Mark Robinson mark.robinson@...............
Sent: Sat, 23 Jul 2011 23:05
Subject: Re: Strange chirps in event data?


This is what you will see when a pulse goes through a filter.
Without knowledge of the exact nature of the filter it is difficult to know=
=20
xactly how it will behave, but in general you will get a time delay and oft=
en=20
xtreme phase effects.
So while the chirp may appear to precede the signal, it may be the signal w=
hich=20
s delayed by the filter, and the chirp may be a filter artefact aligned wit=
h=20
he initial arrival.
It could also be a local electrical pulse (anything being switched on or of=
f)=20
aking the filter ring, so you could try plugging something noisy into the s=
ame=20
ains outlet and switching it on and off to see if you can induce the same=
=20
ehaviour.
On 24/07/11 08:36, Larry Conklin wrote:
 Well, whatever it is that I'm seeing, I'm very sure it has nothing to do w=
ith
 the quake. The signal isn't natural looking at all. It is a nearly sinusoi=
dal
 wave that begins pretty abruptly, sweeps down in frequency from 2 hz to 0.=
4 hz
 and then stops fairly abruptly. I have a history of similar artifacts that=
=20
ave
 showed up from time to time, but I don't think they have shown this sort o=
f
 frequency sweep before. That is what caught my interest this time. In past
 occurrences I would see a minute or so of a very pure looking tone that wo=
uld
 show up in the FFT plot as a very narrow spike around 2 hz. I'd love to kn=
ow
 what's going on, but don't have much of an idea of how to run it down.

 Larry


> both of my Leymans show a sharp spike about 25 seconds before S arrival h=
ere
> in Evansville, IN...largest spike occurred on my E-W unit it is visible o=
n
> the N-S and vertical AS-1 to lesser extent.
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=
Mark Robinson mark.robinso=
n@...............

Sent: Sat, 23 Jul 2011 23:05

Subject: Re: Strange chirps in event data?





This i=
s what you will see when a pulse goes through a filter.

Without knowledge of the exact nature of the filter it is difficult to know=
=20
exactly how it will behave, but in general you will get a time delay and of=
ten=20
extreme phase effects.

So while the chirp may appear to precede the signal, it may be the signal w=
hich=20
is delayed by the filter, and the chirp may be a filter artefact aligned wi=
th=20
the initial arrival.

It could also be a local electrical pulse (anything being switched on or of=
f)=20
making the filter ring, so you could try plugging something noisy into the =
same=20
mains outlet and switching it on and off to see if you can induce the same=
=20
behaviour.

On 24/07/11 08:36, Larry Conklin wrote:
> Well, whatever it is that I'm seeing, I'm very sure it has nothing to =
do with
> the quake. The signal isn't natural looking at all. It is a nearly sin=
usoidal
> wave that begins pretty abruptly, sweeps down in frequency from 2 hz t=
o 0.4 hz
> and then stops fairly abruptly. I have a history of similar artifacts =
that=20
have
> showed up from time to time, but I don't think they have shown this so=
rt of
> frequency sweep before. That is what caught my interest this time. In =
past
> occurrences I would see a minute or so of a very pure looking tone tha=
t would
> show up in the FFT plot as a very narrow spike around 2 hz. I'd love t=
o know
> what's going on, but don't have much of an idea of how to run it down.
>
> Larry
>
>
>> both of my Leymans show a sharp spike about 25 seconds before S ar=
rival here
>> in Evansville, IN...largest spike occurred on my E-W unit it is vi=
sible on
>> the N-S and vertical AS-1 to lesser extent.

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Public Seismic Network Mailing List (PSNLIST)

To leave this list email =
PSNLIST-REQUEST@.............. with=20
the body of the message (first line only): unsubscribe
See htt=
p://www.seismicnet.com/maillist.html for more information.