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.


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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