Excavation Damage Zone Mapping Using EDZ GPR Method

Posiva and SKB are preparing for disposal of spent nuclear fuel deep in the crystalline bedrock. Fuel assemblies are planned to be encapsulated in copper canisters and placed in wells into tunnel floor. Disposal tunnels will be constructed using Drill and Blast method which unavoidably causes EDZ, being one concern in terms of long term safety as it may constitute a hydraulic flow path. As a non destructive and effective method the GPR technique was introduced for EDZ characterization. The GPR EDZ method was tested in several field campaigns in Äspö and in Olkiluoto. High frequency GPR antenna was used in the work on lines parallel to the tunnel. On high frequencies the resistivity is highly dispersive. Detection of the EDZ is based on computing of dispersivity index in a moving window. The EDZ data can be displayed as profiles, maps or volume visualisation. The developed GPR EDZ technique proved to be a quick and effective in locating areas suffering EDZ. The EDZ was indicated to be discontinuous in character. The next stage of method development will use rock sample data and other geophysical methods in verification. Hydraulic conductivity of EDZ volume is also necessary to define.