PSN-L Email List Message

Subject: Re: Caution on low pass filtering emphasis
From: Brett Nordgren brett3nt@.............
Date: Sat, 02 Jan 2010 11:21:37 -0500

Hi Roger,

At 09:37 PM 1/1/2010 -0800, you wrote:
>Hello Thomas,
>I took a look before and after the quake but did not see anything unusual.
>Not unusual for this location is to have a fuzzy trace (~1 Hz High Pass) 
>beginning about 0630 and ending about 1700 local times.  Since I am using 
>a vertical, I have attributed this to the sun warming the 
>atmosphere.  But, the noise does not seem to be present during non-work 
>days which is completely inconsistent with a solar source.

Your description could almost be the definition for man-made, so called 
cultural noise.  Natural phenomena, though some might drop at night, aren't 
usually affected by weekends and holidays.

>  I live about 2 miles from any serious auto traffic which would be on 
> I-90 in Central Washington.  I have no explanation for this "fuzz"

I think that you are almost certainly seeing truck traffic on the 
Interstate.  2 miles is plenty close for a sensitive instrument.

>The "fuzz" is not visible with either the all pass or low pass filters in 
>One puzzle here is that in the past I could get a fairly clean FFT peak at 
>about 6 seconds and little at longer periods.  Lately, I notice a broader 
>peak at 6 seconds and a second peak as low as 44 seconds (both very broad 
>peaks).  I took a look at about 4 hours of data a few moments ago and 
>confirmed what I had previously found.  Would we be seeing the same thing?

The 6-second microseism peak is constantly changing, so changes  there are 
not that surprising.

I agree that the low frequency peak might very well be the atmospheric 
stuff proposed by Chris, though verticals are mostly immune to the 
wind-caused ground tilting that is so annoying with horizontals.  His 
question about the pressure cover is a good one.  Normally the atmospheric 
noise we see on the vertical is in the frequency range of minutes and is 
reduced by a factor of 10 or 20 by adding a sealed cover.  If your vertical 
is small enough to be covered by a cooking pot, that would work nicely.  A 
typical setup would be a very solid base slab, like 2-4" thick 
granite  with an inverted pot or similar container sealed down over 
it.  The granite base is because pressure changes will cause anything much 
less solid to flex under the seismo and generate noise.  Our experience was 
that 3/4" aluminum flexes way too much.  For sealing things, including 
cable openings, Dave Nelson has found that the putty-like windshield 
sealant seems to work 
nicely.  It sticks 
things together very well, remains workable and is designed to not squeeze 
out from between the windshield and car frame or in our case the base slab 
and pressure container.

If a vertical has any long period sensitivity at all, it should be pressure 

Good luck,

Watch our wiggles

or watch some very very good wiggles


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