EXPLOSION PHENOMENOLOGY REVEALED BY FIBER OPTIC DISTRIBUTED ACOUSTIC SENSING MEASUREMENTS AT THE SOURCE PHYSICS EXPERIMENTS PHASE II (DRY ALLUVIUM GEOLOGY) CHEMICAL EXPLOSION SERIES (Invited Presentation)

Fiber optic distributed acoustic sensing (DAS) technology has enabled a cost-effective way to vastly increase the number of ground-motion sensors in any given project. In the case of the Source Physics Experiments (a series of buried chemical explosions conducted at the Nevada National Security Site), we were able to increase the number of ground motion sensors deployed in the source region by over two orders-of-magnitude. The increased number of sensors, and the consequent decrease in spatial aliasing, has allowed for an unprecedented look at the effects of large chemical explosions in an alluvium geology. With the ability to track continuous wavefields, phenomena such as surface spall, source-region damage, and post-explosion microseismicity was observed with much higher fidelity.

The deployment at the Source Physics Experiment Phase II (Dry Alluvium Geology or DAG) site consisted of one surface and two borehole deployments. The two 385-m deep boreholes were 80 meters from the explosives borehole at two azimuths and recorded four explosions ranging from 1,000 to 50,000 kg TNT-equivalent. The explosions were buried from 50 to 385 meters deep, all in alluvium above the water table. These boreholes featured engineered fiber optic cables in which the fiber was wrapped in a helix around a central core. This increases sensitivity to dynamic strain from varying angles of incidence. The surface cable, which was installed in a shallow trench, extended from 40 meters from surface ground zero to just over 2 km. Average channel spacing for the deployment was under 1 meter.

In addition to discussion of spall, S-wave generation, and damage, we will also demonstrate improvements in the characterization of post-explosion seismicity. The DAG-2 explosion (50,000 kg TNT-equivalent at 300-meters depth of burial) exhibited hundreds of microseismic events. The events have been previously located by a surface array of traditional accelerometers and geophones. The inclusion of the DAS data significantly improved the location of these events, especially the depth localization. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.