Abstract
It has long been accepted that occurrences of a known signal are most
effectively detected by cross-correlating the incoming data stream
with a waveform template. Such matched signal detectors have received
very little attention in the field of detection seismology because
there are relatively few instances in which the form of an anticipated
seismic signal is known a priori. Repeating events in highly confined
geographical regions have been observed to produce very similar waveforms
and good signals from events at a given site can be exploited to
detect subsequent co-located events at lower magnitudes than would
be possible using traditional power detectors. Even greater improvement
in signal detectability can be achieved using seismic arrays; running
correlation coefficients from single sensors can be stacked over
an array or network to result in a network correlation coefficient
displaying a significant array gain. If two events are co-located,
the time separating the corresponding patterns in the wave train
as indicated by the cross-correlation function is identical for all
seismic stations and this property means that the correlation coefficient
traces are coherent even when the waveforms are not. We illustrate
the power of array-based waveform correlation using the 1997 August
16 Kara Sea event. The weak event that occurred 4 hr after the main
event was barely detected using an STA/LTA detector on the SPITS
array but is readily detected by signal matching on a single channel.
The main event was also recorded by the far more distant NORSAR array
but no conventional detection can be made for the second event. A
clear detection is, however, made when the correlation coefficient
traces are beamformed over all sensors of the array. We estimate
the reduction in detection threshold of a test signal on a regional
seismic array using waveform correlation by scaling down a master
signal and immersing it into seismic noise. We show that, for this
case, waveform correlation using a single channel detects signals
of approximately 0.7 orders of magnitude lower than is possible using
an STA/LTA detector on the array beam. Waveform matching on the full
array provides an additional improvement of approximately 0.4 magnitude
units. We describe a case study in which small seismic events at
the Barentsburg coal mine on Spitsbergen were detected using the
signals from a major rockburst as master waveforms. Many spurious
triggers occurred in this study whereby short sections of signal
exhibited coincidental similarity with unrelated incoming wave fronts.
We demonstrate how such false alarms can almost always be identified
and screened out automatically by performing frequency-wavenumber
analysis upon the set of individual correlation coefficient traces.
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