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Recent experiments using geophysical tomography in fractured granite

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1 Author(s)
Ramirez, A.L. ; Lawrence Livermore Laboratory, Livermore, CA, USA

This paper provides overviews of recent field experiments where the geophysical tomography method was used to investigate fractures between boreholes and an overview of factors pertinent to accuracy, resolution, and data interpretation. High-frequency electromagnetic waves (5-40 MHz) were used in the field to investigate a fractured granitic rock mass between boreholes separated by up to 30 m. Geophysical tomography images were reconstructed and compared with borehole geophysical data and hydraulic conductivity data. These comparisons suggest that the images are useful in identifying regions in the granite having a high moisture content due to fracture zones. A new technique involving the use of salt-water tracers to delineate fluid flow through fractures has been preliminarily evaluated. Image anomalies created by the tracer can be correlated with fracture data. This technique requires further evaluation because available fracture data are insufficient to delineate flow paths independently. When fractures are sampled from two boreholes, the resolution of the method is a function of the height-to-width ratio of the area investigated, the fracture location within this area, and the orientation of the fractures. The ratio of displacement to conduction currents, the variability of the refractive index in the rock, and the separation between transmitter and receiver are also important parameters affecting imaging results.

Published in:

Proceedings of the IEEE  (Volume:74 ,  Issue: 2 )

Date of Publication:

Feb. 1986

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