PSN-L Email List Message
Subject: Re: Caution on low pass filtering emphasis
From: RSparks rsparks@..........
Date: Thu, 31 Dec 2009 11:29:33 -0800
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:
>> http://www.seismicnet.com/cgi-dos/makegif.exe?/quakes/0912/091230.185135.ebgzx.psn
>> 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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