Abstract
Both phased array techniques for single-component sensors and vectorial
analysis of three-component recordings can provide estimates of the
azimuth and slowness of seismic phases. However, a combination of
these approaches provides a more powerful tool to estimate the propagation
characteristics of different seismic phases at regional distances.
Conventional approaches to the analysis of three-component seismic
records endeavor to exploit the apparent angles of propagation in
horizontal and vertical planes as well as the polarization of the
waves. The basic assumption is that for a given time window there
is a dominant wavetype (e.g., a P wave) traveling in a particular
direction arriving at the seismic station. By testing a range of
characteristics of the three-component records, a set of rules can
be established for classifying much of the seismogram in terms of
wavetype and direction. It is, however, difficult to recognize SH
waves in the presence of other wavetypes. Problems also arise when
more than one signal (in either wavetype or direction) arrive in
the same window. The stability and robustness of the classification
scheme is much improved when records from an array of three-component
sensors are combined. For a set of three-component instruments forming
part of a larger array, it is possible to estimate the slowness and
azimuth of arrivals from the main array and then extract the relative
proportions of the current P-, SV-, and SH-wave contributions to
the seismogram. This form of wavetype decomposition depends on a
model of near-surface propagation. A convenient choice for hard-rock
sites is to include just the effect of the free surface, which generates
a frequency-independent operation on the three-component seismograms
and which is not very sensitive to surface velocities. This approach
generates good estimates of the character of the S wavefield, because
the phase distortion of SV induced by the free surface can be removed.
The method has been successfully applied to regional seismograms
recorded at the medium aperture Warramunga array in northern Australia,
and the two small arrays NORESS and ARCESS in Norway, which were
designed for studies of regional phases. The new wavefield decomposition
scheme provides results in which the relative proportions of P, SV,
and SH waves as a function of time can be compared without the distortion
imposed by free surface amplification. Such information can provide
a useful adjunct to existing measures of signal character used in
source discrimination.
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