RSC-Programme - Interim Report. Approach and Basis for - Posiva

46In this study, the term influence zone is used instead of fault damage zone for severalreasons. An influence zone is based on borehole information. Data used for theinfluence zone includes, not only fracturing but also different types of geological andhydrogeological data presented in the WellCad logs for each drillhole (see Figure 5-5).Hence the term influence zone includes more information, especially thehydrogeological and also hydrogeochemical, than that is generally defined in thegeological context of a fault damage zone. Furthermore, in the nuclear wastecommunity, as well as in mechanical engineering, the term damage zone describes manmadedamage to the host rock, usually fracturing, caused for instance by drilling orblasting. To avoid confusion between man-made artificial damage zones and naturalfault damage zones, the word influence zone is used for the latter. As a summary theterm influence zone is as follows:"Influence zone is used for the 3D volume around a fault or a hydrogeologicallysignificant feature. It is affected by the existence of the fault or hydrogeologicalfeature, and hence considered as a weak or transmissive part of the host rock".More details of influence zone concepts are described in Wikström et al. (2009). Theterm fault damage zone is almost synonymous with influence zone.Areas of the bedrock affected by changing stress field are faults, fault damaged zonearound the fault, local deformation zones and large fractures. The Fennoscandiancrystalline bedrock is very old, 1.8 to 1.9 billion years at the Olkiluoto site, and has beendeformed, altered and fractured in many phases. Existing faults and fractures could havebeen reactivated during the geological history (Mattila et al. 2007b). However, no newfaults formed after the last glaciations have been found in the Olkiluoto vicinity(Lindberg 2007). This hypothesis of the possible reactivation of existing structures hasbeen the basic assumption when defining the rock suitability criteria and respectdistance volumes for main bedrock features. If faults and fault damage zone (theinfluence zone), which is already influenced and having increased fracturing as well aslarge fractures, the seismic disturbance to deposition hole is avoided. Moreover, adamaged zone is often more transmissive than a so called averagely fractured or intactrock, which also helps to fulfil the target for radionuclide retention.In the central investigation area, not only is the amount of information vast, but the sitemodel (Posiva 2009a), including geological and hydrogeological models, has alsoattained a level where interpretation of major features is consistent from one modelversion to another. Furthermore, models from different disciplines support each other.For the layout determining features in the central area (Figure 5-6), due to numerousborehole intersections through each zone and sufficient existing information oncharacteristics of the zone, an influence zone can be definedThe influence zone is defined following geological, hydrogeological or geophysicalmeasurements taken from the core along the borehole in both directions. Whileinvestigating all borehole intersections of one zone, certain characteristics, such asslickenside fractures or alteration, are more distinct in certain zones, but may be missingin or less characteristic of others. Geophysical anomalies seem to be typical of somezones and are believed to represent certain properties like porosity, alteration, sulphidesor water content, unobservable in core logging. All data characterising the influencezone is evaluated together and a limit for the influence zone defined. This process isdescribed in more detail in Wikström et al. (2009) and example is shown in Figure 5-5.This work includes expert judgment. More rules for definition will be developed in the