Abstract
In the monitoring of earthquakes and nuclear explosions using a sparse
worldwide network of seismic stations, it is frequently necessary
to make reliable location estimates using a single seismic array.
It is also desirable to screen out routine industrial explosions
automatically in order that analyst resources are not wasted upon
detections which can, with a high level of confidence, be associated
with such a source. The Kovdor mine on the Kola Peninsula of NW Russia
is the site of frequent industrial blasts which are well recorded
by the ARCES regional seismic array at a distance of approximately
300 km. We describe here an automatic procedure for identifying signals
which are likely to result from blasts at the Kovdor mine and, wherever
possible, for obtaining single array locations for such events. Carefully
calibrated processing parameters were chosen using measurements from
confirmed events at the mine over a one-year period for which the
operators supplied Ground Truth information. Phase arrival times
are estimated using an autoregressive method and slowness and azimuth
are estimated using broadband f-k analysis in fixed frequency bands
and time-windows fixed relative to the initial P-onset time. We demonstrate
the improvement to slowness estimates resulting from the use of fixed
frequency bands. Events can be located using a single array if, in
addition to the P-phase, at least one secondary phase is found with
both an acceptable slowness estimate and valid onset-time estimate.
We evaluate the on-line system over a twelve month period; every
event known to have occured at the mine is detected by the process
and 32 out of 53 confirmed events were located automatically. The
remaining events were classified as 'very likely' Kovdor events and
were subsequently located by an analyst. The false alarm rate is
low; only 84 very likely Kovdor events were identified during the
whole of 2003 and none of these were subsequently located at a large
distance from the mine. The location accuracy achieved automatically
by the single-array process is remarkably good, and is comparable
to that obtained interactively by an experienced analyst using two-array
observations. The greatest problem encountered in the single array
location procedure is the difficulty in determining arrival times
for secondary phases, given the weak Sn phase and the complexity
of the P-coda. The method described here could be applied to a wide
range of locations and sources for which the monitoring of seismic
activity is desirable. The effectiveness will depend upon the distance
between source and receiver, the nature of the seismic sources and
the level of regional seismicity.
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