An overview of ahead geological prospecting in tunneling

Abstract

Prediction of abnormal geological structures ahead tunnel drilling (ahead geological prospecting) has become an essential routine work in the tunnel excavation, offering particularly important priori information to ensure safe, economic and efficient tunneling. During the last 50years, ahead prospecting has evolved from drilling and geological analysis to include geophysical prospecting and integrated interpretation. By integrated interpretation, we mean an interpretation combined of several different geophysical methods or an intra-method integration involving modifications of a single method. By analyzing the available related literature on this topic, a detailed overview is provided. We provided a review of the principles, technical levels, trends, key problems and applications. The basic approaches, such as surface geology mapping can always provide rough and qualitative results with low resolution. Destructive in-tunnel ahead prospecting techniques have been frequently used in early ages of tunnel drilling to provide high resolution and accurate knowledge, mainly including pilot tunnel digging and borehole drilling. Logging measurements have been used to expand the use of boreholes either on earth surface or on the tunnel face. These measurements include geophysical and geochemical properties. Drilling and logging methods are destructive, time consuming and expensive. They can be justified in complicated geological scenarios and in areas that feature geo-hazards. Non-destructive geophysical methods have been developed and widely used in practical projects, including seismic methods (e.g., Horizontal Seismic Profiling method, Tunnel Seismic Prediction method, and True Reflection Tomography method), electromagnetic methods (e.g., Ground Penetrating Radar and Transient Electromagnetic Method) and electrical methods (e.g., Focused Resistivity or Induced Polarization). This paper analyzes the characteristics, advantages and applicability of a variety of techniques. Ahead prospecting should aim to determine the properties of the rock ahead of the tunnel face instead of assessing the structure. Besides, ahead geological prediction in the case of Tunnel Boring Machines (TBM) is more complicated than in the case of drilling-and-blasting methods. And, special requirements, such as rock mass parameters evaluation, also play an important role in TBM tunneling. Some ahead prospecting techniques related to the TBM-tunnel are not extensively used because the technology is not mature enough, facing some limitations in its operability. Relatively new approaches, such as 3D seismic methods, focused electrical methods and directional borehole-GPR (Ground Penetrating Radar) methods, feature higher resolution and may be better suited to complex geology. In order to address complex problems, decrease the degree of non-uniqueness and inaccuracy, the joint, multidisciplinary integrated interpretation of ahead prospecting methods is required. It can achieve relatively better results than a single prediction method. Moreover, joint inversion of the prospecting data, simultaneously inverting disparate data sets, should increase the resolution and decrease ambiguities. This has been demonstrated by using a generalized joint inversion method called structural constrained inversion. In the end, two key challenges are discussed as key problems and future research focus. One is the quick acquisition and processing of ahead prospecting data, especially in TBM tunnels, and the other is the 3D virtualization of detection results and their intellectualized interpretation based on empirical prospecting results and expert system.