Fracture characterization in granite using ground probing radar

CIM Bulletin, Vol. 83, No. 940, 1990

Arthur L. Holloway and James C. Mugford, Atomic Energy of Canada Ltd., Whiteshell Nuclear Research Establishment, Pinawa, Manitoba

The Underground Research Laboratory (URL) is located in southeastern Manitoba, in the Lac du Bonnet granite batholith of the Canadian Shield. It was constructed by Atomic Energy of Canada Limited (AECL) to conduct geotechnical research for the Canadian Nuclear Fuel Waste Management Program, assessing the concept of permanent disposal of nuclear fuel waste deep in plutonic rock. One of the primary objectives of the research program at the URL is to develop a nondestructive rock mass characterization method that can be used during siting, design and construction of a used fuel waste disposal vault. An experimental program was established to assess radar reflection profiling as a technique for fracture characterization in granite. The highly resistive, homogeneous nature of the Lac du Bonnet granite is ideal for the propagation of electromagnetic energy at the radar range of frequencies, and water-filled fracture zones act as excellent reflectors because of their contrasting electrical properties. Radar reflection profiling was undertaken to characterize a major fracture zone located between 5 m and 10 m below Room 211 on the 240 m level of the URL. The profiling was done at a frequency of 120 MHz, on a 1 m grid laid out on the bare rock floor of the room. Reflectors identified in the analog data were correlated between the profile lines, digitized, and a three-dimensional surface for each recognizable reflector was constructed. These surfaces were then compared with data obtained from the examination of numerous cores from adjacent boreholes intersecting the fracture zone. A very good correlation was found between the major radar reflectors and the most intensely fractured regions of the zone. Modelling studies also showed that a series of prominent shallow radar reflectors were caused by a borehole drilled close to Room 211.

Mots Clés: Rock mechanics, Underground Research Laboratory (URL), Radar reflection, Granite fracturing.