A new 3D observation system designed for a seismic ahead prospecting method in tunneling

The tunnel seismic ahead prospecting method is usually used to estimate adverse geology ahead of a tunnel face. The observation system plays an important role in tunnel seismic ahead prospecting, which helps to obtain accurate imaging results. Optimizing the observation system is a key issue for seismic ahead prospecting. In this paper, observation layouts are evaluated and a new three-dimensional (3D) observation system is proposed by imaging results analysis. For linear and 3D layouts, imaging result analysis in this paper indicates that the mirror artifact caused by the linear layout (Which sets sources and geophones on a straight line) can be suppressed by the 3D layout, due to its energy concentration and convergence. Moreover, reflections using 3D layout identify real abnormal bodies better than the linear layout. Moreover, conventional observation layouts often use shot point on the tunnel face or front sidewall, which makes it difficult to filter interference waves from the back of the tunnel face. To solve this problem, typical numerical examples are conducted, and imaging results of observation layouts with shot point on front sidewall, tunnel face or back sidewall are studied. Results show shot point on the back sidewall helps to separate waves, suppress interference waves and extract effective waves (reflected P-waves by discontinuities) by apparent velocity filtering method, so shot point is designed on the back sidewall to get high-quality records. To balance record quality and application convenience, a new 3D observation system is proposed with 12 geophones and 10 shot points in a special arrangement. Compared with a linear layout, practical testing and application using the new 3D system indicate the main fractured zones are accurately identified and located, which proves the reliability and practicability of the new observation system.