PSN-L Email List Message

Subject: RE: Microseisms and the need for PSN to look closer
From: Jack Ivey ivey@..........
Date: Fri, 25 Feb 2005 10:28:36 -0500


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

__________________________________________________________

Public Seismic Network Mailing List (PSN-L)

  for more information. 


##################################/ John C. Lahr
#################################/ Emeritus Seismologist 
################################/ U.S. Geological Survey
===========================/ Geologic Hazards Team, MS966
##############################/ PO Box 25046
#############################//##############################
############################//###############################
     Denver, Colorado 80225-0046 /################################
             Phone: (303) 215-9913 /=============================
               Fax: (303) 273-8540 /##################################
                     lahr@........ /###################################
 
/####################################
                                 http://jclahr.com/science/
 
























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

__________________________________________________________

Public Seismic Network Mailing List (PSN-L)

eudora=3Dautourl>http://www.seismicnet.com/maillist.html for more information.


##################################/ John C. Lahr
#################################/ Emeritus Seismologist
################################/ U.S. Geological = Survey
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D/ Geologic Hazards Team, MS966
##############################/ PO Box 25046
#############################//##############################
############################//###############################
     Denver, Colorado 80225-0046 /################################
            = Phone: (303) 215-9913 = /=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D
            =    Fax: (303) 273-8540 /##################################
            =          lahr@........ /###################################
            =             =             =       /####################################
            =             =          http://jclahr.com/science/


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