44 research outputs found
Observations of multiple seismic events
Two or more dispersed wave trains each with constant amplitude will interfere giving a resultant
wave train which is amplitude modulated, if the individual waves have their principal
energies in a common frequency band and if the trains arrive with time separations small
compared to their total length. The dispersive characteristics of the trains need not be the
same. If the component trains are of comparable magnitude, the modulation due to interference
becomes significant and a "beat" phenomenon occurs. Multiple trains of dispersed
seismic surface waves may occur because of a temporal and/or spatial distribution at the
source or because of multipath propagation. Each of these causal mechanisms influences the
amplitude and phase spectra of the resultant wave train; derived properties such as phase
velocities and amplitude ratios are also influenced. In the case of multipath propagation,
wavelength dependent time delays may occur. Two cases of twin earthquakes are analyzed,
and the significant features of interference are demonstrated. In one case, estimates are obtained
for the amplitude ratio and time delay of the second shock with respect to the first.
The interpretation of seismograms and spectra influenced by multiple events is discussed
An axisymmetric time-domain spectral-element method for full-wave simulations: Application to ocean acoustics
The numerical simulation of acoustic waves in complex 3D media is a key topic
in many branches of science, from exploration geophysics to non-destructive
testing and medical imaging. With the drastic increase in computing
capabilities this field has dramatically grown in the last twenty years.
However many 3D computations, especially at high frequency and/or long range,
are still far beyond current reach and force researchers to resort to
approximations, for example by working in 2D (plane strain) or by using a
paraxial approximation. This article presents and validates a numerical
technique based on an axisymmetric formulation of a spectral finite-element
method in the time domain for heterogeneous fluid-solid media. Taking advantage
of axisymmetry enables the study of relevant 3D configurations at a very
moderate computational cost. The axisymmetric spectral-element formulation is
first introduced, and validation tests are then performed. A typical
application of interest in ocean acoustics showing upslope propagation above a
dipping viscoelastic ocean bottom is then presented. The method correctly
models backscattered waves and explains the transmission losses discrepancies
pointed out in Jensen et al. (2007). Finally, a realistic application to a
double seamount problem is considered.Comment: Added a reference, and fixed a typo (cylindrical versus spherical
Criminal Procedure and Criminal Law: Virginia Supreme Court Decisions During the 70\u27s
The purpose of this note is to examine the decisions of the Virginia Supreme Court during the period between 1970-1980 in the area of criminal procedure and substantive criminal law. Legislative changes will not be dealt with in depth except as they have affected these decisions. Because of space constraints, a complete review of all areas is impossible; therefore, review has been limited to those issues most likely to be of interest to the practicing attorney. The discussion will also attempt to establish the position of the Virginia Supreme Court on these matters in relation to the United States Supreme Court and the majority of state courts