Characterization of fractured rock in the vicinity of tunnels by the swept impact seismic technique

Abstract

The diversity of the applications of high-resolution seismics requires investigations in the range of hundreds of meters to be performed in very diverse conditions. We found the swept impact seismic technique (SIST) to be a viable solution for high-resolution surveys in hard rocks. Swept impact seismic technique combines the Vibroseis swept frequency and the Mini-Sosie multi-impact ideas. Several variations of the method have been studied leading to improved resolution and efficiency. The development was partly funded by the Finnish Center for Technical Development (TEKES) and partly by the French National Agency for the Management of Radioactive Waste—ANDRA. A test programme was carried out at the Grimsel test site, operated by NAGRA—the Swiss National Cooperative for the Disposal of Radioactive Waste. Measurements were also carried out with single-pulse sources, but data of acceptable quality could not be obtained. Surface and tunnel-wall, as well as borehole SIST sources have been developed and tested. The ability of the seismic techniques to detect and characterize rock discontinuities was proven by investigating a rock block delimited by two parallel boreholes and a tunnel perpendicular to them, involving source–detector distances of 100–200 m. The characterization included the determination of the 3-D positions and orientations of rock features and the tomographic mapping of seismic velocities.