[SAC-HELP] Polezero rewrite
Renate Hartog
renate at ess.washington.edu
Mon Apr 27 10:49:49 PDT 2009
Hello Everyone,
I don't have time to work on a true edit, but I do applaud you all for
working on a better explanation of the POLE ZERO option for TRANSFER since
it seems to generate a lot of confusion.
Here are the few points that add to the confusion in my opinion:
1) Transfer's SAC HELP says that the NONE type is Displacment.
If NONE is as I interpret it, namely, don't deconvolve or convolve with
anything, then there is no longer a units issue. You can just use the
native response units as specified in de SEED file and get out the units
you'd expect. You could even leave out the extra zero(s) and get velocity
or acceleration, whichever you want.
2) The fact that SAC defaults to Displacement in nm, is only relevant if
you want to put in, or take out, one of the pre-programmed instrument
types.
3) rdseed does add the extra zeros to its SAC_PZ files to result in a
Displacement response, however, it outputs a CONSTANT that is A0*S that
results in the signal that are usually (THANK GOODNESS) in SI, m/s or
m/s**2 (depending on the units-in field of the SEED file) if you use
TRANSFER FROM POLEZERO rdseed-output-file TO NONE.
To make all this clear, you don't only need to edit the POLEZERO part of
the transfer help, but also the other parts of TRANSFER. And Rdseed could
use some more information about the SAC_PZ output file in its man-page.
-Renate
Renate Hartog
PNSN/UW Seismology lab.
On
Mon, 27 Apr 2009, George Helffrich wrote:
> Dear All -
>
> Here's my rewrite of the polezero response removal text to clarify
> some of the issues raised in the version proposed earlier. For sanity, use a
> fixed-pitch font to view the text.
>
> ------
>
> POLEZERO OPTION:
> One of the instrument types is called POLEZERO. This type lets you
> describe a general instrument response by specifying a file which contains
> its poles and zeros. The options in the file are keyword driven and the
> numbers are in free format. You may specify a multiplicative scaling
> constant A0 by putting a line in the file containing the keyword "CONSTANT"
> followed by a floating point number. The default for this constant is 1.0
> if
> you omit this line. You specify the number of poles by putting a line in
> the
> file with the keyword "POLES" following by an integer number. The next
> lines
> in the file until another keyword is read become the poles for this
> instrument. Each such line contains two floating point numbers specifying
> the real and imaginary parts of each of the poles. If you have fewer lines
> specifying poles than you stated on the "POLES" line, the remaining poles
> are
> assumed to be numerically zero. You specify the zeros in the same way with
> a "ZEROS" keyword line following by lines specifying the non-zero zeros.
> You
> may specify up to 15 poles and 15 zeros. For example, the following is the
> specification for the SRO broadband seismometer:
>
> ZEROS 4
>
> -0.125 0.0
>
> -50.0 0.0
>
> POLES 4
>
> -0.13 0.0
>
> -6.02 0.0
>
> -8.66 0.0
>
> -35.2 0.0
>
> CONSTANT -394.0
>
> Notice that since two of the zeros are at the origin, they don't have to be
> specified in the file. Also notice that the options may appear in any order
> in the file. If there are any nonzero imaginary parts to a pole or zero,
> they
> must appear as conjugate pairs to make the response purely real.
> SAC uses the omega convention for the pole-zero response: the
> frequency
> w = 2*pi*i*f, where f is frequency in Hz and i is the imaginary unit
> (sqrt(-1)). Thus, if an instrument manufacturer specifies that a pole is at
> 1
> Hz on the real axis, the SAC pole's real and imaginary parts would be
> (6.28, 0). The response H(w) is the ratio of the product of the difference
> between w and each of the np poles and nz zeros:
>
> (w-z )*(w-z )*...*(w-z )
> 1 2 nz
> H(w) = -------------------------
> (w-p )*(w-p )*...*(w-p )
> 1 2 np
>
> For conversion of a seismometer's analog output to a physical unit
> (m, m/sec, m/sec**2), there are two specific frequencies that are important:
> the normalization frequency wn, and the sensitivity frequency, ws. The
> normalization frequency is the frequency at which the product A0*H(wn) = 1.0
> A0 is the CONSTANT value specified in the response. The sensitivity S of
> the
> instrument is the value by which A0*H(w) should be multiplied to get the
> appropriate physical unit. To avoid unnecessary complications in applying
> instrument corrections, wn and ws should be equal. If they are, then by
> multiplying the signal by S before applying the transfer function, the
> correct
> physical units are produced. If not, you must multiply by S*H(ws)/H(wn).
> If
> convenient, you can combine S and A0 and set CONSTANT to be their product
> S*A0.
> SAC assumes that when you apply your pole-zero instrument correction,
> it will yield displacement. If this is not the physical unit yielded in
> your
> instrument response, dividing or multiplying by w will respectively
> integrate
> or differentiate the response. Thus if your instrument response is velocity
> (m/sec), then adding an extra zero with value zero will yield displacement.
> Conversely, if your instrument response is displacement and you want
> velocity
> as your physical unit, add an extra pole with value zero (but this is not
> what SAC would expect of the response).
> 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 "polezero" subdirectory
> of the "sacaux" directory. By putting a file in this global directory,
> anyone
> on your system can easily use it.
>
> REFERENCES:
> Hutt, C. R., "Specifying and using channel response information",
> Appendix
> C, SEED Reference Manual, version 2.4, 2006; available online from
> <http://www.iris.edu/software/downloads/seed_tools/>.
> Nakanishi, K., "Computer code for the transfer function of seismic
> systems", Lawrence Livermore National Lab., UCID-18071, 1979.
>
> 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
>
> ------
>
> George Helffrich
> george at geology.bristol.ac.uk
>
> _______________________________________________
> sac-help mailing list
> sac-help at iris.washington.edu
> http://www.iris.washington.edu/mailman/listinfo/sac-help
>
More information about the sac-help
mailing list