GeoSIG AS-12, Руководство пользователя

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GeoSIG Ltd. GS_GSR_GCR_UserManual_V21.doc / 18.09.2009
Page 18 GSR / GCR / AS User Manual
The average time for the STA and LTA may be set as follows:
•
STA 0.1 to 10 seconds
•
LTA 1 to 100 seconds
a
The LTA may not be set smaller than the STA.
The ratio of STA to LTA, ALPHA, is defined as:
10
STA
ALPHA 20 log
LTA
= ×
The probability of having a false trigger due to noise can 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 stored in the Pre-event Memory.
If not, the initial portion of an event may not be recorded.
The average period for the LTA should be chosen short enough to adapt to any changes in the level of
background noise. However, the LTA time should be long compared to the STA time, otherwise, the
LTA will adapt to an event as if it was simply a change in the background noise level.
Please note that it is possible to select a combination of STA, LTA and ALPHA, which will not meet the
trigger conditions, and no internally generated trigger will occur. For example with
STA 1.0
LTA 10
ALPHA 23 dB
=
=
=
the STA/LTA ratio of 14.125 (23 dB) never could be reached because the LTA adapts to the signal
before the STA value gets to the trigger level.
In this case, the theoretical maximum value for ALPHA would be
10
2048
ALPHA 20 log 20 dB
205
= × =
This would indicate that any value of ALPHA greater than 20 dB could never be used for any
LTA period/STA period ratio equal to 10. Some sample STA/LTA values and the greatest possible
ALPHA that would allow a trigger are given hereafter.
Table 6. Maximum values of ALPHA for some sample STA/LTA ratios.
STA LTA Maximum ALPHA
2.0 10 14 dB
1.0 10 20 dB
0.5 10 26 dB
0.5 20 32 dB
0.2 20 40 dB
If the LTA Update is enabled, the long term average value (LTA) will be periodically updated even
during an event. The recording of a long event could be cut off since the LTA value increases until the
ratio of STA/LTA has reached a value where the trigger condition is no more fulfilled. With an
adequate "Post-Event Time" the loss of data can be minimised.
If the LTA Update is disabled, the long term average value (LTA) will be "frozen" during recording.
With a permanent seismic noise signal where the input signal does not decrease for a long time, the
short term average value (STA) will no more reach a value below the trigger level any more after
triggering. The GSR will not stop recording until its memory is full. Therefore it is recommended to
have the LTA Update "enabled" for standard applications.
Three examples of typical applications and the suggested trigger parameters are given below