Geologic Characterization of the Deep Gneissic Bedrock at Chalk River Laboratories (Ontario) Using Oriented Drill Core and Integrated Borehole Surveys

Abstract

As part of an overall feasibility study, multidisciplinary surface and borehole investigations are being conducted on the Chalk River Laboratories (CRL) property in order to characterize the rock mass and to assess the technical viability of siting a deep Geologic Waste Management Facility (GWMF) for permanently managing CRL's low- and intermediate-level radioactive waste at a reference depth of 500-1000 m. The current borehole program has included the drilling of seven cored boreholes to lengths of up to 1200 m and vertical depths of one kilometre. This paper presents some of the borehole techniques used to obtain the data required for bedrock characterization and the results of geologic interpretations.

Detailed logging of oriented core provides the most direct information on fracture and lithologic characteristics and is supplemented by data from borehole geophysical surveys that include:

• 1. electrical logs (normal and single-point resistivity, and magnetic susceptibility),
• 2. natural gamma logs,
• 3. fluid temperature and resistivity logs,
• 4. 3-arm caliper logs,
• 5. image (optical and acoustic televiewer) logs, and
• 6. flow-meter logs.

The electrical logs and natural gamma logs corroborate the visual generalization of rock types and fracturing into lithostructural domains and assemblages. Some fracture filling minerals are preferentially distributed in the broader rock units. Pyrite fracture filling occurs preferentially in a garnetiferous quartz-monzonitic assemblage, which has a low magnetic signature, whereas hematite filling occurs predominantly in a magnetite-rich and garnet-poor granitic assemblage. Graphite-bearing rocks are reflected by a markedly low resistivity. Diabase dykes, represented by positive magnetic anomalies at surface, have similar signatures in borehole intersections.

Fracture orientation patterns determined from borehole image logs are similar to those from core measurements, but are more tightly clustered. There is a predisposition for a brittle fracture set to develop along pre-existing low-dip ductile fabrics although other fracture sets also occur. The identification of open fracture locations, used to select intervals for hydraulic tests, is more confidently established using caliper and image logs as compared to drill core logging, although the core is required to determine the mineralogical character of these features. Heat-pulse flow- meter logging can confirm the hydraulic significance of fractures. Borehole fluid properties are also used to locate open fractures in the boreholes. The core logs and geophysical survey logs supplement each other and provide independent verification of rock mass conditions.