Unknown
In:
Pure and Applied Geophysics, Oxford and Edinburgh, Blackwell Scientific
Publications, vol. 159, no. 4, pp. 831-863, pp. 1484, (ISBN: 0534351875, 2nd
edition)
Publication Date:
2002
Description:
This paper reports results from two recent monitoring experiments in
Wyoming. Broadband seismic recordings of kiloton class delay-fired cast blasts and
instantaneous calibration shots in the Black Thunder coal mine were made at four
azimuths at ranges from 1° to 2°. The primary focus of this experiment was to observe
and to explain low-frequency signals that can be seen at all azimuths and should
routinely propagate above noise to mid-regional distances where most events will be
recorded by International Monitoring System (IMS) stations.The recordings clearly
demonstrate that large millisecond delay-fired cast blasts routinely produce seismic
signals that have significant spectral modulations below 10Hz. These modulations are
independent of time, the azimuth from the source and the orientation of the sensor.
Low-frequency modulations below 5Hz are seen beyond 9°. The modulations are not due to
resonance as they are not produced by the calibration shots. Linear elastic modeling of
the blasts that is guided by mine-blast reports fails to reproduce the fine detail of
these modulations but clearly indicates that the enhanced ``spectral roughness'' is due
to long interrow delays and source finiteness. The mismatch between the data and the
synthetics is likely due to source processes, such as nonlinear interactions between
shots, that are poorly understood and to other effects, such as variations of shot time
and yield from planned values, that are known to be omnipresent but cannot be described
accurately. A variant of the Automated Time-Frequency Discriminant (Hedlin, 1998b),
which uses low-frequency spectral modulations, effectively separates these events from
the calibration shots.The experiment also provided evidence that kiloton class cast
blasts consistently yield energetic 2-10 second surface waves. The surface waves are
strongly dependent on azimuth but are seen beyond 9°. Physical modeling of these events
indicates that the surface waves are due mainly to the extended source duration and to a
lesser extent to the slap-down of spalled material. The directionality is largely a path
effect. A discriminant that is based on the partitioning of energy between surface and
body waves routinely separates these events from the calibration shots.The Powder River
Basin has essentially no natural seismic activity. How these mining events compare to
earthquake observations remains to be determined.
Keywords:
K Low frequency
;
Discrimination explosions
;
Teleseismic events
;
Seismology
;
Nuclear explosion
;
PAG
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