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Shallow Soil Response to an Explosion With Geophones and Distributed Acoustic Sensing

Description: 

Soft sediments have been observed to respond non-linearly to large dynamic strains over a wide range of scales - from laboratory experiments at centimeter scales, to active source studies at local scales (i.e., m to km), and from measurements of ground shaking during large earthquakes at regional scales (i.e., km to hundreds of km). This study focuses on the near-field response of shallow soils to the shaking from a series of planned underground chemical explosion, which occurred at the Nevada National Security Site during the second phase of the Source Physics Experiment (SPE). We examine in detail the recordings of an explosion emplaced in weak rock which was detonated at a depth of ~300 m in the Dry Alluvium Geology (DAG) borehole with a TNT equivalent yield of 50, 997 kg (i.e., DAG-2). The explosion was densely recorded by approximately 460 geophones and 2200 Distributed Acoustic Sensing (DAS) channels within 2 km from ground zero. The frequency content of the signal recorded by both types of sensors is significantly modified by the explosion. To quantitatively characterize the temporal evolution of the shallow soil properties in the hours following the explosion, we develop a stretching method which uses the high-frequency (10-30 Hz) content of the Fourier amplitude spectra at each sensor. We observe a pronounced recovery phase following the ground shaking from DAG-2, which we interpret to be indicative of a healing process within the soils (e.g., slow dynamics). The sensors located close to ground zero (within 400 m) exhibit particularly strong modulations of several percent which then recover within 12 hours following a log-type recovery. We interpret the healing process of shallow soils to be associated with damage generated by the ground spall. Sensors located far from ground zero (>1.5 km), for which ground motion levels were moderate, show that the soil properties are not significantly affected by the explosion. Finally, co-located geophones and DAS channels display similar results, which validates the use of DAS to infer the non-linear behavior of shallow soils at local scale.

Session: Exploiting Explosion Sources: Advancements in Seismic Source Physics

Type: Oral

Date: 4/19/2023

Presentation Time: 03:00 PM (local time)

Presenting Author: Loic Viens

Student Presenter: No

Invited Presentation: 

Authors