Imaging ahead of a tunnel boring machine with DC resistivity: A laboratory and numerical study

Abstract

Tunnel Boring Machines (TBMs) are used for tunneling and underground construction, by excavating material and subsequently installing a segmental concrete tunnel liner for support. However, unknown ground conditions pose a significant risk to tunneling operations and any damage to the machine can be disastrous to a project. There is a need for tools which look ahead of the TBM for potential hazards during tunneling, such as water saturated zones, faults, boulders and metal pipes. Geophysical methods offer the capability to image unexcavated material in order to avoid such hazards and thus improve tunneling operations. In particular, the DC resistivity method is useful because it is sensitive to a large range of conductivity variations in geological and man-made materials.The research presented in our paper consists of three parts: (1) a laboratory study of a DC resistivity system mounted on a scale model TBM within a simulated tunneling environment, (2) a series of forward models studying different DC resistivity survey designs, and (3) the inversion and imaging of synthetic DC resistivity data under different constraints. We introduce several new survey designs that attach electrodes on a probe (or probes), which are then pushed into the earth in front of the machine each time excavation stops. Our laboratory data and forward modeling results show that using probes reduces interference caused by the metallic TBM body, and increases the distance ahead of the machine at which a target may be detected. The TBM influence on the data is significantly reduced once the probe is pushed 25% of the TBM diameter ahead of the machine and negligible once the probes are pushed 50% ahead of the machine. Depending on the specific survey design, targets can be detected from up to 70% of the TBM diameter away. Finally, we invert synthetic data to produce ahead-of-tunneling images using different amounts of prior information (e.g. TBM geometry and host resistivity) and also study time-lapse inversion. Numerical results show the target can be imaged with these methods from distances up to 45% TBM diameter.