Greetings,
Could someone please clarify the physical meaning of freqlimits when using
TRANSFER function in SAC? I have read the documentation but for me it still
is a little fuzzy. Does it mean that when we use certain freqlimits, for
example freqlimits 0.01 0.015 100 200, the result we see is a narrow band
passed record with corner frequencies of, in this example 0.15Hz and 100Hz?
If that were the case and if I wanted to have the broadest bandpass
possible, what would be the limits to use? When I use lower freqlimits like
0.001 0.005 the resulting record seems to have a trend and sometimes
amplified long period signals. Therefore, I am forced to narrow the
freqlimits to 0.01 0.15 (lower end) 100 200 (higher end). I believe if
freqlimits is acting as a bandpass I am loosing some lower and higher
frequency information. Any feed back is much appreciated!
Thank you!
Januka - using SAC 101.3b
Could someone please clarify the physical meaning of freqlimits when using
TRANSFER function in SAC? I have read the documentation but for me it still
is a little fuzzy. Does it mean that when we use certain freqlimits, for
example freqlimits 0.01 0.015 100 200, the result we see is a narrow band
passed record with corner frequencies of, in this example 0.15Hz and 100Hz?
If that were the case and if I wanted to have the broadest bandpass
possible, what would be the limits to use? When I use lower freqlimits like
0.001 0.005 the resulting record seems to have a trend and sometimes
amplified long period signals. Therefore, I am forced to narrow the
freqlimits to 0.01 0.15 (lower end) 100 200 (higher end). I believe if
freqlimits is acting as a bandpass I am loosing some lower and higher
frequency information. Any feed back is much appreciated!
Thank you!
Januka - using SAC 101.3b
-
Januka,
I suggest looking at the help file for transfer that was included with
101.4 and is posted on the IRIS SAC Web site at
http://www.iris.edu/software/sac/commands/transfer.html. I think/hope
the description is clearer than it was earlier, although the functionality
is probably the same.
Dealing with your example: freqlimits 0.01 0.015 100 200. Assuming your
Nyquist is above 200.0, what you have here is a filter that has unit
weight between f2 and f3: 0.015 and 100.0. That is, f2 and f3 are not
corners as in a "normal" bandpass. There is a cosine taper from unity to
0 between 100.0 and 200.0 and between 0.015 and 0.01. The response is
"zero" for frequencies below 0.01 and above 200.0. I put quotes around
the zero, because unless your npts is a power of 2, because (apparently)
of the nature of the FFT, the response will not be zero.
I hope this helps. If not, let me know.
Arthur
On Mon, 13 Sep 2010, Januka Attanayake wrote:
Greetings,
Could someone please clarify the physical meaning of freqlimits when
using TRANSFER function in SAC? I have read the documentation but for me
it still is a little fuzzy. Does it mean that when we use certain
freqlimits, for example freqlimits 0.01 0.015 100 200, the result we see
is a narrow band passed record with corner frequencies of, in this
example 0.15Hz and 100Hz? If that were the case and if I wanted to have
the broadest bandpass possible, what would be the limits to use? When I
use lower freqlimits like 0.001 0.005 the resulting record seems to have
a trend and sometimes amplified long period signals. Therefore, I am
forced to narrow the freqlimits to 0.01 0.15 (lower end) 100 200 (higher
end). I believe if freqlimits is acting as a bandpass I am loosing some
lower and higher frequency information. Any feed back is much
appreciated!
Thank you!
Januka - using SAC 101.3b
-
Thanks Arthur, the process is clear now. However, one of the questions I
raised still remains. That is how would you determine what low/high pass
frequencies (f1 f2 f3 f4) would you use? Documentation says "for a Nyquist
frequency of 0.5, set f3=10. and f4=20". When processing seismograms in bulk
with different instruments
(1) you wouldn't know what the Nyquist freq. is
(2) If the answer to (1) is " yes you don't" how would you determine
f1,f2,f3,f4 for a given Nyquist freq on general terms.
Thanks again !
Januka.
On Mon, Sep 13, 2010 at 2:28 PM, Arthur Snoke <snoke<at>vt.edu> wrote:
Januka,
--
I suggest looking at the help file for transfer that was included with
101.4 and is posted on the IRIS SAC Web site at <
http://www.iris.edu/software/sac/commands/transfer.html. I think/hope
the description is clearer than it was earlier, although the functionality
is probably the same.
