## 1. Introduction

[2] It is a well-known fact that in the west-south of China, there are large numbers of mountains which lead to low economy development in the area, therefore, constructing railway is one of the most important tasks for government and designing a lot of tunnels is a good choice for railway construction. But the geological structure is very complex in the area. Especially, water-filled structure brings more difficulties to excavation. So, it is crucial to carry out tunnel prediction before excavation.

[3] At present, tunnel prediction based on geology is mainly attained through the combination of drill exploration and geophysical methods including Tunnel Seismic Prediction (TSP) [*Dickmann and Sandeter*, 1996] True Reflection Tomography (TRT) etc. However, there is no efficient way to detect water-filled geological features during tunnel based on geology prediction.

[4] Recently, the transient electromagnetic method (TEM) has been proven to be an efficient geophysical method in both environmental investigation [*Buselli et al.*, 1986; *Chen*, 1998] and mineral exploration [*Christen and Sorensen*, 1998; *Zhang and Xiao*, 2000]. Unfortunately, the required theory for Tunnel Forecasting in TEM has not been fully addressed.

[5] TEM response is sensitive to low resistivity bodies and the received signal corresponding to the peak of the secondary field can provide useful information about the target. Because TEM can provide excellent resolution of conductive layers, the imaging method is capable of detecting thin low resistivity water-filled zones (such as fractures and faults).

[6] S-inversion is an interpretation method for transient electromagnetic (TEM) data using the second derivative of the conductivity parameter based on the moving thin sheet approach. The S in word “S-inversion” is a physical parameter, i.e. the conductance of belowground medium which will need to be calculated in this paper. The “inversion” means reversion transformation. Therefore, the S-inversion means the reversion transformation for the conductance of belowground medium. The S-inversion is a fast-imaging technique for TEM data interpretation based on the thin sheet model approach in earth-surface exploration. The foundations of the thin sheet model approach for conductivity-depth imaging depend on the theory of EM induction in thin conducting plate. This theory was first developed by *Price* [1949] and *Sheinmann* [1947]. The theory for such models was extended by *Berdichevsky and Zhdanov* [1984], *Zhdanov* [1993], *Tartaras and Zhdanov* [1996], and *Singer and Green* [1998]. In this technical method, the observed electromagnetic signal is used to invert the physical parameter of belowground medium, i.e. the conductance *S*(*t*) using the algorithm proposed in this paper. Then, the deposited distribution of ground conductivity medium can be interpreted using the analysis on the magnitude and distributing of the conductance *S*(*t*).

[7] In this paper, the conductive finite plate model [*Singer and Green*, 1998; *Markku and Saurabh*, 1998; *Efthimios and Michael*, 2000] is introduced to tunnel based geological prediction. A tunnel forecasting TEM method has been developed, where the response from a transmitter coil placed vertically on the front tunnel wall are measures by the established facility system. But there is the few report about the application in tunnel predication.

[8] Tunnel forecasting TEM has been carried out to predict water-filled structure in west of HuBei province in south-west of China. Figure 1 is the view of the tunnel location where the working area crosses interline of HuBei province and Chong Qing city. The main harmful geological bodies are crack fault and water-filled cave which may lead to water-jet and mud-jet.