Interpretation of structural domains in discontinuity data from Nirex deepboreholes at Sellafield

Knowledge of the fracture network is fundamental to the fluid transport and geotechnical modelling necessary to assess the suitability of a rock mass to host a potential underground repository for the disposal of radioactive waste. However, in the early stages of a site investigation programme, site -specific fracture data are frequently available only from boreholes. This was the case at Sellafield, where characterization of borehole fracture dat a presented a number of inter-related problems including the volume of data , two data sets for each borehole and borehole sampling effects. A solution to the presentation and interpretation of such data was found in simple gr aphical display which allows rapid appraisal of a range of fracture attribu tes in relation to depth, lithostratigraphy and structure or in any other c ontext. The succession sampled by the boreholes in the immediate vicinity of the pr oposed repository at Sellafield comprises three major units. The Borrowdale Volcanic Group, of Ordovician age, is unconformably overlain by a Permo-Tr iassic cover sequence com prising up to 100 m of sedimentary breccia (Brock ram) succeeded by c. 450 m of the fluvial St Bees Sandstone to rockhead, Fr acture data from these are available from logging of the borehole core and from interpretation of resistivity and acoustic images of the borehole wall , providing accurate frequency and orientation information respectively. Th ese show that the discontinuity character of each of the major units is dis tinctive, indicating overall lithological control on the fracture network. Fracture frequency in the sedimentary cover rocks is relatively low, especi ally in the Brockram, and orientation patterns are dominated by bedding. Th e fracture frequency in the volcanic rocks is generally high and orientatio n typically very variable in the volumetrically dominant welded ignimbrites . However, interbedded units of non-welded lapilli tuff and breccia have lo wer fracture frequency and simpler orientation patterns. In all units, vari ations in fracture frequency and orientation character on a scale of a few metres to 100 m can be correlated with fault intersections indicating a str uctural overprint. Borehole sampling effects are illustrated by comparison of data from differ ently orientated boreholes and the underlying causes reviewed. Orientation bias is a well known problem usually addressed by weighting according to th e probability of the borehole intersecting a planar feature in a particular orientation. However, data from borehole wall imagery are subject to addit ional sampling effects which reduce the effectiveness of such weighting. A different approach is suggested, using the probability density function of the population from non-censored parts of the sample.