Pete,
What I do effectively is to 'bandpass' my helicord records for purpose =
of seeing mostly teleseismic earthquakes. For example, in the 'integrated'=
(true velocity) output from the VolksMeter, I operate with a 1-pole-each-p=
air of high-pass and low-pass filters. The WinSDR high-pass cutoff is 0.05=
Hz (20 s period, sitting "on top of " the typical Rayleighh wave) and a lo=
w pass cutoff at 0.5 Hz (2 s period). This way I have a good chance of se=
eing teleseismic events whose signals are above my SNR threshold. To see l=
ocal , usually small earthquakes (which I'm not interested in, since I don'=
t live in a place like California, where you could see them almost daily, I=
would want to operate with the 'raw' signal (displacement proportional to =
acceleration) output from the VM. I would set the low pass for that case a=
t probably 10 Hz, since the 'signature' of seismic waves received from suc=
h are much higher frequency than what is left after the waves travel large =
distances through the Earth.
I don't save anything from my instrument for later analysis using sample =
rates greater than one per second, since I don't care about looking at loca=
lly generated 'noises'. If you look at Larry Cochrane's VM output you can=
see (for reason of his filter settings) when San Francisco is awake as opp=
osed to asleep.
My buddy at another place in CA, who's also using a VolksMeter (on Cobb=
Mtn, I leave it to you readers to see if you can figure out his name from=
his extensive webpages) has been doing something that list-serve readers m=
ight find interesting. The data that is presented in his animations at htt=
p://flyingsnail.com/AmateurRadio/Indonesia7-2-fft-csp.html
Involves records for which the high-pass cutoff frequency was 0.0005 Hz.
I am working with an internationally distributed group to look at something=
at even lower frequencies, that is really exciting. It appears that Volks=
Meter tilt data is highly correlated with magnetic field variations of the =
Earth derived from flux-gate magnetometers. One cannot for such studies ha=
ve any high-pass filter whatsoever. The VolksMeter is ideally suited to th=
ese investigations, since it responds all the way down to d.c. Probably th=
e reason what we're seeing has not been observed many times in the past, is=
because seismologists have operated mostly with instruments that don't go =
that low in frequency.
Randall
Pete,
What I do effectively is to ‘=
bandpass’
my helicord records for purpose of seeing mostly teleseismic earthquakes.&n=
bsp;
For example, in the ‘integrated’ (true velocity) output from th=
e
VolksMeter, I operate with a 1-pole-each-pair of high-pass and low-pass fil=
ters.
The WinSDR high-pass cutoff is 0.05 Hz (20 s period, sitting “on top =
of “
the typical Rayleighh wave) and a low pass cutoff at 0.5 Hz (2 s
period). This way I have a good chance of seeing teleseismic ev=
ents
whose signals are above my SNR threshold. To see local , usually smal=
l earthquakes
(which I’m not interested in, since I don’t live in a place lik=
e
California, where you could see them almost daily, I would want to operate =
with
the ‘raw’ signal (displacement proportional to acceleration) ou=
tput
from the VM. I would set the low pass for that case at probably 10 Hz=
,
since the ‘signature’ of seismic waves received from such=
are
much higher frequency than what is left after the waves travel large distan=
ces through
the Earth.
I don’t save anything from my instrument =
for
later analysis using sample rates greater than one per second, since I don&=
#8217;t
care about looking at locally generated ‘noises’. I=
f
you look at Larry Cochrane’s VM output you can see (for reason of his
filter settings) when San Francisco is awake as opposed to asleep.
My buddy at another place in CA, wh=
o’s
also using a VolksMeter (on Cobb Mtn, I leave it to you readers to se=
e if
you can figure out his name from his extensive webpages) has been doing
something that list-serve readers might find interesting. The data th=
at
is presented in his animations at http=
://flyingsnail.com/AmateurRadio/Indonesia7-2-fft-csp.html
Involves records for which the high-pass cutoff
frequency was 0.0005 Hz.
I am working with an internationally distributed group=
to
look at something at even lower frequencies, that is really exciting. =
;It
appears that VolksMeter tilt data is highly correlated with magnetic field =
variations
of the Earth derived from flux-gate magnetometers. One cannot for suc=
h
studies have any high-pass filter whatsoever. The VolksMeter is ideal=
ly
suited to these investigations, since it responds all the way down to d.c.&=
nbsp;
Probably the reason what we’re seeing has not been observed many time=
s in
the past, is because seismologists have operated mostly with instruments th=
at
don’t go that low in frequency.
Randall