Towards an Improved Understanding of the Signal Generated by an Underground Explosion:Case Study – Wookey Hole Caves, Somerset, UK

Abstract:

In order to continue to improve our understanding of the dynamics of underground explosions, we analyse blast data from the recent expansion of the Wookey Hole cave system in Somerset, UK (July 2015). Such data has clear relevance and implication to the work of the Comprehensive Nuclear Test Ban Treaty Organization (CTBTO) and for verification of suspicious seismic events.

We first analyse the pre- and post-blast seismic noise in order to characterize the resonant modes of the cave systems and identify any sources of persistent noise in the data. An unexpectedly high persistent noise source at 119.47Hz is thought to relate to electrical noise from machinery near to the recording site.

With only a single three-component seismometer, ascertaining variations in the radiation patterns from the explosions was impossible. However by rotating the seismograms into their radial and transverse components, we were able to determine that both P- and S-waves were generated during the blasting. The source of these S-waves is under ongoing investigation, but scattering close to the source caused by the impedance contrast between the cavity and the surrounding bedrock, or fracturing of the bedrock during blasting, are both possibilities^[1].

Frequency analysis on a sample of the data collected shows that for each blast, the dominant recorded frequency on the East component was around 35-40Hz for both the P- and S-arrivals. Dominant frequencies on the N and Z components was much more variable, up to 120Hz. For each blast, spectrograms show significant energy up to high frequencies across the duration of the recording, up to 125Hz, showing that little attenuation has occurred.

[1] - Imhof, M. G., and M. N. Toksöz. "Scattering from tunnels near sources and their effect on seismic signatures of explosions." Bulletin of the Seismological Society of America 90.1 (2000): 229-242.