The Redmond Salt Mine Monitoring Experiment: Observations of Infrasound Resonance

Abstract

ABSTRACT From October 2017 to July 2019, we collected seismic recordings of nighttime mining explosions occurring at Redmond Salt Mine. In addition to seismic data, infrasound recordings of the explosions were collected from October 2017 to June 2018. Mine operators provided logs of explosion locations during the period from May 2018 to November 2018, along with a detailed survey of the mine structure. Seismic signals from these explosions exhibit a very complex character resulting from the heterogeneous subsurface of the region. We were able to cluster the observed seismic waveforms into several groups based on waveform similarity, which were then associated with specific regions of active mining using the operator logs. Two of these clusters were associated with several explosive events that generated infrasound easily detected out to 20 km. Because all these events occurred at night, the observed similarity of the infrasound waveforms across all the stations may result from the presence of an inversion layer that traps acoustic energy near the surface. Fully understanding the influence of atmospheric and meteorological conditions on the propagation of these infrasound signals is not possible without additional surveys. However, the recorded infrasound signals exhibit a harmonic structure that appears to result from acoustic reverberations within the mine tunnels. Application of a simple model to explain these results gives a factor of two agreement between the actual and estimated dimension of a mine tunnel whose southern end is open to the atmosphere. The observation of harmonic infrasound signals highlights the increased usefulness of infrasound in monitoring at local distances, relative to monitoring at farther offsets. Although these signals cannot be reliably recorded, presumably due to the requirement of a relatively simple atmosphere at the time of recording, we nonetheless encourage the routine collection of infrasound data in local monitoring applications.