PSN-L Email List Message

Subject: Re: Caution on low pass filtering emphasis
From: Thomas Dick dickthomas01@.............
Date: Fri, 01 Jan 2010 10:38:55 -0600

I did download your files...but that muddy the waters a little. My 
interests here have turned to the microscisms at about  period 6 sec and 
35 sec. USIN (locally) and my own equipment have shown these 
"irritating" noises more frequently this Fall than in the past. In fact 
I pull down USIN's record and compared it to mine. Two hours before the 
quake and about seven hours after the quake's arrival the "noise" was 
still very prevalent. I don't see any sign of this "problem" on your 
record. Could you comment; would this be a regional thing, electronics 
or period of the seismic units involved?

RSparks wrote:
> Thomas Dick wrote:
> are your sure the bottom is the raw data?? it looks a lot like my 
> filtered data
> Hello Thomas,
> Yep, I am sure.  You can take a better look at the data by downloading 
> the PSN file and opening it with WinQuake.  Then look at each 
> seismograph by selecting from the View menu "Display Record".
> I am using the Saum A/D device with some modifications intended to set 
> the on board filter to low pass about 4.5 Hz.  When I say "raw data", 
> I mean data as it comes from the Saum device.
> The Saum device also uses a very long period high pass filter that 
> acts to center the data automatically.
> My own practice is to use a high pass filter (> 1 Hz) on my helicorder 
> screen so that I can easily see local quakes. I am careful to also 
> view the screen using a low pass filter (< 0.2 Hz) as well (so it 
> takes two looks for each "check") because I occasionally find a quake 
> that has almost no energy displayed above 1 Hz.  Unusual but it 
> happens.  The local quakes usually become invisible with the LP filter 
> in place unless they are very large.
> Roger
> RSparks wrote:
>>> Hello All,
>>> I sometimes think that we amateur seismologist over-emphasize the 
>>> need for low pass filtering, with the unexpected result that we 
>>> destroy the fidelity of the earthquake signal.  To illustrate what 
>>> we might be losing, I posted my recording of this mornings Baja 
>>> quake using both raw data and HIGH PASS filtered data in the 
>>> WinQuake volume format.  The high pass was done using WinQuake 
>>> filters set at 1 Hz, two pole.
>>> Here is a link to the posting: 
>>>    The trace on the bottom is the raw data.
>>> As you can see, the quake was about 1677 km from this station but 
>>> there was still lots of high frequency (> 1 Hz) in the data.   If 
>>> filtered low pass below 1 Hz, this information would be compromised, 
>>> and as you can see, there is a lot of information to be lost.
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