PSN-L Email List Message

Subject: Measuring Time Delay
From: Karl Cunningham karlc@.......
Date: Sun, 09 Jan 2000 12:41:35 -0800

SDR has a time delay parameter that can be entered for each channel, to
compensate for time delay in the seismometer.  I'd like to make some
measurements to be able to enter accurate values for these, and I'm curious
how time delay of a seismometer is measured.  I can easily see how it could
be done with a shaker table, but not having that luxury, what are some
other methods?

One could, of course, test the electronics using a function generator, but
I would really like to include the mechanical components as well.

If all the filtering in the seismomter electronics is of the equal time
delay (Bessel) characteristic, time delay is probably not a function of
frequency in these circuits.  But this probably doesn't apply to the
mechanical response or to the overall response of the instrument to ground

What about measuring the response to dropping some large object on the
floor near the seismometer?  The falling object could easily actuate a
contact closure as a time reference when it hits the floor, but how are the
results to be interpreted?

What about a time delay measurement using real earthquakes?  Could a simple
reference instrument (geophone?) be used to do the calibration?  Could a
calculated time delay for a geophone be trusted?

My questions are:

1.  Given that the instrument probably does not have equal time delay
response, is there a standardized frequency for which seismometer time
delay measurements are made?

2.  If pulse response is used (dropping the object on the floor), can the
seismometer output just be bandpass filtered to the desired frequency, and
the delay measured?  If so, what kind of bandpass filtering should be used,
and should its delay be considered?

3.  In doing the pulse response measurement, is the proper delay
measurement made to the first noticable change in the output, or to some
other point in the output waveform?

Any help or suggestions are appreciated.

Karl Cunningham
La Mesa, CA.
PSN Station #40


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Larry Cochrane <cochrane@..............>