Guralp Systems CMG-DM24 - Manual (Page 37)

Operator's guide
STA/LTA – Triggering by average values
The STA/LTA algorithm applies a simple short-term average – long-
term average calculation to the triggering stream. It works by
identifying sections of an incoming data stream when the signal
amplitude increases. The purpose of taking a short term average,
rather than triggering on signal amplitude directly, is to make it less
likely that spurious spikes will trigger the device. Averaging also
introduces an element of frequency selectivity into the triggering
process.
You can select which tap is tested for the trigger from the Data source
drop-down menu. The tap does not have to output data to Scream! for
you to be able to use it here.
Any or all of the channels available at that tap may be used to
determine a trigger. You can select which channels are considered by
checking the boxes in the Channel column of the table. If any of the
checked channels passes the trigger condition, the trigger will activate,
and will remain active until all of the checked channels have fallen
below their respective ratio values.
The STA and LTA columns allow you to set the intervals over which
the two averages are calculated, in seconds. Typically, the time
interval for the short term average should be about as long as the
signals you want to trigger on, while the long term average should be
taken over a much longer interval. Both the STA and LTA values are
recalculated continually, even while a trigger is active.
The Ratio column determines by what factor the STA and LTA must
differ for the trigger to be passed. Finding the ratio most suited to your
needs is best done by experiment. 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. Like the averages, their ratio is
continuously recalculated for all components. Note that none of the
boxes are allowed to be empty, and so you will need to enter the new
value before removing the old one. Alternatively, you can use the up
and down cursor keys to change the values.
October 2009 37