Ultrasonic Tomography of the Excavation Damage Zone at the Onkalo, Nuclear Waste Repository Facility, Finland

Ultrasonic velocity surveys in the frequency range of 20kHz to 120kHz were carried out in the Excavation Damage Zone (EDZ) of the ONK-TKU-3620 niche, excavated through drill and blast, to obtain a P-wave tomographic image of the top 0.8 m of the tunnel floor as part of the geophysical characterisation of the EDZ. The P-wave tomography covered a three-dimensional volume of the EDZ field sampled by multiple two-dimensional planes defined by drillholes, providing 30 2-D sections along the North-South, East-West, North West-South East and North East-South West directions. A total of 24 sensors were deployed in the field with a spacing of 0.05 m, performing four surveys at different positions for each tomographic section in order to provide a homogeneous spacing of 0.05 m. The sensors were able to switch between pulsing and receiving mode and provide up to 552 ray-paths for each tomographic plane. Only high-quality cross-hole P-wave first arrivals were used to produce the tomographic velocity images. The results showed that the P-wave velocity varies between 4.8 km/s and 6.5 km/s, with higher velocities at the northern end of the EDZ field. A general trend of increasing velocities with depth was observed, with lower velocities confined to the top 0.5 m associated with a higher degree of fracturing or damage. The modest variation of velocity with direction also indicates weak anisotropy. The higher velocities in the NE-SW direction may suggest oriented fracturing developed in its normal direction NW-SE. Some low velocity anomalies near a number of drill-holes can also be observed in the tomographic image, and may be associated with local damage around the drill-holes. The combination of tomography surveys with other geophysical measurements is able to characterise the rock deformation in the EDZ and provides better evaluation of the long-term safety and integrity of the rock mass around engineered openings in the nuclear disposal facility.