnels excavated with TBM. Novel characteristics are also obtained that the horizontal components which have not been used in tunnels may be helpful in identifying large water bearing structures.A survey line is usually configured on tunnel face in traditional TEM survey and the loop is moved point by point to form a cross section. Its limation lays in the bad space distribution abilities in three dimensions. This work references the large fixed loop configuration and gives a three dimensional multi-component and multi-array configuration in tunnels. Based on the1D electromagnetic theory, this work studies the computation methods of double Bessel function integrals in frequency domain for both vertical and horizontal components with small loops. The polynomial form of central vertical frequency response is used as the basis function to define the responses with offsets. The coefficients of the polynomials are solved with a least square fitting. Time domain responses are obtained by inverse Fourier transformation. In the late time approach, the resistivity definitations are derived for both vertical and horizontal components. The acquired data can be transformed to resistivity for interpretation. A vector resistivity synthesizing method is also given based on the layered earth resistivity characteristics. A homogeneous half space model, four types of layered models and a field data are used to demonstrate the method. The field data is also verified by drilling.These two parts provide a basis for future research of TEM detection. The modified TEM configuration acquires data in three dimensions and provides a basis for future three dimensional inversions together with the three dimensional modeling.