Abstract
An analysis of data from our deployments and ground truth collection
in northern Fennoscandia and northwestern Russia shows systematic
variations in the P/S ratios of different types of explosions. The
fact that this fundamental discriminant varies with firing practice
is not in itself surprising - such variations probably contribute
to the spread in P/S ratios normally observed for ripple-fired explosions.
However, the nature of the variations is sometimes counterintuitive.
Last year (Harris, 2003), we found that the P/S ratios of small compact
underground explosions in mines of the Khibiny Massif are systematically
lower than the P/S ratios of large ripple-fired surface explosions.
We had anticipated that smaller underground shots would be more like
single well-coupled explosions, thus having higher P/S ratios than
large ripple-fired explosions. We are now performing a more extensive
analysis of the data, including compact and large ripple-fired explosions
at additional mines and different types of explosions: small surfaces
shots and large ripple-fired underground explosions. Our data are
more complete as a result of an additional year of collection and
allow a more complete sampling of the signals in range from the source.
As of this writing, we have measured Pn/Lg ratios on a larger number
of explosions of three types: compact underground explosions, surface
ripple-fired explosions, and now underground ripple-fired explosions.
We find that both types of underground explosions have systematically
lower P/S ratios than surface ripple-fired shots; this effect is
most pronounced in the 4-8 Hz frequency band. This result appears
to be due to relatively diminished shear-wave excitation by the surface
explosions. We speculate that the relatively large shear phases in
underground explosions may be caused by large amounts of rockfall
in these events, which are designed to collapse the ceilings of tunnels.
We have continued comprehensive ground truth collection at the Khibiny,
Olenegorsk, Kovdor, and Zapolyarnyi mines, and now have basic information
on 2,052 explosions. In addition, we have more detailed information
for select surface and underground ripple-fired explosions and underground
compact explosions. This information ultimately may help the community
understand the observed P/S ratio offsets. We also are continuing
the compilation of ground truth and associated regional waveform
data into a database as a prelude to further analysis. We have operated
two lines of stations extending from the Khibiny Massif to the ARCES
array and to the north of the ARCES array for a second year. This
deployment provides the principal data for measuring the rangedependence
of near-regional phase amplitudes. Data collection has been more
robust in this second season of the deployment. Preparations are
now underway for concluding the line deployment in late August or
early September 2004 and for redeploying the stations to the Kiruna,
Malmberget, and Aitik mines in northern Sweden. Like the Khibiny
mines, these mines provide an interesting natural laboratory for
examining questions about the phase characteristics of mining explosions
because they include both underground and surface operations.
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