DAS monitoring of large and small chemical explosions in variable geologic media

We compare Distributed Acoustic Sensing (DAS) observations from chemical explosions in dry alluvium and in rhyolitic tuff. The Dry Alluvium Geology (DAG) experiment took place as part of the Source Physics Experiment at the Nevada National Security Site in a thick layer of homogeneous alluvium well above the water table. The Blue Canyon Dome (BCD) experiments are taking place in the Socorro mountains of New Mexico where the geology of the testbed is predominantly composed of rhyolitic tuff. Optical televiewer, DAS and cross-hole seismic data reveal the presence of a sub-horizontal fracture zone that extends throughout the BCD testbed, which has a strong influence on explosion-induced wave propagation. Here, we compare DAS strain-rate measurements from the third DAG explosion (DAG-3) and two smaller explosions that took place at the BCD testbed. DAG-3 had a yield of approximately 900 kg and was detonated 153 m below ground, and DAS observations were made in two 385-450 m deep monitoring wells 80 m away from the ground zero well. In contrast, the BCD explosions both had a yield of 5 kg and were detonated between 55 and 61 m below ground in a water filled borehole. DAS observations were made in a 90-m-deep monitoring well 35 m away. Data from the two different test sites show strong differences in frequency content, amplitude and complexity that are due to differences in both geologic structure and materials, as well as the size of the explosions. The DAG-3 data have a peak frequency between 20 and 50 Hz, while peak frequencies in the BCD data range between 50 and a few thousand Hz. The DAG-3 data show obvious P and S phase arrivals, surface reflections and surface spalling events. The BCD data are much more difficult to interpret, but contain obvious P phases, tube waves, and low-frequency interface waves traveling along the low velocity fractured layer. In both experiments, DAS data provided a more detailed picture of the wavefield compared to the more traditional point measurements (geophones and accelerometers).