Guralp Systems Scream User Manual

Page 82

Scream! 4.5

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 or generally elevated background noise 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
ticking the boxes in the Channel column of the table. If any of the

ticked channels passes the trigger condition, the trigger will activate,
and will not de-trigger until all of the ticked channels have fallen

below their respective ratio values.

The STA and LTA columns allow you to set, in seconds, the intervals

over which the two averages are calculated. 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 during a trigger.

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: 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

and

cursor keys to change the values.

For example, setting the STA to 1 second, the LTA to 10 seconds and
the Ratio to 4 would give rise to the trigger behaviour depicted in the
following illustration:

Issue K