GeoSIG Ltd.
GS_GSR_GCR_UserManual_V21.doc / 18.09.2009
Page 16
GSR / GCR / AS User Manual
Pre-Event Event Post-Event
Time
Trigger
Figure 5. Typical Recorded Data for 1 Channel
If the user sets up trigger parameters that are continually met by the environment, the system will
trigger continuously. This will be indicated by the yellow event LED which is always on. The user can
connect a PC to the Instrument unit to interrupt the recording and select a different set of trigger
criteria.
5.2.1. Internal Triggering
Internal triggering requires that incoming sensor data satisfy certain pre-set criteria for recording to
begin. The digitised and digitally filtered sensor data are used as input to the internal triggering
system.
Either of two criteria may be used for internal triggering. The Threshold Trigger simply determines if
the signal amplitude has exceeded a pre-set threshold. The Short Term Average/Long Term Average
(STA/LTA) Ratio Trigger determines if there is a sudden increase in signal amplitude. A combination of
both Threshold and STA/LTA triggers may also be selected so that the Instrument will start recording
when either trigger criterion is met.
5.2.1.1. Threshold Trigger
The Threshold Trigger compares the signal amplitude of the filtered trigger signal to a threshold value.
The sensor data are first processed by a digital high-pass filter with a corner frequency of about 0.1 Hz
to remove any DC offset. The data are then filtered and rectified to give an absolute-value signal
amplitude ranging from 0 to a maximum of 2048. Data recording is initiated when the amplitude
exceeds the pre-set threshold value.
The user may enter the threshold values in the selected channel unit. GeoDAS calculates it to raw
counts (LSB) and puts the value into the recorder. If the threshold is set to 10 counts, then a signal
greater than 10 counts will cause a trigger.
The standard GSR-18 has the following trigger filters:
•
Low-pass filter of 2
nd
order Butterworth filter with corner frequency at 12Hz.
•
High-pass filter of 2
nd
order Butterworth filter with corner at 0.1Hz.
•
These filters are used simultaneously (not individually) and can be turned on and off in GeoDAS.
5.2.1.2. STA/LTA Ratio Trigger
The STA/LTA ratio trigger uses an algorithm, which will filter out background noise and allow only
those data, which pass through its filters to trigger the record function of the Instrument. This filter
method uses a Long Term Average of data inputs to set the background noise base level and
compares this to a Short Term Average of data which is more responsive to changes in the data input
stream. During the steady state of the system, the STA and the LTA will be nearly equal. The shorter
the STA averaging period, the more quickly it will change with the input. The trigger condition occurs
when the ratio of the STA versus the LTA exceeds a present level, ALPHA.
The probability of having a false trigger due to noise will be minimised if a long STA averaging time is
selected. Obviously, the STA should not be chosen longer than the shortest event of interest. In
addition, the STA should be shorter than the block of pre-event data stores in the Pre-event Memory.
If not, the initial portion of an event may not be recorded.
