PSN-L Email List Message

Subject: Re: seismograph calibration?
From: Karl Cunningham karlc@.......
Date: Tue, 14 Nov 2000 15:41:34 -0800

Hi Dave

I expect your calibration idea will work, but a couple of thoughts come to

1.  I think this would only test the system at the natural period of the
Lehman.  The response of the coil / magnet arrangement may not be flat with
frequency, and the electronics may not be either.

2.  I assume your amplitude measurement is visual.  If the magnitude of the
response of the coil / magnet is not proportional to the amplitude of the
swinging pendulum over the range of operating amplitudes, the gain may be
different when the amplitude is large enough to see than when it is in the
nanometer range due to earthquakes.  I think this might be a possibility
since, in some arrangements, the field gradient of the magnet may not be
very uniform.

Wishing to get the largest dynamic range I can, I run my gain so that the
lowest background noise is about +/- 4 counts.  And it's a rare day that it
gets that low.  Typical is +/- 20 counts, ranging to +/- 75 counts during

One thing that may make your record different from others is frequency
response, both in your equipment and in your location (soil type).  Another
is the orientation of your seismometer (NS vs EW).

Hope this helps.

Karl Cunningham
PSN Station #40

At 05:07 PM 11/14/2000 -0500, you wrote:
>Suppose you just take the damping off a Lehman pendulum and 
>give it a little shove.  It will begin to oscillate and if you
>know the period and the amplitude then you can calculate
>the peak velocity in nm/s.  Then you read the peak value
>from your ADC and that allows you to compute the
>velocity sensitivity of your seismograph in nm/s per bit.
>By observing your backgraound noise level in bits during normal
>damped operation you can convert it to nm/s.   
>Or am I making a stupid mistake?
>And what is a good value for sensitivity and background noise level?
>When I do the tests above, it looks like I have a sensitivity of
>about 120nm/s/bit and my noise level is about +/- 3.  I have
>a 16 bit ADC so should I crank up the analog gain a tad?
>The reason I am trying to calibrate my Lehman is that I just
>got it working and I can not figure why it detects some quakes
>well and others poorly.  I am comparing my results with those
>posted on seismicnet.  Especially the results from Herndon, VA
>which is just a few miles away from me at Bailey's Xroads, VA.
>I am trying to figure out whether I need more gain or is my location
>on a office building basement slab next to a busy street just too noisy? 

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