PSN-L Email List Message

Subject: RE: What is your advice?
From: "Steve Hammond" shammon1@.............
Date: Sun, 11 Jul 2004 11:16:01 -0700

Boy, this could become an ugly discussion. I'm sure there is a few people
that don't agree with your comments and parts of them will be taken very
personally. For example, if you are talking about the AT1 and AT2 sensors in
Aptos, I will be the first to tell you that they are crap and I'm very
unhappy with their operation. I was forced to cut the booms down in size
because I didn’t have the physical space needed for 38-in. booms, which is
what I had used in the San Jose location. Additionally, I only have one spot
in my small yard for the seismic box and the cover box will only align off
the N/S axis, which limits the boom direction to be off the N/S axis. Let me
put it this way, getting the existing hardware to a period of greater than
12-14 seconds will require divine intervention. My point is this; I don’t
care if the person submitting the data provides it from a magnet daggling
from a string, if they wish to contribute to the dataset, they can. This is
the “Public Seismic Network” and all are welcome to take part, regardless of
how crappy their equipment is.

Instead of stating hard fast rules, I think we should describe desired
results as goals. I would like to discuses the goal of sample rates. I
believe the teleseimic sample rate goal should be 25 SPS. While Ed Cranswick
of the USGS is not here to defend himself, I base this goal on a discussion
I had with him in 1991. Ed said, and this is not an exact quote, “any data
sampled below 25 samples per second is worthless for future study. Most
researchers prefer to extrapolate the needed research data themselves. They
would also prefer the data be unfiltered so that they can apply the needed
filters. Data that has been highly processed is suspect because the
processing algorithms are unknown and not documented and therefore the data
may not represent the actual event.”

Regards, Steve Hammond  PSN San Jose --  Aptos, CA

-----Original Message-----
From: psn-l-request@..............
[mailto:psn-l-request@................. Behalf Of rem11560@...........
Sent: Sunday, July 11, 2004 9:53 AM
To: psn-l@..............
Subject: What is your advice?

Hi Larry,

    I know that I am a fuss pot, but I would like to submit the following
advice to the PSN List. I need your input before I do. I practice what I
preach, and I think the files that I now regularly submit are as good or
better than most that I see. I am especially annoyed that hardly anyone
takes the trouble to calibrate their sensors. Maybe there should be a
discussion among list members and a concensus reached on what advice to
amateurs would be best to prominently and permanently post on your web site.

    I also don't like overly large over-sampled files. I would think that
you would rather not have to archive such files.


Bob McClure
Locust Valley, NY

>Re: What is your advice?
>Hi Bob,
>I don't have a problem with you sending out your request to the PSN list.


1. Be sure that your STATION COORDINATES are correct. You can use Microsoft
Streets and Trips, or online MapQuest for the purpose. Also align horizontal
sensors to true North or East, or else give true direction in sensor
comments box.

2. LOCK your timing to GPS or WWV or a crystal clock slaved to WWVB.

3. CALIBRATE YOUR SENSOR. You would not have much use for a voltmeter with
no scale on it, would you?. For open loop sensors, I have two methods. One
uses the raw sensor output of pendulum movement between fixed stops, the
other measures the force exerted by the pendulum in response to a known
current. I can tell you more if you ask.

4. DO NOT submit files with high sample rates on distant teleseisms. High
frequencies are attenuated with distance, and files with excessive sample
rate only take up bandwidth and archival storage space. Decimate before
submitting. One to five samples per second should handle most teleseisms
without loss of waveform detail. Also, do not cover an overly large time
span after the L wave onset.

5. DO NOT use any more FILTERING than absolutely necessary. Let some
microseisms come through. Leave it to the downloader of files do more
filtering if they wish.

6. If possible, adjust your sensor's NATURAL PERIOD to least 16 seconds if
you record and report teleseisms. If that is not practical, I have written
an application program for WinQuake files which can digitally extend the
effective period of your sensor by up to a factor of five. I use it
routinely on my sensors which have natural periods of 5, 8, and 14 seconds
to extend their response to 24 seconds.

7. Control your sensor DAMPING. The barest amount of overshoot on a
displaced pendulum is about right.

8. Use as little AMPLIFIER GAIN as possible to avoid clipping on major
events. Most of the files I see on the seismicnet site have been recorded at
far more gain than necessary. You may have to make component value changes
in your amplifier to accomplish this.

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