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.
ithout 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.
o while the chirp may appear to precede the signal, it may be the signal wh=
ich=20
s delayed by the filter, and the chirp may be a filter artefact aligned wit=
h=20
he initial arrival.
Hi Mark,=20
Larry said :-
    I was looking over the data I recorded overnight from the 6.5 quake off=
=20
apan.  In the process I applied a band-pass filter (0.4 - 2.0 hz) to=20
ee what, if anything, would show up.  What showed up were two strange=20
chirps" that are all but hidden in the noise. They are substantially=20
identical, lasting for about a minute and 20 seconds each, and decending fr=
om=20
around 2 hz to .4 hz.=20
.
    This is NOT the response of a heavily damped filter to a transient puls=
e !!=20
Check http://neic.usgs.gov/neis/travel_times/ttgraph.html for maybe 90 to 1=
00=20
degrees. There are several S wave variants, so the signals could be real.
.
    Regards,
    Chris Chapman=20
=20















=
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 i=
t is difficult to know=20
exactly how it will behave, but in general you will get a ti=
me delay and often=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 arte=
fact aligned with=20
the initial arrival.

Hi Mark, 
Larry said :-
identical, la=
sting for about a minute and 20 seconds each, and decending from 
around 2 hz to .4 =
hz. 
.
    This is=
 NOT the response of a heavily damped filter to a transient pulse !! 
Check http://neic.usgs.g=
ov/neis/travel_times/ttgraph.html for maybe 90 to 100 
degrees. There are severa=
l S wave variants, so the signals could be real.
.
    Regards,
    Chris Chapman 
=