Chapter 19
Characterization of Fracture Networks Using Data from Boreholes: An Example from the Concealed Borrowdale Volcanic Group in West Cumbria, U.K.

Robert P. Barnes
British Geological Survey
Edinburgh, U.K.

Stephen J. Dee
Badley Technology Ltd.
Hundleby, U.K.

David J. Sanderson
T. H. Huxley School of Environment
Earth Sciences and Engineering
Imperial College, London, U.K.

ABSTRACT

The Borrowdale Volcanic Group (BVG) in west Cumbria, northwestern England, comprises a thick sequence of pyroclastic rocks dominated by relatively homogeneous, welded ignimbrite units. Between 1989 and 1997, the group was investigated intensively by UK Nirex Ltd. as the potential host rock for an underground repository for radioactive waste. At the proposed repository location, near Sellafield, the volcanic rocks are concealed beneath about 450 m of Permo-Triassic cover. Site-specific fracture data were acquired from logging of cores and from processed electrical and acoustic wall images of 28 vertical or steeply inclined boreholes. Cores and imagery produced large fracture datasets, but the datasets could not easily be combined for purposes of interpretation. A simple graphic method allowed rapid appraisal of a range of fracture attributes in relation to depth and geological context. However, full characterization of the fracture network relied on orientation data derived from borehole imagery.