Explosion Coupling in Frozen and Unfrozen Rock: Experimental Data Collection and Analysis

Nuclear monitoring agencies often use seismic amplitudes to estimate the yields of underground nuclear tests. Any emplacement phenomena that can alter those amplitudes and lead to bias in estimated yields must be considered in the analysis. One condition that might cause such a bias is detonation in frozen rock. Laboratory analyses (Mellor, 1971; Miller and Florence, 1991) have shown that frozen rock has faster seismic velocity and greater compressive strength than unfrozen rock. This increased strength is hypothesized to reduce the seismically estimated yield of an explosion in frozen rock. To test this hypothesis, we conducted the Frozen Rock Experiment (FRE), a series of explosions in frozen and unfrozen rock, in central Alaska during August 2006. Over 120 seismic instruments were deployed to record six detonations—three in frozen and three in unfrozen-dry media—at a wide range of distances and azimuths. The data acquired show that the frozen test site explosions had significantly larger amplitudes for all phases ( P , S , and surface waves) above 8–10 Hz. These data confirm that the frozen rock medium was stronger and resulted in a smaller seismic source radius for the explosions, thus increasing the corner frequency when compared to the unfrozen rock explosions. Between 3 and 9 Hz, the unfrozen shots produced slightly larger S and surface waves resulting in different P / S spectral ratio plots for the frozen and unfrozen shots, possibly affecting regional phase discrimination. We show that the observed amplitude differences for these shots can be effectively modeled using the Mueller-Murphy (1971) explosion source and the in situ P - and S -wave velocities for the two test sites. Differences in the velocities at the frozen and unfrozen rock test sites are caused by minor metamorphic facies changes, saturated versus dry conditions, and the presence of ice in the pores and fractures at the frozen test site. Extrapolation of the results of this study to synthetic nuclear explosions suggests there may not be significant coupling differences between explosions in frozen and unfrozen hard rock.