EFFECTS OF FROZEN ROCK ON EXPLOSIVE COUPLING

Laboratory studies have demonstrated that frozen rock is significantly stronger than unfrozen rock, and it has been hypothesized that this increased strength can significantly alter seismically estimated yield. Weston Geophysical Corporation conducted the Frozen Rock Experiment (FRE) in central Alaska in August 2006 to provide empirical data in order to test the hypothesis. This region of Alaska has abrupt lateral boundaries in discontinuous permafrost, and we detonated 3 shots in frozen rock and 3 shots nearby in unfrozen rock ranging in size from 200 to 350 lbs. Nearly 125 accelerometers and seismometers were deployed specifically for this experiment at distances of 10 m to over 20 km. Videographic and velocity of detonation data were also collected. The data from this experiment are being analyzed to aid in determining actual yield for explosions in permafrost regions. We developed a 1-D velocity model for the test site region using refraction analyses and inversion of surface wave dispersion and receiver functions. During development of this model, we found evidence of seismic anisotropy in the shallow crust. This model, with inclusion of the anisotropy, fit the observed Pand S-wave arrivals well, but predicted a later arrival for the surface waves than was observed. Therefore, we initiated development of a 3-D velocity model for the region that could account for lateral velocity changes. Laboratory analysis of test site rock samples document the rheologic differences of the test site medium when both frozen and unfrozen. Velocities increased ~12% for Vp and Vs when the sample was frozen to below -10 °C. Elastic moduli also showed significant increases in the frozen samples. These results are in agreement with prior studies and are indicative of the increased strength of frozen rock. The explosive source is being defined in various ways. Theoretical Mueller-Murphy (MM) source spectra were generated using the test site seismic velocities determined in-situ. Ratios of the unfrozen/frozen spectra were computed. Although there are no calibrated MM rock properties for gneiss or schist, which are found in the test site, modeling indicates that MM spectra generated from a rhyolite model fits the observed data well when the differences in the test site velocities are considered. In addition, recordings on close-in accelerometers and seismometers provide data for moment tensor inversions, which help define the symmetry and phenomenology of the explosions. Dr. Charlie Sammis has performed extensive analysis modeling the damage around an explosive source. His model involves multiple zones of rock pulverization and fracturing. It is hypothesized that displacement along small fractures, integrated over the entire damage volume, can produce the large shear waves observed from explosions. The model is dependant on the rock strength, which has been shown to increase in frozen rock. We are collaborating to provide him with data to validate his model. Spectral analyses were conducted to determine the differences in seismic phases generated by the explosions. Spectral ratios of the full waveform for equal yield detonations show that frozen shots produced larger vertical spectral amplitudes at all frequencies, except between approximately 4 and 9 Hz. In this frequency range, the unfrozen shots were up to 1.4 times larger, while above 10 Hz, the frozen shots were 4 times larger. Large shear waves are observed in the FRE data. Broadband transverse components, as close as 3 km, show a large amplitude arrival just prior to the Rayleigh wave, which could be a Love or SH wave. Record sections of the Texan recordings clearly show an arrival we determined to be a shear wave. The P/S ratio appears to vary depending on the shot type and may therefore provide important information on the mechanics of shear wave generation from an explosive source. 29th Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies