Characterization of S-waves Generated from Aboveground and Underground Explosions in Alluvium

Thursday, 18 December 2014
Jessie Lafayette Bonner, Weston Geophysical, Lufkin, TX, United States, Vanessa Napoli, Weston Geophysical Corporation Lexington, Lexington, MA, United States and Robert Reinke, Defense Threat Reduction Agency, Albuquerque, NM, United States
The source of S-waves from explosions is debated within the nuclear explosion monitoring community. The HUMBLE REDWOOD III (HRIII) experiment series in 2012 located in New Mexico provides a unique dataset to further our understanding of the generation of S-waves in alluvium by comparing and contrasting co-located aboveground and underground explosions. Two 91 kg explosions were detonated in alluvium at the same location, but HRIII-1 was detonated at 2 m aboveground and HRIII-2 was detonated at 7 m belowground and was fully coupled. A semicircular seismic network of 21 stations was deployed at 1 km to record the explosions. We determined that the belowground HRIII-2 explosion generated P-waves that were 2.4x larger than the aboveground HRIII-1 shot, thus we scaled the aboveground signals by this factor. Visual inspection of the waveforms after scaling showed similar P-wave, higher mode (HM) Rayleigh-wave, and fundamental mode Rayleigh-wave amplitudes for both shots on the vertical and radial components; however the largest differences were observed on the transverse components. Additional Love wave (or SH) energy is generated by the underground explosion that is not accounted for in the P-wave scaling factor. Spectral ratios confirm that the increased SH generation occurs at frequencies between 10-100 Hz. Future work includes modeling of these waveforms to explain the larger SH waves as well as analysis of S-wave generation from a similar set of above- and below-ground explosions in limestone.