Microfracturing in Postshot Gasbuggy Core GB-3

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Abstract
Microfracturing in shocked sedimentary rocks near to the 29-kt Gasbuggy nuclear explosion has been microscopically examined in samples from postshot core (hole GB-3). Of four reentered or drilled postshot holes, GB-3 makes the closest approach to the shot point (198-ft radial distance or ~2.47 times the cavity radius). The amount of fracturing in rock>200 ft from the shot point is small and shows little correlation with distance. Calculated maximum peak stresses for the rock are in the 6 to 8 kb range and are below the laboratory-measured yield strengths. Comparison of shock effects in brittle granodiorite and in the semibrittle Gasbuggy rocks at the same peak radial stresses indicates that matrix fracturing in the granodiorite is many times greater than in the Gasbuggy rocks. It points up the important role played by weak, ductile cementing minerals in the latter in determining the mode of yielding of the whole.

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