Dealing with your example: freqlimits 0.01 0.015 100 200. Assuming your
Nyquist is above 200.0, what you have here is a filter that has unit weight
between f2 and f3: 0.015 and 100.0. That is, f2 and f3 are not corners as
in a "normal" bandpass. There is a cosine taper from unity to 0 between
100.0 and 200.0 and between 0.015 and 0.01. The response is "zero" for
frequencies below 0.01 and above 200.0. I put quotes around the zero,
because unless your npts is a power of 2, because (apparently) of the nature
of the FFT, the response will not be zero.
I hope this helps. If not, let me know.
Arthur
On Mon, 13 Sep 2010, Januka Attanayake wrote:
Greetings,
Could someone please clarify the physical meaning of freqlimits when using
TRANSFER function in SAC? I have read the documentation but for me it still
is a little fuzzy. Does it mean that when we use certain freqlimits, for
example freqlimits 0.01 0.015 100 200, the result we see is a narrow band
passed record with corner frequencies of, in this example 0.15Hz and 100Hz?
If that were the case and if I wanted to have the broadest bandpass
possible, what would be the limits to use? When I use lower freqlimits like
0.001 0.005 the resulting record seems to have a trend and sometimes
amplified long period signals. Therefore, I am forced to narrow the
freqlimits to 0.01 0.15 (lower end) 100 200 (higher end). I believe if
freqlimits is acting as a bandpass I am loosing some lower and higher
frequency information. Any feed back is much appreciated!
Thank you!
Januka - using SAC 101.3b
"Nothing can start to exist nor can cease to exist, only transformation is
possible"
Januka Attanayake
Solid Earth Geophysics & Economics
University of Connecticut
Beach Hall U-2045
354,Mansfield Rd;
Storrs.CT 06269
Tel: +1 860 486 0475 (Office)
+1 860 486 3914 (Lab - P405)
URL: http://sites.google.com/site/janukaattanayake/
-
Comments interspersed.
On Tue, 14 Sep 2010, Januka Attanayake wrote:
Thanks Arthur, the process is clear now. However, one of the questions I
In the help file that comes with 101.4 and which can bde accessed from the
raised still remains. That is how would you determine what low/high pass
frequencies (f1 f2 f3 f4) would you use? Documentation says "for a
Nyquist frequency of 0.5, set f3=10. and f4=20". When processing
seismograms in bulk with different instruments (1) you wouldn't know
what the Nyquist freq. is (2) If the answer to (1) is " yes you don't"
how would you determine f1,f2,f3,f4 for a given Nyquist freq on general
terms.
IRIS SAC manual pages, it says:
If you want to do a low-pass filter but have no filtering at low
frequencies, one way is to set f1=-2 and f2=-1. If you want to do a
high-pass fiter but have no filtering at the high frequencies, for a
Nyquist frequency of 0.5, set f3=10. and f4=20.
The point of this passage is how do you use freqlimits if you want to have
no filtering at one end of the spectrum but you do at the other. The
program as written says that if f2 > f1, but f2 is less than df, there is
no filtering at low frequencies (a negative frequency is always less than
any df.). Similarly, for a Nyquist of 0.5 Hz, if f4 > f3 and f3 is
greater than 0.5, there is no filtering at high frequencies (10 is much
greater than 0.5). Note that it is important that f2 > f1, or the program
wil think this is dealing with the high-frequency end.
You ask how one knows the Nyquist if one has many instruments, etc. DF
and the Nyquist are related to the time series itself. Here I read in a
time series and do an lh:
SAC> r N11A.lhz
SAC> lh
FILE: N11A.lhz - 1
--------------
NPTS = 3101 B = -9.980005e+01
E = 3.000200e+03 IFTYPE = TIME SERIES FILE
LEVEN = TRUE DELTA = 1.000000e+00
DEPMIN = -1.928006e+03 DEPMAX = 1.540401e+03
Now I do an fft and a lh
SAC> fft
DC level after DFT is -724.33
SAC> lh
FILE: N11A.lhz - 1
--------------
NPTS = 4096 B = 0.000000e+00
E = 5.000000e-01 IFTYPE = SPECTRAL
FILE-AMPL/PHASE
LEVEN = TRUE DELTA = 2.441406e-04
DEPMIN = -1.928006e+03 DEPMAX = 1.540401e+03
In the first lh, we see that there are 3101 points with a sampling rate of
1 sps for a total time, then, of (NPTS-1)*DELTA =3100 seconds. After the
fft, there are
4096 points because the fft adds zeroes to the time serie to the next
power of 2. The lowest frequency is 0, and E is now the Nyquist of 0.5 Hz
(0.5/DELTA). Now, DELTA = 2.441406e-04 is DF, the frequency interval.
