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 13 μm s −1  Pa −1 . For the precursor seismic wave, a coupling coefficient of 0.16 μm s −1  Pa −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 100 m 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 25 mm s −1 will be exceeded when the peak positive pressure exceeds 480 Pa (147.6 dB) or 1 kPa (154.0 dB), respectively. Either of these levels is much higher than the current U.S. Army overpressure damage criterion of 159 Pa (138 dB). 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.
A porosity-based Biot model for acoustic waves in snow
Phase velocities and attenuation in snow can not be explained by the widely used elastic or viscoelastic models for acoustic wave propagation. Instead, Biot's model of wave propagation in porous materials should be used. However, the application of Biot's model is complicated by the large property space of the underlying porous material. Here the properties of ice and air as well as empirical relationships are used to define the properties of snow as a function of porosity. Based on these relations, phase velocities and plane wave attenuation of shear- and compressional-waves as functions of porosity or density are predicted. For light snow the peculiarity was found that the velocity of the first compressional wave is lower than the second compressional wave that is commonly referred to as the "slow" wave. The reversal of the velocities comes with an increase of attenuation for the first compressional wave. This is in line with the common observation that sound is strongly absorbed in light snow. The results have important implications for the use of acoustic waves to evaluate snow properties and to numerically simulate wave propagation in snow.
Frank Must Marry Money: Men, Women, and Property in Trollope's Novels
There is a continuing debate about the extent to which women in the 19th century were involved in economic life. The paper uses a reading of a number of novels by the English author Anthony Trollop...