[SAC-HELP] polezero option

George Helffrich george at gly.bris.ac.uk
Sat Apr 25 06:14:53 PDT 2009


Dear All -

	I will provide a rewrite suggestion after some further research.  Here 
are my initial comments, however.

----

POLEZERO OPTION:

%%%Comments indicated flagged with this notation.
%%%This paragraph is jumbled.  It confusingly combines information about
%%%the concepts underlying a pole-zero representation with the way to 
specify
%%%a response to SAC.  They should be separated.

POLEZERO is an instrument type that can be used to put in or take out 
the
(analog) seismometer response.  A good reference is Appendix C in the 
SEED
manual.  The current version can be downloaded from IRIS at URL
<http://www.iris.edu/software/downloads/seed_tools/>, and a good way to 
get a
polezero file in the correct format is to download data as a SEED 
volume and use
program rdseed to extract the data, the polezero file(s) and the 
response file.
Program rdseed can be downloaded from the same Web site.  The response 
file for
a specified channel/station is useful to see the "response in" and 
"response
out" units for the transfer function.  Typically, the "response in" 
listed in
the response file is velocity, in m/s, but the polezero file has a 
"response in"
of displacement.

%%%As Kuang He points out, this equation is syntactically incorrect.  
Pole-zero
%%%filters aren't causal, either!
%%%Using TeX-style expressions isn't a good idea -- it assumes the 
reader knows
%%%TeX, when many use nothing more than Word to typeset equations.  Lay 
them
%%%out graphically in text, assuming a fixed-pitch font.

       A polezero file is a listing of complex zeros and poles of a 
causal filter
that represents the seismometer.  The transfer function is of the form

(s-z1)(s-z2)...*s-zn)
_____________________

s-p1))(s-p2)...*s-pm)

where the z1 ... zn are the N zeros and the p1 ... pm are the m poles 
of the
transfer function.  The convention used is a Laplacian transform, with  
s = 2\pi
i f with f the frequency in Hz.  The sign convention is e^{-st} for the 
forward
transform (from the t domain to the s domain).
       Here is a polezero file for the LHZ channel from station JCC 
returned by program rdseed for a 28 June 2007 teleseism:

ZEROS 3
POLES 5
-0.0370  0.0370
-0.0370  -0.0370
-118.7520  423.4880
-118.7520  -423.4880
-251.3270  0.0000
CONSTANT 3.056572e+16

%%%The information about the input units and output units needs to be 
presented
%%%more clearly.  The description here jumbles what rdseed does with 
what SAC
%%%does.

For this transfer function, there are five poles, for which the complex 
values
are listed on the five lines following the line POLES 5.  Also there 
are three
zeros.  None are listed, and the convention is that an unlisted zero 
has the
value of zero.  Hence if there were five zeros, for which three were 
actually
zero, one could either write out five lines for which three of them 
would be
0.00 0.00 and the other two the complex, nonzero values, or one could 
have ZEROs
5 with only the two nonzero zeros explicitly written out.  In the 
response file
for this channel, there are only two zeros because the "response in" is 
velocity
but rdseed "knows" that the user wants to work with waveforms corrected 
to
displacement. The CONSTANT is a multiplicative factor that rescales the
"response in" units to the "response out" units at the normalization 
frequency.
The derivation of CONSTANT is described in the SEED manual and can be 
pieced
together for a particular channel from that channel's response file.

      The default for CONSTANT is 1.0, the options can be written in any 
order,
and one can add a comment line by starting that line with a * 
(asterisk).

       To use this option you specify the type to be POLEZERO and the 
subtype to
be the name of the file.  This may be a file in the current directory 
or in some
other directory if you specify the absolute or relative pathname.  It 
may also
the the name of a global file contained in the sac subdirectory
sac/aux/polezero/.  By putting a file in this global directory, anyone 
on your
system can easily use it.

EXAMPLE:  suppose the file was named sro.pz and you want to remove the
instrument response from station ABC.Z.

       u:  READ ABC.Z

       u:  TRANSFER FROM POLEZERO SUBTYPE SRO.PZ TO NONE

On 24 Apr 2009, at 20:25, Arthur Snoke wrote:

> Unfortunately, the file I sent was the unaltered "transfer' help file, 
> rather than my revised "polezero" subset of that file.  The desired 
> file (I hope) is attached.
>
> ---------- Forwarded message ----------
> Date: Fri, 24 Apr 2009 14:36:39 -0400 (EDT)
> From: Arthur Snoke <snoke at vt.edu>
> To: SAC-help Listserv <sac-help at iris.washington.edu>
> Subject: [SAC-HELP] polezero option
>
> We are preparing the next update for SAC, and I have been looking at 
> some "help" files called from within SAC (and in the computer 
> accessible manual), and have changed a few.  Because there were 
> several e-mail exchanges regarding the polezero option of transfer, I 
> looked closely at
>
> SAC> help transfer
>
> I am attaching my first draft for a replacement of that section of the 
> transfer help command.  Please share comments/corrections, etc.
>
> Others I have already worked on are 04graphics, 09file_format, 
> begindevices, enddevices.
>
> If there are others you think need updating, please share.  It is more 
> likely they will be included if you send me a suggested revision!
>
> Finally, I have written a C program for doing endian swapping for .sgf 
> files (SAC Graphics Format files).  I have tested it on several 
> platforms, but if someone would like to try it -- or just want an 
> advance copy -- let me know.
>
> Arthur Snoke
> snoke at vt.edu<polezero>_______________________________________________
> sac-help mailing list
> sac-help at iris.washington.edu
> http://www.iris.washington.edu/mailman/listinfo/sac-help
>
                                     George Helffrich
                                     george at geology.bristol.ac.uk




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