Larry
I wrote the Quick Basic acquisition program back when storage size was an i=
ssue :)=A0 Initially , before I changed the program to an FFT analysis, bec=
ause of the computer speed=A0 I ran what was called a Walsh transform. It w=
as similar to an FFT but you used square waves. The math was faster and I c=
ould write it in assembly language. What I thought was that a=A0 quake sign=
al is seen as a change in amplitude and the frequency composition of the ba=
ckground signal. The "short term-long term" trigger and the like routines l=
ook at signal amplitude only and not changes in frequency. What I did was t=
o run an FFT on a sort of small size window ( one I could run between the a=
cquisition of two data points) maybe around 512 points. At a fairly slow sa=
mple rate of about 5 hz ( for teleseismic events) it covered a reasonable f=
requency range and I could do it with my "slow" 286 computer. I chose certa=
in frequencies to observe (based on FFTs I had run=A0 on typical
 previously recorded events ) and ran the short term/ long term style routi=
ne on a weighted sum of these frequency amplitudes. The thought was that th=
e trigger would pick up the P wave arrival.=A0 I then started recording beg=
inning with a moving buffer of certain # of data points. It would ignore mo=
st transients. The "noise" I had the most trouble with was wind ( not on th=
e sensor), I could protect from that. It was fluctuating wind gusts that ge=
nerated movement in the slab the sensor was on. My trigger threshold varied=
 in amplitude based on running data. It could correct for most of this wind=
 also. The interesting thing was I could "catch " confirmed events that I c=
ould barely see with my eye "in the grass". It all became academic when it =
was easier to record continuously and look for events based on what showed =
up on say the USGS web site. Events you can't see very well aren't that int=
eresting to look at. ---- You asked :} =A0=A0 I don't remember right
 now the numerical specifics, but I can look it up if you are interested.
Regards
Barry
http://www.seismicvault.com

--- On Tue, 12/29/09, Larry Conklin  wrote:

From: Larry Conklin 
Subject: Re: Sensor noise
To: psn-l@..............
Date: Tuesday, December 29, 2009, 1:16 PM

Hi Barry,

It sounds like you've been messing around with something similar to a proje=
ct I started but didn't get around to finishing.=A0 I added a waterfall FFT=
 plot to my data logging program, but never finished debugging it.=A0 In pa=
rt because I wasn't all that confident that the plot would be worth all tha=
t much.

I'd be interested in a little more information regarding how you are collec=
ting the FFT and some of the design decisions you use (number of samples in=
 the window, how frequently you do a conversion, etc.)=A0 Did you write you=
r own code or are you using some 3'rd party software?

Larry Conklin
Liverpool, NY
lconklin@............


On 12/29/2009 12:12 AM, Barry Lotz wrote:
> All
> I have successfully used a running fft to sense signal frequency
> component changes and used this as a trigger mechanism for an event.
> Could one use the ( I guess you call it ) power spectrum of the signal
> just before and during the event to remove the noise? I guess you would
> have to use the same time window so the frequencies would compare. Could
> it be a simple subtraction of the "before" from the "during"? This would
> assume that the background noise didn't change in the period during an
> event. This could be better than trying to shape a multi pole filter to
> eliminate the noise. I have found that often a portion of the event
> signal is in the same frequency range as the noise.
>=20
> Regards
> Barry
> http://www.seismicvault.com
>=20
__________________________________________________________

Public Seismic Network Mailing List (PSN-L)

Larry
I wrote the Quick Basic acquisition =
program back when storage size was an issue :)  Initially , before I c=
hanged the program to an FFT analysis, because of the computer speed  =
I ran what was called a Walsh transform. It was similar to an FFT but you u=
sed square waves. The math was faster and I could write it in assembly lang=
uage. What I thought was that a  quake signal is seen as a change in a=
mplitude and the frequency composition of the background signal. The "short=
 term-long term" trigger and the like routines look at signal amplitude onl=
y and not changes in frequency. What I did was to run an FFT on a sort of s=
mall size window ( one I could run between the acquisition of two data poin=
ts) maybe around 512 points. At a fairly slow sample rate of about 5 hz ( f=
or teleseismic events) it covered a reasonable frequency range and I could =
do
 it with my "slow" 286 computer. I chose certain frequencies to observe (ba=
sed on FFTs I had run  on typical previously recorded events ) and ran=
 the short term/ long term style routine on a weighted sum of these frequen=
cy amplitudes. The thought was that the trigger would pick up the P wave ar=
rival.  I then started recording beginning with a moving buffer of cer=
tain # of data points. It would ignore most transients. The "noise" I had t=
he most trouble with was wind ( not on the sensor), I could protect from th=
at. It was fluctuating wind gusts that generated movement in the slab the s=
ensor was on. My trigger threshold varied in amplitude based on running dat=
a. It could correct for most of this wind also. The interesting thing was I=
 could "catch " confirmed events that I could barely see with my eye "in th=
e grass". It all became academic when it was easier to record continuously =
and look for events based on what showed up on say the USGS web
 site. Events you can't see very well aren't that interesting to look at. -=
--- You asked :}    I don't remember right now the numerical spec=
ifics, but I can look it up if you are interested.
Regards
Barry
<=
span style=3D"text-decoration: underline; color: rgb(64, 64, 255);">http://=
www.seismicvault.com

--- On Tue, 12/29/09, Larry Conklin <=
i><lconklin@............> wrote:
=

From: Larry Conklin <lconklin@............>
Subject: Re: Senso=
r noise
To: psn-l@..............
Date: Tuesday, December 29, 2009, 1:=
16 PM

Hi Barry,

It sounds like you'v=
e been messing around with something similar to a project I started but did=
n't get around to finishing.  I added a waterfall FFT plot to my data =
logging program, but never finished debugging it.  In part because I
 wasn't all that confident that the plot would be worth all that much.
<=
br>I'd be interested in a little more information regarding how you are col=
lecting the FFT and some of the design decisions you use (number of samples=
 in the window, how frequently you do a conversion, etc.)  Did you wri=
te your own code or are you using some 3'rd party software?

Larry Co=
nklin
Liverpool, NY
lconklin@............

On 12/29/2009 12:12 AM, Barry Lotz wrote:
> All
> I have =
successfully used a running fft to sense signal frequency
> component=
 changes and used this as a trigger mechanism for an event.
> Could o=
ne use the ( I guess you call it ) power spectrum of the signal
> jus=
t before and during the event to remove the noise? I guess you would
>=
; have to use the same time window so the frequencies would compare.
 Could
> it be a simple subtraction of the "before" from the "during"=
? This would
> assume that the background noise didn't change in the =
period during an
> event. This could be better than trying to shape a=
 multi pole filter to
> eliminate the noise. I have found that often =
a portion of the event
> signal is in the same frequency range as the=
 noise.
> 
> Regards
> Barry
> http://www.seismicvault.com
&g=
t; 
__________________________________________________________

Pu=
blic Seismic Network Mailing List (PSN-L)

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