Local P-wave amplitudes from explosive sources from the Foulness Seismo-acoustic Coupling Trials (FSCT)

Abstract

Over the past century, global seismic monitoring has allowed us to detect powerful signals
such as terrorist attacks and clandestine nuclear bomb tests. These near surface explosions
generate seismic and acoustic waves with the energy partitioning each to the atmosphere or
the Earth being typically determined by two factors: yield and Height of Burst or Depth of
Burial (HOBDOB). Several experimental studies have been conducted in the past to explore
the changes in seismoacoustic coupling using explosives of varying yields, detonated at small
HOBDOBs (<8m above or below the Earth’s surface) with recording equipment arranged at
regional distances (< 10km). These have yielded empirical models for calculating yield and
HOBDOB, but have largely been limited to dry, alluvial environments.
To supplement previous investigations, explosive tests were carried out in Foulness, Essex
as part of the Seismoacoustic Coupling Trials (FSCT) with the purpose of generating seismic
and overpressure data in a soft, saturated geological environment. Six shots had known yields
of 10 or 100kg TNT equivalent and were fired at a range of Heights of Burst (maximum of
1.39m) and Depths of Burial (maximum of 2.32m). Broadband seismometers and geophone
nodes were placed at distances between 360 - 6960m from the shotpad. In this thesis we focus
on the P-wave signals which were recorded for the shots.
We firstly measure a set of P-wave travel-times and use a seismic raytracing forward modelling
approach to produce synthetic seismograms to test and refine a velocity profile for the subsurface
in Foulness. We then analyse the initial P-wave amplitude relationship with distance and observe
a localised increase in amplitude or ’hump’ in the trend, found within 350-500m distance from
the shotpad, contrary to previous studies which exhibit a monotonic decay. We explore the
cause of this with full-waveform synthetics, using the models derived from the travel-time
analysis.
We suggest the potential cause of the ’hump’ trend to be an increasing velocity gradient
in the shallow geology of the test area, though we are unable to completely reproduce it,
suggesting further investigation is merited. We compare our amplitude measurements with the
predictions of an earlier empirical model relating measured P-wave displacement, yield and
HOBDOB, and derive new calibration parameters suitable for the soft, saturated sediments at
Foulness to improve its predictions.

Date of Award	17 Jun 2025
Original language	English
Awarding Institution	University of Bristol
Sponsors	AWE Blacknest
Supervisor	David Green (Supervisor), Stuart Nippress (Supervisor), James M Wookey (Supervisor) & Nicholas A Teanby (Supervisor)