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From: "Len Polucci" 
Sent: Friday, October 15, 2010 8:51 PM
To: 
Subject: Re: Period of seismic units

> Great, Great question! Since I have no idea and have wondered in the past..I'd 
> love to see some answers!
> 
> 
> 
> 
> ________________________________
> From: Thomas Dick 
> To: psnlist@..............
> Sent: Fri, October 15, 2010 10:32:41 AM
> Subject: Period of seismic units
> 
> I asked Larry about this and he wanted me to post it here. I think he was trying 
> to get rid of me. It probably shows how little I know and that I am missing 
> something. This question came out of the activity in the Guy area which is about 
> 500 miles west of me as well as the New Madrid system which is closer. It seems 
> the records are better on the 2-4 Hz units. Here was my question;
> 
>> It is about this geophone vs lehman or other long period instruments. In
>> California you have many local quakes. These quakes have frequencies
>> higher than long distance quakes. What is the logic of (just/or popularity of) 
>>long
>> period instruments out there? What precipitated this trend? The 1-5 Hz units 
>>should be better on local events -- which you have a lot.
> There are many California quakes and there is the activity in Washington State 
> and Yellowstone but you never see local seismic detectors being posted with 
> periods of 1-4.5 Hz -- except possibly tchannel.  Shouldn't these units help 
> differentiate human noises like quarry activity from smaller local earthquake 
> activity?

Unless you are plagued by vehicular or other human activity you
should be able to open your spectrum all the way
out to the anti alizing frequency.

No matter what you need to filter out all above
1/2 or 0.5 the sample rate or you will get unwanted artifacts.
You need to filter this way at some stage prior to
the sampling of the data.

So like if your samp[le rate is 100 then filter
out all over 50Hz and if sample rate is
20 all over 10 Hz but if you are plagued by
machinery on the ground or vehicular activity
then the weaker seismic signals will require you
filter at like two hertz no matter what.
Like you live in the middle of a crowded place.

You can see both regional and teleseismic but not
footfalls or those higher disturbances that make
the picture so perfect.

The ability to reproduce a square wave is in my opinion
the best way to test the over all response of any system.
You need to resilve many harmonics to be able to do this.

Not sure how to test this with a geophone
but you might inject a 1HZ square wave into
the sensor itself. Most probably disconnecting the
geophone all together for the test.

The more faithful your recording the better
your system.

I would call footfalls close to your sensor
a local event and if you can see footfalls
you must have a good setup.
Footfalls being a human walking by or
some heavy animal like a bunny rabbit
hopping by.

???

If the seismic thing which you look for has no
alizing frequencies then I say, why any filter at all.
Maybe only equalization for a SPZ.
Except possibly a DC blocker at 30 seconds or
0.03333 Hz. .
But it has to be located in a very quiet area.
On granite bedrock.
Underground.
Away from any human habitation/villages maybe
several feet away from any human foundation/home/work.

Still, I have never seen a better SPZ than the 500 lb
squeegee sharpener thing which was a velocity device.
It has a mass of like over 100Lbs and a very heavy flat
coiled [prestressed?] spring. Maybe there is a picture on the
internet of this, but, I have not yet seen any.
The manufacturer was a Texas Company I think.

Your device is the most portable vertical
I have ever seen. Just put it in a vacuum
chamber and shield it from electrical disturbances.
All which adds to the expense.

geoff






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