Thread: Correct the clock drift of a SAC file

Started: 2017-01-11 21:41:36
Last activity: 2017-01-12 18:49:37
Topics: SAC Help
Roberto Cabieces Diaz
2017-01-11 21:41:36
Hello everyone,

So, I have a problema with my file.sac data, the thing is,

Because of my data have been a bad GPS time syncronization, every sample of
my file dosn't correspond with the real GPS time. But I have a time factor
correction to move every data to his real simple. I mean, for example in
the original file for the first simple correspond 01h03min33 seconds and i
have to rest one second to this simple (and the same operation to all the
seismogram)

How I have to apply this correction,

For example if I have 1 day seismogram of data???

It is sac able to do this task??

thnak you to the people tah give some light or any idea to solve the
problem.

  • Geo .
    2017-01-12 07:33:00
    Hi,
    It's not exactly clear to me what you want to do. If, however, you want to
    change the origin date/time of the file, you could play with headers,
    NZYEAR, NZJDAY, NZHOUR, NZMIN, NZSEC, and NZMSEC.

    Januka

    Januka Attanayake
    Postdoctoral Research Associate

    Institut für Geophysik
    Westfälische Wilhelms - Universität Münster
    CorrensstraBe 24
    48149 Münster
    Germany

    University email: jattanayake<at>uni-muenster.de
    Homepage: http://sites.google.com/site/janukaattanayake/

    On 11 January 2017 at 17:43, Roberto Cabieces Diaz <rcabdia<at>gmail.com>
    wrote:

    Hello everyone,

    So, I have a problema with my file.sac data, the thing is,

    Because of my data have been a bad GPS time syncronization, every sample
    of my file dosn't correspond with the real GPS time. But I have a time
    factor correction to move every data to his real simple. I mean, for
    example in the original file for the first simple correspond 01h03min33
    seconds and i have to rest one second to this simple (and the same
    operation to all the seismogram)

    How I have to apply this correction,

    For example if I have 1 day seismogram of data???

    It is sac able to do this task??

    thnak you to the people tah give some light or any idea to solve the
    problem.


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  • George Helffrich
    2017-01-12 18:49:37
    Dear Roberto -

    From your description, it is not clear what type of clock problems you have. There are three possibilities: a completely unsynchronized clock; a clock that periodically locks to GPS and causes a time tear (gap or overlap) when synchronization is achieved; and discrete offsets due to leap seconds not handled properly by buggy/faulty datalogger software.

    The last case requires knowledge of how the datalogger behaves and correction of the time stamps in the data (probably in the MSEED headers). SAC cannot help with this; you have to write a program to do the correction. Consult an expert on your datalogger.

    The other two problems SAC can help with. To fix them you can change the value of DELTA in the header (use CHNHDR) to represent the real sample rate and then use INTERPOLATE to convert it to the desired sample rate. Split your data up into segments with known start and stop times and assume a linear clock drift rate within each segment to calculate the “real” DELTA. After repair, you can look at the end time of one segment and the start time of the next segment to see if you got the correction right. There should be a 1 sample gap between them (the DELTA separating the end of one and the start of the next).

    On 12 Jan 2017, at 01:43, Roberto Cabieces Diaz <rcabdia<at>gmail.com> wrote:

    Hello everyone,

    So, I have a problema with my file.sac data, the thing is,

    Because of my data have been a bad GPS time syncronization, every sample of my file dosn't correspond with the real GPS time. But I have a time factor correction to move every data to his real simple. I mean, for example in the original file for the first simple correspond 01h03min33 seconds and i have to rest one second to this simple (and the same operation to all the seismogram)

    How I have to apply this correction,

    For example if I have 1 day seismogram of data???

    It is sac able to do this task??

    thnak you to the people tah give some light or any idea to solve the problem.

    ----------------------
    SAC Help (http://ds.iris.edu/message-center/topic/sac-help/)

    Sent from the IRIS Message Center (http://ds.iris.edu/message-center/)
    Update subscription preferences at http://ds.iris.edu/account/profile/



    George Helffrich
    george<at>elsi.jp


17:14:41 v.22510d55