Thread: Correct the Lag time

Started: 2021-05-20 12:36:35
Last activity: 2021-08-31 10:50:03
Topics: SAC Help
Aqeel Abbas
2021-05-20 12:36:35
Dear All,
I found lag time around 2 seconds for each day in my data by using cross correlation.
1: I modified the KZDATE and KZTIME based on lag time.
2: Later, when I want to compare arrival times for all stations in SAC for teleseismic event modifying header according to event time, (I modified the station files according to events info such as evla, evlo,evdp,kztime and kzdate)
3. But it gives the same arrival time for the corrected and original sac file.

What is the possible way to correct the SAC files for few seconds or milliseconds? Or where I am doing some mistakes?
  • George Helffrich
    2021-06-04 15:18:02
    Dear Aqueel -

    A possible solution to your problem is that you must change NZHOUR NZMIN NZMSEC to change KZTIME - it is derived from those other header quantities. Try a test with a single file, change NZHOUR, NZMIN and NZMSEC, write it out, then re-read and check KZTIME.

    Other than this suggestion, perhaps you aren’t writing out the file before re-reading it and using the changed header information.

    On 4 Jun 2021, at 15:04, Aqeel Abbas (via IRIS) <sac-help-bounce<at>lists.ds.iris.edu> wrote:

    Dear All,
    I found lag time around 2 seconds for each day in my data by using cross correlation.
    1: I modified the KZDATE and KZTIME based on lag time.
    2: Later, when I want to compare arrival times for all stations in SAC for teleseismic event modifying header according to event time, (I modified the station files according to events info such as evla, evlo,evdp,kztime and kzdate)
    3. But it gives the same arrival time for the corrected and original sac file.

    What is the possible way to correct the SAC files for few seconds or milliseconds? Or where I am doing some mistakes?

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    George Helffrich
    george<at>elsi.jp


    • Pritesh Soni
      2021-07-28 16:41:39
      respected sir,

      sir i am also facing this problem, the original SAC files NZHOUR and other headers have different values than event origin time so in order to change them and set them to event origin time i have followed the same steps you have suggested in your above solution,
      using "ch O GMT" (also tried using "CHNHDR O GMT") command and writing over the files using "wh over" ( "w over" ) command. but still header values are not changed.
      • Milton Plasencia
        2021-08-22 21:20:16
        Hi,

        Your ch O GMT is incomplete,
        Try,
        sac> ch O GMT 2021 234 21 28 00.000
        where 234 is Julian day (today)
        21=Hour; 28=Minutes, 00.000=second.milliseconds.

        HTH,

        Milton


        Dott. Milton P. PLASENCIA LINARES
        Centro Ricerca Sismologiche (CRS)
        Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS
        Borgo Grotta Gigante n. 42/c
        34010 Sgonico (Trieste) - Italia
        Tel. +39 040 2140 256 /156 Cell. +39 331 6481 935
        www.inogs.it


        On Sun, 22 Aug 2021 at 18:15, PRITESH SONI (via IRIS) <
        sac-help-bounce<at>lists.ds.iris.edu> wrote:

        respected sir,

        sir i am also facing this problem, the original SAC files NZHOUR and other
        headers have different values than event origin time so in order to change
        them and set them to event origin time i have followed the same steps you
        have suggested in your above solution,
        using "ch O GMT" (also tried using "CHNHDR O GMT") command and writing
        over the files using "wh over" ( "w over" ) command. but still header
        values are not changed.

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        • Pritesh Soni
          2021-08-29 12:58:50
          Hi,

          SAC throws an error while trying your command,

          "ERROR interpreting command: ch O GMT 2012 001 05 27 55.980
          ILLEGAL OPTION:"
      • Geo .
        2021-08-31 10:50:03
        Hi Pritesh,
        In addition to what others have suggested, you also will have to take time
        offsets into account as your seismogram appears to start at a different
        time than the origin time. So here's the complete solution:

        CHNHDR O GMT 2012 001 05 27 55 980
        CHNHDR allt (0 - &1,o&) IZTYPE IO
        WRITE OVER

        Make sure to keep a copy of your original seismogram since you're doing a
        WRITE OVER.

        Januka

        Januka Attanayake
        Research Fellow/Lecturer | Earthquake Seismology Lead
        Homepage: *https://sites.google.com/site/janukaattanayake
        https://sites.google.com/site/janukaattanayake*

        School of Geography, Earth & Atmospheric Sciences
        University of Melbourne | Parkville 3010 VIC
        Australia




        On Mon, 23 Aug 2021 at 02:13, PRITESH SONI (via IRIS) <
        sac-help-bounce<at>lists.ds.iris.edu> wrote:

        respected sir,

        sir i am also facing this problem, the original SAC files NZHOUR and other
        headers have different values than event origin time so in order to change
        them and set them to event origin time i have followed the same steps you
        have suggested in your above solution,
        using "ch O GMT" (also tried using "CHNHDR O GMT") command and writing
        over the files using "wh over" ( "w over" ) command. but still header
        values are not changed.

        ----------------------
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