I hope this helps. If you still have questions ...
-
GREAT ! Thanks again Arthur, I haven't got a better explanation than the one that you just gave me! May be you should include this in SAC documentation.
Serenity isn't freedom from the storm, but peace within the storm
Januka Attanayake Solid Earth Geophysics & Economics
The University of Connecticut
Beach Hall - U2045
354, Mansfield Rd;
Storrs, CT 06269
Tel : 860 486 0475 (Office)
860 486 3914 (Lab - P405)
URL: http://sites.google.com/site/janukaattanayake/
--- On Tue, 9/14/10, Arthur Snoke <snoke<at>vt.edu> wrote:
From: Arthur Snoke <snoke<at>vt.edu>
Subject: Re: [SAC-HELP] TRANSFER freqlimits
To: "Januka Attanayake" <jattanayake<at>gmail.com>
Cc: sac-help<at>iris.washington.edu
Date: Tuesday, September 14, 2010, 10:54 AM
Comments interspersed.
On Tue, 14 Sep 2010, Januka Attanayake wrote:
Thanks Arthur, the process is clear now. However, one of the questions I raised still remains. That is how would you determine what low/high pass frequencies (f1 f2 f3 f4) would you use? Documentation says "for a Nyquist frequency of 0.5, set f3=10. and f4=20". When processing seismograms in bulk with different instruments (1) you wouldn't know what the Nyquist freq. is (2) If the answer to (1) is " yes you don't" how would you determine f1,f2,f3,f4 for a given Nyquist freq on general terms.
In the help file that comes with 101.4 and which can bde accessed from the IRIS SAC manual pages, it says:
If you want to do a low-pass filter but have no filtering at low frequencies, one way is to set f1=-2 and f2=-1. If you want to do a high-pass fiter but have no filtering at the high frequencies, for a Nyquist frequency of 0.5, set f3=10. and f4=20.
The point of this passage is how do you use freqlimits if you want to have no filtering at one end of the spectrum but you do at the other. The program as written says that if f2 > f1, but f2 is less than df, there is no filtering at low frequencies (a negative frequency is always less than any df.). Similarly, for a Nyquist of 0.5 Hz, if f4 > f3 and f3 is greater than 0.5, there is no filtering at high frequencies (10 is much greater than 0.5). Note that it is important that f2 > f1, or the program wil think this is dealing with the high-frequency end.
You ask how one knows the Nyquist if one has many instruments, etc. DF and the Nyquist are related to the time series itself. Here I read in a time series and do an lh:
SAC> r N11A.lhz
SAC> lh
FILE: N11A.lhz - 1
--------------
NPTS = 3101 B = -9.980005e+01
E = 3.000200e+03 IFTYPE = TIME SERIES FILE
LEVEN = TRUE DELTA = 1.000000e+00
DEPMIN = -1.928006e+03 DEPMAX = 1.540401e+03
Now I do an fft and a lh
SAC> fft
DC level after DFT is -724.33
SAC> lh
FILE: N11A.lhz - 1
--------------
NPTS = 4096 B = 0.000000e+00
E = 5.000000e-01 IFTYPE = SPECTRAL FILE-AMPL/PHASE
LEVEN = TRUE DELTA = 2.441406e-04
DEPMIN = -1.928006e+03 DEPMAX = 1.540401e+03
In the first lh, we see that there are 3101 points with a sampling rate of 1 sps for a total time, then, of (NPTS-1)*DELTA =3100 seconds. After the fft, there are 4096 points because the fft adds zeroes to the time serie to the next power of 2. The lowest frequency is 0, and E is now the Nyquist of 0.5 Hz (0.5/DELTA). Now, DELTA = 2.441406e-04 is DF, the frequency interval.
I hope this helps. If you still have questions ...
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