Chapter 3—Containing Underground Nuclear Explosions ¢ 53 Figure 3-10—Radius of Decrease in Rock Strength — —_ ——<— — en, — “ “XN Surtace Depth = 400 y—- La y Observation radius 165 Vy Micro-fractions Seismic radius s00\/y Seismic measurements and measurements taken from drili-back samples indicate a seemingly contradictory (but in fact consistent) radius of decrease in rock strength. SOURCE: Office of Technology Assessment, 1989. continue to be low outto a distance of three cavity radii, seismic velocity measurements and strength radii. The decrease in seismic shear velocity indi- tests typically show no change from their pre-shot decreased. At distances greater than three cavity planes are occasionally seen when the tunnels are cates that the rock has been stressed and the strength values, although small disturbances along bedding