GS_GSR_GCR_UserManual_V21.doc / 18.09.2009
GeoSIG Ltd.
GSR / GCR / AS User Manual
Page 17
If the LTA is allowed to update during the event, and the event is long relative to the length of the LTA
period, then the instrument will determine that the event is actually part of the background noise and
the trigger condition will terminate. The user has the option of enabling or disabling the LTA update.
5.2.2. Time Triggering
The Instrument may be set up to begin recording data at predetermined times. Up to four dates and
times may be programmed into the unit during set-up. A recording duration for each recording time is
also specified. Once set, the Instrument uses its internal clock to keep track of the date and time-of-
day until reset again.
5.2.3. Software Triggering
The Instrument may be set up to begin recording data by command of the connected PC. The
program supports triggering by key stroke. If the Software Trigger in the Trigger Settings window is
enabled, you may start recording by clicking Trigger Now in the Directory of Instrument Files. The
Instrument records Pre- and Post Event Time. This feature is useful for measuring seismic noise
levels.
5.2.4. External Triggering
(option)
The Instrument may be externally triggered by connecting a trigger signal to the Interconnection
connector (option). The external signal may come from another Instrument recorder, from a Control
Centre or from some other source. The Instrument begins recording data when the external trigger
signal goes to a low (0 V) on a falling edge. Several data recorders may be connected in "bus" fashion
and triggered simultaneously by one trigger signal.
Whenever it is triggered, regardless of trigger source, the Instrument puts out a signal on the
Master/Slave connector to be used as a trigger by other units, e.g., one Instrument may be set up to
trigger internally on incoming sensor signals, and can then activate several other units to begin
recording.
5.3. Triggering Guidelines
It is important to note that the input signal for the trigger channel is digitally high-pass filtered to
remove any DC offsets. The digital high-pass filters have a corner frequency of approx. 0.1 Hz.
5.3.1. Threshold Trigger
The threshold trigger compares the digitally low-pass filtered and rectified input (sensor) signal with
the value of the threshold level in the
Trigger
menu. The corner frequency of the filter has to be set in
the
Filter
menu.
5.3.2. STA/LTA Ratio Trigger
The STA/LTA (Short Time Average/Long Time Average) ratio trigger computes the short term and
long term averages of the rectified input (sensor) signal. When the STA exceeds a pre-selected
multiple of the LTA, the
GSR
begins to record data. The advantage of this trigger type is that the
trigger sensitivity adapts to the seismic background signal. With an increasing noise level the trigger
sensitivity decreases and therefore less false triggering occur.
To use the STA/LTA ratio trigger, four following information must be supplied:
•
the short term time period
•
the long term time period
•
the ratio dB of short term average to long term average which will trigger recording
•
if you want to have the value of the long term average updated during an event.
