Ground vibrations produced by surface and near-surface explosions

Abstract

Measurements of seismic signatures produced by airborne, near-surface detonations of explosive charges over a variety of ground types show two distinct ground vibration arrivals. In all cases, the earlier arrival (precursor), has a time of arrival consistent with a predominantly underground path and coupling of blast sound to the ground close to the source and is always much smaller than the later vibration, the time of arrival of which is consistent with coupling from the air blast arrival at the receiver. The ratio of the seismic particle velocity to the acoustic pressure at the surface for the air-coupled seismic wave is constant with respect to distance and maximum pressure at a given location, but varies from site to site, with values usually between 1 and 13μms−1Pa−1. For the precursor seismic wave, a coupling coefficient of 0.16μms−1Pa−1 was measured.A numerical code enabling calculations of the fields due to an impulsive source above a layered poroelastic ground is described. Predictions of the air pressure spectrum above ground and the vertical and radial components of solid particle velocity near the ground surface are found to compare tolerably well with the measured spectra and waveforms of acoustic and seismic pulses at about 100m range in seismically-hard and -soft soils and with a snow cover present. The predicted seismic responses in ‘soft’ soil confirm that the existence of a near-surface S-wave speed less than that in air is responsible for the observed ‘ringing’, i.e. a long low-frequency wavetrain associated with coupling to the dispersive Rayleigh wave. The predicted seismic pulses in the presence of the (shallow) snow cover explain the observed phenomenon whereby a high frequency ground vibration is modulated by a lower frequency layer resonance.An empirical equation relating ground vibration from explosions to distance predicts that the commonly-used vibrational damage peak velocity criterion of 12 or 25mms−1 will be exceeded when the peak positive pressure exceeds 480Pa (147.6dB) or 1kPa (154.0dB), respectively. Either of these levels is much higher than the current U.S. Army overpressure damage criterion of 159Pa (138dB). Thus in most situations damage from blast overpressure will occur long before damaging levels of ground vibration are reached, so it is likely that civilian perceptions of vibration are produced by coupling from the airblast.