Chapter 3—Containing Underground Nuclear Explosions « 53 Figure 3-10—Radius of Decrease in Rock Strength —_ —_— “ Surface Depth = 400 y>— y Observation radius 165 Vy Micro-fractions Seismic radius 500N/y_ Seismic measurements and measurements taken from drill-back samples indicate a seemingly contradictory (but in fact consistent) radius of decrease in rock strength. SOURCE: Office of Technology Assessment, 1989. radii. The decrease in seismic shear velocity indi- radii, seismic velocity measurements and strength tests typically show no change from their pre-shot decreased. At distances greater than three cavity planes are occasionally seen when the tunnels are continue to be low out to a distance of three cavity cates that the rock has been stressed and the strength values, although small disturbances along bedding