Fracture analysis from borehole geophysical techniques in crystalline rocks

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Abstract
Summary: Geophysical borehole logging data from three 300 m boreholes in the Strath Halladale Granite in northeast Scotland have been analysed in terms of fracture delineation. The geophysical techniques which were found to be of most use in determining the location, nature and orientation of fractures are outlined, and the results used for defining fracture indices for the rock mass. A comparison is made between different fracture indices, and a simple geotechnical assessment of the rock mass is made using fracture measurements, on the complete core, and dynamic values of the bulk modules. Specific sections of the core material are compared with the responses of the geophysical sondes to illustrate the use of borehole geophysics in determining fracturing of the rock. Potential flow zones predicted from neutron and gamma-gamma logging are compared with those measured by temperature/conductivity logging, and are related to the fracturing of the rock and to its hydraulic conductivity.