Seismic coupling of surface waves in clay sediments from subsurface and subaerial explosions

Abstract

Chemical explosions close to the surface generate waves that travel through the atmosphere as blast waves and through the ground as seismic waves. The amount of energy converted into seismic energy is determined by the height-of-burst (or depth-of-burial) as well as the geological medium beneath (or surrounding) the explosion. Near surface explosions can be better characterised by understanding the wavefields produced and the corresponding variations in seismic coupling induced by different geological media. Forensic evaluations of unknown explosions depend on understanding this seismic coupling.

Particularly little is known about seismoacoustic coupling for near-surface explosions above and within saturated sediments, which are common environments in coastal and estuary settings. To fill in this knowledge gap, five 100kg charged TNT-equivalent explosions (between 1.39m above and 2.32m below ground) were carried out and recorded at four stations at 1.2–3.3km distance. Here, we analyse the surface wave portion of the resulting waveforms. We identify three distinct components: the expected Rayleigh wave, an inversely-dispersed wave and an air-to-ground coupled wave.

To understand the propagation pattern observed, and gain an insight into the velocity structure, we apply a range of forward modelling approaches to model different characteristics of the surface wave packet. We use the wavenumber integration method to generate synthetic seismograms, which we compare with analytically-predicted group velocity curves. This is supplemented with horizontal vertical spectral ratio (HVSR) modelling. Together, these approaches converge on a 1D velocity model which reproduces the features observed in the real data. Additional work has explored the ability to predict explosive charge weight (and/or yield) from surface wave amplitudes, and that the surface waves may be less influenced by multi-pathing when compared to the initial P-wave amplitudes that are commonly used in current procedures.

Date of Award	18 Jun 2024
Original language	English
Awarding Institution	University of Bristol
Supervisor	David Green (Supervisor), James M Wookey (Supervisor), Stuart Nippress (Supervisor) & Nicholas A Teanby (Supervisor)