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DOCUMENT 302200, REVISION I
sps as the sensor limits the bandwidth. Historically, strong motion recording
has been at 200 sps, while seismological recording has often used the 100
sps rate. Obviously, an event file recorded at 200 sps will be almost twice as
long as one recorded at 100 sps and will take almost twice as long to
transfer over a communications link. You must evaluate this trade-off.
Note:
200 samples per second was used historically when the
instrument's effective bandwidth was limited by the roll off of the
analog anti-alias filters. For a 200 Hz sampling rate the filter cut
off was normally 50 Hz. With a digital anti-alias filter the cutoff at
200 Hz samples will be 80 Hz, a much higher effective bandwidth.
Supported sampling rates are 20, 40, 50, 100, 200 and 250 sps.
PRE-EVENT TIME
The setting allows you to determine how many seconds of data before the
trigger criteria were met will be recorded in the event file. For strong
motion applications this is normally set to a few seconds so you can
determine the noise before the start of the event. The factory default
is ten seconds.
For weak motion recording, especially if you expect the system to be
triggered on the S-waves, the situation is more complex. In this case, the
pre-event should be sufficiently long to allow any P-waves from an event
within the region of interest to be recorded. This time can be estimated by
the travel times of the P and S waves from the most distant point of interest.
The difference between these times, with some allowance for a true preevent time, gives the setting for the pre-event time. The maximum allowed
setting is dependent on the amount of RAM in the recorder and the number
of channels being recorded. A table showing the maximum settings is
contained in the Using QuickTalk & QuickLook Software User’s
Guide/QuickLook manual in the Setting Stream Parameters section.
POST EVENT TIME
The post event time determines how many seconds after the system has detriggered will be recorded in the file. It also determines how likely events
are to be split into separate files. This is because if the system re-triggers
during the post event the file will just be extended. If the post event is set
too short, several files could be created from the same event. We set the
factory default at ten seconds, which is a reasonable value for strong motion
recording. For weak motion recording this time should be set according to
the goals of the study. Generally, a time of 30 seconds or more should be
considered.
MINIMUM RUN TIME
This is the minimum time the recorder will record once an event is
triggered. Formerly, it was used to ensure a complete sequence of time code
was recorded with the event, but this is not required anymore. Generally,
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