Operator's Guide
probability of triggering on spurious spikes or short duration
transients, and to introduce some element of frequency
selectivity into the triggering process. As a rule of thumb, the
short term average should be set to the dominant frequency of
the events the trigger is designed to catch. The purpose of the
long term average is to provide a measure of the variation in
the background seismic noise, so it should be set to some value
longer than the period of the lowest frequency seismic signal of
interest. Obviously there is some element of trade-off in
setting a value for the trigger ratio. Too high a value will result
in events being missed, while too low a value will result in
spurious non-seismic noise triggering the system producing
false alarms. Determining an appropriate value in any given
situation which maximises the number of seismic events
detected while minimising the number of false alarms is a
matter of experiment.
In order to capture all of a seismic event, some seconds of
buffered data from before the trigger is declared are recorded.
This facility is particularly useful for emergent type signals
where the system triggers on a phase after the first arival.
Furthermore, some seconds of data after the system stops
triggering are also recorded to ensure the coda of an event is
not missed.
There are 5 parameters directly associated with the STA/LTA
triger algorithm:
sta
defines the length of the Short Term Average window in
seconds. Takes 3 arguments which are the values to use for
each of the three seismometer components (Z, N/S, E/W)
Example:
1 1 1 sta
lta
defines the length of the Long Term Average window in
seconds. Takes 3 arguments which are the values to use for
each of the three seismometer components (Z, N/S, E/W)
Example:
1 1 1 lta
ratios
defines the STA/LTA ratio above which the system
will declare an event. Takes 3 arguments which are the values
to use for each of the three seismometer components (Z, N/S,
E/W)
Example:
4 4 4 ratios
July 2016
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Summary of Contents for CMG-DM16R8
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