A 15 kHz cross-hole seismic survey across a fracture at the AECL Underground Research Laboratory

Abstract

A 15 kHz cross-hole seismic tomography survey was completed between two boreholes that cross a wellknown subvertical fracture called the "Room 209 fracture" on the 240 Level of the Atomic Energy of Canada Limited Underground Research Laboratory. This survey measured in situ P-and S-wave velocity and amplitude relations across the fracture between the two boreholes. A velocity anisotropy of about 1.5% (peak-to-peak) exists in the rock mass for both P and S waves. Consistently faster velocities were observed for rays oriented roughly parallel to the Room 209 fracture. This anisotropy is consistent with geological and geotechnical models of microcrack populations for the region. Tomographic velocity images for both the P-and S-waves show a distinct change across the fracture. Higher velocities occur east of the fracture, whereas lower velocities occur to the west. The velocity differences indicate minor changes in the rock structure from one side of the fracture to the other which correlate with observed differences in alteration and fracture frequency. There is a slight reduction in velocity at the fracture itself. A study of seismic amplitudes was also completed. A weak anisotropy also exists in the amplitude data, with higher amplitudes for rays that parallel the fracture direction. The P-wave attenuation ranges between 0.03 and 0.7 dB/m across the survey panel.The region of the fracture shows a subtle increase in the P-wave attenuation of about 0.04 dB/m above the local background. These measurements provide a useful demonstration of the sensitivity of the technique. The results correlate well with geological models and simple physical models for seismic wave propagation in sparsely fractured granite.Résumé : Une étude de tomographie sismique 15 kHz de trou à trou a été complétée entre deux trous de forage traversant la fracture subverticale connue sous l'appellation «fracture de la chambre 209» du niveau 240 du Laboratoire souterrain de recherches, de l'Énergie atomique du Canada Limitée. Cette étude a permis de mesurer in situ les relations entre les vitesses et les amplitudes des ondes P et S de chaque côté de la fracture entre les deux trous de forage. On observe dans la masse rocheuse une anisotropie des vitesses des ondes P et S de l'ordre de 1,5% (pic-à-pic). Les rayons avec orientation grossièrement parallèle possèdent des vitesses qui sont régulièrement plus rapides. Cette anisotropie est compatible avec les modèles géologique et géotechnique représentant la densité des microfissures dans la région. Les images de la tomographie révèlent qu'il se produit en traversant la fracture un changement distinct des vitesses des ondes P et S. Les vitesses les plus rapides apparaissent à l'est de la fracture, tandis que les vitesses les plus lentes se manifestent du côté ouest. Les différences de vitesse sont dues à des changements mineurs dans la structure de la roche d'un côté à l'autre de la fracture, elles reflètent les différences observées dans l'altération et la fréquence des fractures. Il se produit dans la...

Appraisal of waveform repeatability for crosshole and hole-to-tunnel seismic monitoring of radioactive waste repositories

“…Many projects involve one of two types of underground facility: repository candidates such as the Olkiluoto site in Finland ͑Vira, Switzerland ͑Lieb, 1989;Thury and Bossart, 1999͒. Diverse seismic methods have proven to be suitable for exploring the structure and physical properties of potential host-rocks. These include 2D and 3D surface-reflection seismic imaging ͑Mair and Green 1981; Birkhäuser et al, 2001;Schmelzbach et al, 2007;Juhlin and Stephens, 2006͒, multiazimuth and multioffset VSP surveying ͑Enescu et al, 2004͒, crosshole seismic investigations ͑Vasco, 1991Tura et al, 1992;Maurer and Green, 1997;Vasco et al, 1998;Hayles et al, 1999͒, andsurface-to-tunnel studies ͑Gritto et al, 2004͒. Once the relevant host rock is characterized and judged to be suitable, effective monitoring strategies need to be devised for the various stages of a repository's lifetime ͑i.e., preexcavation, excavation, HLRW emplacement, backfilling and sealing, and 100-300 years of postclosure͒. While there is access to the repository, a wide range of suitable geophysical methods exists for monitoring the host rock.…”