Xiu Li

Three-dimensional Modeling of Transient Electromagnetic Responses of Water-bearing Structures in Front of a Tunnel Face

We present a finite-difference time-domain (FDTD) approach for the simulation of threedimensional (3-D) transient electromagnetic diffusion phenomena for the detection of waterbearing structures in front of a tunnel face. The unconditionally-stable du Fort-Frankel difference discrete method is used and an additional fictitious displacement current is introduced into the diffusion equations to form explicit difference equations. We establish a new excitation loop source which considers Maxwell’s equations in source media to overcome the limitations of the precondition that the near-surface resistivity of the model is uniform in the well-known 3-D FDTD algorithm demonstrated by Wang and Hohmann in 1993. The algorithm has the ability to simulate any type of transmitting current waveforms and arbitrarily complicated earth structures. A trapezoidal wave is used to simulate a step-off source. The fictitious permittivity is allowed to vary during the computation to ensure the stability and optimize an efficient time step. Homogeneous full-space models with different resistivities are simulated and compared with the analytical solutions to demonstrate the algorithm. Transient electromagnetic (TEM) responses of a tunnel with and without a water-filled vertical fault in front of the tunnel face are simulated and compared. 3-D models with water-filled fault and karst caves in front of a tunnel face are simulated with different parameters considered.

Multi-component and multi-array TEM detection in karst tunnels

Emerging applications of transient electromagnetic methods (TEM) in tunnelling require higher resolution on the distributions and shapes of low resistivity bodies, such as karst water and karst pipes, using multi-component and multi-array receivers. However, there are no apparent resistivity definitions for both vertical and horizontal components with offsets inside the loop. Although the raw field can show the differences of the earth electric structure, it is not straightforward. Apparent resistivity is very convenient and easy for engineers. We have developed a method for multi-component and multi-array TEM which can be applied in tunnelling and defined the expressions of apparent resistivity. This method takes advantage of the difference in resolution among components. A homogeneous half-space model and four typical three-layered models are used to test the effectiveness of the new definition. A field case history is carried out and analysed to demonstrate the viability of this technique. The results suggest that it is feasible to use the technique in tunnelling, especially for identifying the spatial distribution of karst water and karst pipes.

Practice of TEM tunnel prediction in Tsingtao subsea tunnel

Summary Introduce the TEM tunnel prediction and its application in Tsingtao subsea tunnel. Give an introduction about the apparent longitudinal quadratic differential conductance imaging method. It has the characteristic of more sensitivity to geological surfaces. Finally, give a field test case of TEM prediction in Tsingtao subsea tunnel. The result forecast two water bearing structure ahead of the tunnel face successfully.

Inverse transformation algorithm of transient electromagnetic field and its high-resolution continuous imaging interpretation method

We introduce a new and potentially useful method for wave field inverse transformation and its application in transient electromagnetic method (TEM) 3D interpretation. The diffusive EM field is known to have a unique integral representation in terms of a fictitious wave field that satisfies a wave equation. The continuous imaging of TEM can be accomplished using the imaging methods in seismic interpretation after the diffusion equationxa0is transformed into a fictitious wave equation. The interpretation method based on the imaging of a fictitious wave field could be used as a fast 3D inversion method. Moreover, the fictitious wave field possesses some wave field features making it possible for the application of a wave field interpretation method in TEM to improve the prospecting resolution.Wave field transformation is a key issue in the migration imaging of a fictitious wave field. The equationxa0in the wave field transformation belongs to the first class Fredholm integration equation, which is a typical ill-posed equation. Additionally, TEM has a large dynamic time range, which also facilitates the weakness of this ill-posed problem. The wave field transformation is implemented by using pre-conditioned regularized conjugate gradient method. The continuous imaging of a fictitious wave field is implemented by using Kirchhoff integration. A synthetic aperture and deconvolution algorithm is also introduced to improve the interpretation resolution. We interpreted field data by the method proposed in this paper, and obtained a satisfying interpretation result.

Technology of transient electromagnetic synthetic aperture method in tunnel prediction

Summary As an electromagnetic induction method, TEM (transient electromagnetic method) is sensitive to low resistivity water-filled fracture zone, and has made some achievements in the advanced prediction. However, as the geological and hydro geological conditions encountered in the excavation of buried long tunnel become more and more complicated, it is difficult to forecast the water hazard problems in the construction process of complex geological conditions, so new interpretation methods that fit to such conditions is in urgent need. In this paper, the principle of transient electromagnetic synthetic aperture method is introduced. Firstly, we get longitudinal conductivities with equivalent conductive plane method. Then, by borrowing ideas from synthetic aperture radar, correlation superposition can be used in the processing of differential conductance signals. Lastly, draw profiles with synthesized signals and give a joint interpretation combined with apparent resistivity sections. The results of imaging and interpretation for measured data have shown that this method is sensitive to geological disasters in front of the tunnel face, and compared with previous methods, the resolution has improved greatly.