Three-dimensional analysis of fractured zones ahead of tunnel face using seismic reflection

Abstract

Recently, a geophysical exploration technology is more frequently utilized in the field of civil engineering such as underground construction, subway construction, subsurface investigation, etc., as well as resource survey for mining. Two 3-dimensional data processing techniques to predict the fractured zone ahead of tunnel face by tunnel seismic survey were proposed so that the geometric formation of the fractured zone could be estimated, i.e., the angle between tunnel axis and discontinuity zone, and the dip. The first 3-dimensional data processing was developed based on the principle of ellipsoid. The input data for the 3-D migration can be obtained from the 2-dimensional tunnel seismic prediction (TSP) test where the TSP test should be performed in each sidewall of a tunnel. The procedure of 3-dimensional migration is as follows: (1) conduct the 2-dimensional migrations independently based on the principle of ellipse with the data obtained in each sidewall; (2) determine the 3-dimensinal discontinuity plane which simultaneously contacts two cones which are made by rotation of the 2-dimensional discontinuity plane to the axis of receiver-sources line. The second 3-D migration technique was developed based on the concept of wave travel plane. The data needed for this migration technique can be obtained from the receivers installed in each sidewall of tunnel while the sources are operated only in one sidewall. The procedure of 3-dimensional migration is as follows: (1) based on the principle of ellipse, conduct the 2-dimensional migration using the data obtained from the receiver, R1, installed in the same sidewall where sources were operated; (2) based on the relationship between discontinuity plane and wave travel plane, perform the 3-dimensional data processing using the data obtained from the receiver, R2, installed in the opposite sidewall to the receiver R1. A numerical simulation using finite element method was performed to demonstrate the applicability of the proposed migration technique. For numerical modelling, wavelet type, element size, analysis time step, rise time of blasting and boundary condition were selected through initial simulation. Example problems on a hypothetical site showed the possibility that the dip angle obtained from the 2-dimensional TSP could be significantly different from the one obtained by the 3-dimensional data processing technique proposed in this paper. New migration technique was applied to an in-situ tunnelling site. The test tunnel for tunnel seismic prediction was 'Jukryung' tunnel at Kyungbuk province in Korea. Two twin tunnels were constructed in parallel and separated by 30 m (center to center). Tunnel seismic predictions were carried out several times in both tunnels where fault zones were expected ahead. The 3-dimensional migration was performed using field data and compared with geological investigation results that were monitored during tunnel construction. It could be seen that even though the 2-D migration could estimate almost exactly the location of faults, the strikes and dip angles of faults were significantly different from actual geometric formation of faults. Meanwhile, the 3-D migration showed relatively similar results to the actual geometric formation of faults. Finally, it was revealed that the proposed migration technique could successfully reconstruct the discontinuity planes. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.