PSN-L Email List Message

Subject: Re: Why we like verticals
From: Brett Nordgren brett3nt@.............
Date: Wed, 09 Oct 2013 17:12:58 -0400

Hi Chris,

Yes, sorry.  I clipped off the time markers when I grabbed the 
picture.  From what I can estimate now, the entire trace 
represented  about one hour.

Dave later reported that, contrary to what I'd thought, it was not 
unusually windy at the time and the weather was quite typical with 
microseisms relatively low.  I can see similar traces any time I 
look.  Also the vault was relatively stable at the time with nothing 
out of the ordinary, and bear in mind that the three traces are 
derived from three identical Galperin-oriented sensors, so you would 
expect that many possible noise sources would affect all three axes 
in a similar way.  The motion we were seeing was truly either 
horizontal or tilting of the ground.  I am currently suspecting that 
tilt noise is mainly due to large-scale atmospheric pressure changes 
deforming the ground, though I think it's a subject that's still 
being studied.

I uploaded a 1Hr barometric pressure trace recorded at the same time 
as the ZNE traces.  At the moment I don't have the sensitivity value 
available, but it is extremely high in that recording.  At times we 
have seen 10-minute pressure oscillations that could be 10x what you 
see there, possibly greater.  The barograph has flat response down to 
about 3600 seconds.  I don't see any obvious 

Microseisms, which appear to have been small at the time, had already 
been removed with a 17-sec low-pass filter, and the instrument 
response is otherwise flat down to 120 seconds.


>Hi Brett,
>      It would be great if you could put some time markers on the
>trace, please ? It is difficult to get a good idea of the wave periods
>without this. Maybe just tell us the total trace time ?
>      Could we have a 'good' trace from a low wind / low sea day, for
>comparison, please ?
>      Did Dave leave the door to the seismometer vault open, or
>something ? I get signals like this if I leave the insulating cover off
>my Lehman / don't seal the case to the floor with foam tape.
>     Regards,
>     Chris Chapman
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