John,
Yes, the algorithms are published =
and not
difficult. A nice user interface would be a =
bit of work, but it’s =
definitely
doable.
=
Jack
=
=
From:
psn-l-request@.............. [mailto:psn-l-request@............... =
On Behalf Of John or Jan Lahr
Sent: Friday, February =
25, 2005
10:20 AM
To: psn-l@..............
Subject: RE: Microseisms =
and the
need for PSN to look closer
Hi Jack,
We used "spectrograms" of the sort that you described in =
studying the
1989-1990 eruption of Mt. =
Redoubt,
Alaska. They were =
the best
method that we found to monitor and distinguish "LP" events =
(with
emergent arrivals and lower frequency content) from "VT" =
events
(volcano-tectonic events with higher-frequency content and sharp
arrivals). Swarms of LP events tended to precede eruptions.
It would be great to have a tool available to amateur seismologists and
educators that would allow spectrograms to be easily generated. =
Do you
think this could be done without the use of Matlab?
Cheers,
John
At 07:01 AM 2/25/2005, you wrote:
There's a different way to look at seismic records that is =
particularly
interesting for microseisms. For a long time I've been using the =
specgram function of Matlab to look at both microseisms and =
quakes.
Specgram essentially divides the signal record into blocks of time
and performs an FFT on each block. It then displays the FFT =
amplitude
as a gray scale (or other color map). The Y axis of the display =
is
increasing frequency, the X-axis is time, and brightness of each pixel =
corresponds to the amplitude of the signal at that time and =
frequency.
Essentially you get an image showing how each frequency component =
changes
with time.
This type of display is frequently used in speech analysis, passive =
sonar,
and probably other fields. This is not to be confused with the =
simple
FFT function implemented by many of the data acquisition programs =
that
gives a line of amplitude versus frequency, and which is useless
by comparison.
I was amazed at the different information available in this type
of display compared with looking at a time series. You can see =
amplitude and frequency shifts of the microseisms (presumably as
storms change location and intensity). You can see frequency =
shifts
of the (dispersed) surface waves of a quake as it arrives.
I have identified quakes by looking at the specgram display that I
couldn't make out looking at the time series because they were buried =
in high-frequency noise.
You can also see interesting higher-frequency signals, including line =
spectra that shift and come and go mysteriously (probably cultural =
noise
of some type).
The representation allows you to easily distinguish body and surface =
waves
by their spectra, but because the FFT is done on blocks of data it is =
not useful for calculating very accurate arrival times.
If anyone's interested I can dig out some old data and post a =
picture. It
would be pretty easy to implement the algorithms in one of the data
=
acquisition/display programs....
Jack
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