Guralp Systems CMG-DM24 - Manual (Page 64)

Operator’s Guide DM-24 Digitiser
Issue G January 2003 62
This section describes the triggering algorithm and gives several examples of typical
system configurations that may be used. It is intended to be read along with the
Digitiser Configuration Section of the Operator’s Guide.
USING THE TRIGGERED SYSTEM -- OVERVIEW
The Digitiser Configuration Section of the Operator’s Guide describes in detail how to
configure the system to output continuously at up to 3 different sample rates, selectable
within certain constraints by the user. Typically, you may only wish to record one
sample rate as continuous data, but it may be useful to record selected time periods at a
higher sample rate
The triggering algorithm used is a standard STA/LTA ratio test on a bandpass filtered
signal. An individual channel, corresponding to an individual seismometer component
can be defined as the trigger channel, or triggers can be permitted on any channel.
Details of the various parameters related to the triggering process are given in the
following sections.
PRETRIGGER BANDPASS FILTERING
As the digitiser is normally used with a Güralp Systems Limited broadband
seismometer the raw data is very broadband. To enhance the performance of the
triggering algorithm, the raw input data is bandpass filtered prior to running it through
the triggering system. This filtering serves to maximise sensitivity within a specific
frequency band of interest, and to reject noise outside this band, for example from
oceanic microseisms. The system is provided with a choice of 3 inbuilt generic
bandpass filters (wide, medium and narrow) which are slaved to the defined tap output
sample rates defined using the samples/sec command. The filters all have a low pass
corner at 90% of the Nyquist frequency of the selected tap, and the wide, medium and
narrow filters have high pass corner frequencies at 10%, 20% and 50% of the Nyquist
frequency respectively. For example, if we consider the 100 samples/sec tap defined
above (Tap#1) the low pass corner for each filter will be at 45Hz, and the high pass
corners will be at 5Hz, 10Hz and 25Hz.