[ANSS-netops] Strong motion calibrations

[David S. Croker]

Hi Nathan,

There was a vast silence from this question.  I can't really answer 
question 1, and given the response from Kinemetrics, I think you 
answered your own question.  As for number 2, in California we have a 
distinct advantage over some other regional networks:  We always have 
earthquakes.  At any time, I can pull up a waveviewer and look at a 
handful of earthquakes and see that the SM sensor is functioning 
properly.  Unfortunately this method is just a subjective evaluation, 
but after looking at hundreds of earthquakes, it becomes clear what's 
working and what's not.  Also, of course, this method is by no means 
a calibration of the sensor either.

For locations away from earthquakes, Gray suggested developing a 
small solenoid that could clamp onto the sensor that you could 
remotely release to see what kind of response you get.  Of course, 
then you'd have to calibrate the solenoid and have one more thing to 
maintain.  Just a side note:  we have seen energy on our EpiSensors 
from large teleseisms now, so you wouldn't necessarily have to wait 
for a local event.  We saw energy on most of our EpiSensors from the 
Honduran earthquake on May 28.

The only calibration, per se, that we do on our SM sensors (and 
usually only when there is a question about it's performance) is to 
hook them up to a digitizer and turn them on their sides and 
up-side-down to see if we get the proper DC offset.  For example, we 
turn the EpiSensor up-side-down and hope to see -2g = x V depending 
on your settings.  You could do this easy test before you install the 
sensor, then do it any time later and compare numbers.  It's crude, 
but it's cheap and easy.

I don't know if this helped, but I thought I'd chime in with 
something.  Don't know what ASL does to calibrate SM sensors.

Dave Croker
NCSN

At 04:50 PM 6/17/2009, Nathan L Edwards wrote:
>Hey all,
>
>We are attempting to establish some procedures for calibrating 
>strong motion instruments.  There seems to be varying opinions as to 
>what can be claimed with these results.  With our EpiSensor 
>stations, we initially planned on providing a step voltage from the 
>calibrator and recording output to confirm amplitude 
>response.  There is a similar procedure for the K2 with an internal 
>EpiSensor that is referred to as a "functional test" (not claimed to 
>be a calibration).  These are reproducible, however we are 
>rethinking what these results really mean after these comments from 
>Kinemetrics tech support:
>
>"Calibration functions [of the datalogger] are provided for transfer 
>function frequency
>response measurement, and cannot be used in any electronic force
>feedback sensor [like the EpiSensor] for meaningful gain 
>measurements. This is an aspect of
>all electronic force feedback sensors, not an aspect of any calibration
>source."
>
>Regarding strong motion sensors (not just the EpiSensor) my questions are:
>1.  Does the step function test reliably tell you anything useful 
>other than that there's something alive at the end of the sensor 
>cable (i.e. to what degree is it a true calibration)?
>2.  What are other labs doing to remotely confirm sensor 
>function/calibration on their strong motion sensors?
>
>Thanks,
>Nathan Edwards
>Development Technician
>Nevada Seismological Laboratory
>University of Nevada Reno
>Reno, NV 89557-0174
>o:  775-784-1106
>m:  775-303-5899
>f:  775-784-4165
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__________________________________________________________
USGS - Earthquake Hazards Team - NCSN Field Operations Manager
David S. Croker                                         office (650) 329-4697
345 Middlefield Rd, MS 977                           fax (650) 329-4732
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email: croker at usgs.gov
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