Thread: how does SAC define the header variable CMPINC, exactly?

Started: 2008-10-07 20:35:54
Last activity: 2008-10-08 07:19:21
Topics: SAC Help
Hello SAC users,

I have a dataset from a station that was NOT placed perfectly level -
e.g. the sensor was placed on a ledge that had a little bit of tilt,
such that Z is not perfectly up/down, and N + E have some down or upward
component in the data. I'm now trying to analyze that data, but have
yet to find a good definition of cmpinc in the manual (all it says is
"Component incident angle (degrees from vertical)"). CMPINC has no
inherent orientation (like CMPAZ, from north, and looking down with 90
to the right e.g. east), but I can think of only one good, logical way
to define cmpinc. Although, I'm probably missing something, and would
like to verify that this is correct, hence the email to you all.

I can infer the following things from pulling the data from an
earthquake seismology station (cmb.bk):

* +Z is probably up, (not down like in the oilfield): Up = 0, hence
Down = 180.
* the other components have cmpinc = 90 (N + E)
* Therefore, the direction of the cmpinc vector *must* be relative
to the cmpaz vector AND that cmpinc vector corresponds to apparent
tilt along that vector (not absolute/maximum tilt for the whole
instrument in whatever direction that happens to be).
* It follows other axes would have to be defined as follows:
o DIR CMPAZ CMPINC
o south 180 90
o west 270 90
o n + a little up 0 75
o e + a little down 90 110

Can anyone tell me if this reasoning is correct, and if not, point me to
some documentation with a clear definition?

Oh, and if any of you know how Antelope defines the <vang> vector, I'm
also trying to figure that out (I think that Antelope's definition of
"vertical" is different, e.g. Up = 180 and Down = 0, although, I cannot
confirm it).

Thanks for your time!
Valerie Zimmer

  • Dear Valerie

    as far as I know your interpretation is essentially correct except that I
    am not entirely clear with what you mean by
    * Therefore, the direction of the cmpinc vector *must* be relative
    to the cmpaz vector AND that cmpinc vector corresponds to apparent
    tilt along that vector (not absolute/maximum tilt for the whole
    instrument in whatever direction that happens to be).

    If you think of each component as a vector then cmpinc is the angle the
    vector makes with vertical up, and cmpaz is the azimuth clockwise from
    north of the horizontal projection of the component vector. It does not
    make sense to talk about the cmpinc or cmpaz vector. For cmpinc=0 or
    cmpinc=180 the value of cmpaz is irrelevant, of course. As far as I know
    nowhere in SAC is cmpinc actually used except to check whether components
    are horizontal for application of the "rot to gcp" command.

    A few years back I wrote a SAC command to rotate an arbitrarily oriented
    sensor (3 components needed to be perpendicular) into either VRT, ZNE, or
    LQT system, and this command used cmpinc. This used the extern mechanism of
    sac2000 but I am not sure whether this way of loading up user-defined
    commands works with the more recent versions of sac. If you are interested
    I can send you the source.

    Regards
    Frederik








    On Oct 7 2008, Val Zimmer wrote:

    Hello SAC users,

    I have a dataset from a station that was NOT placed perfectly level -
    e.g. the sensor was placed on a ledge that had a little bit of tilt,
    such that Z is not perfectly up/down, and N + E have some down or upward
    component in the data. I'm now trying to analyze that data, but have
    yet to find a good definition of cmpinc in the manual (all it says is
    "Component incident angle (degrees from vertical)"). CMPINC has no
    inherent orientation (like CMPAZ, from north, and looking down with 90
    to the right e.g. east), but I can think of only one good, logical way
    to define cmpinc. Although, I'm probably missing something, and would
    like to verify that this is correct, hence the email to you all.

    I can infer the following things from pulling the data from an
    earthquake seismology station (cmb.bk):

    * +Z is probably up, (not down like in the oilfield): Up = 0, hence
    Down = 180.
    * the other components have cmpinc = 90 (N + E)
    * Therefore, the direction of the cmpinc vector *must* be relative
    to the cmpaz vector AND that cmpinc vector corresponds to apparent
    tilt along that vector (not absolute/maximum tilt for the whole
    instrument in whatever direction that happens to be).
    * It follows other axes would have to be defined as follows:
    o DIR CMPAZ CMPINC
    o south 180 90
    o west 270 90
    o n + a little up 0 75
    o e + a little down 90 110

    Can anyone tell me if this reasoning is correct, and if not, point me to
    some documentation with a clear definition?

    Oh, and if any of you know how Antelope defines the <vang> vector, I'm
    also trying to figure that out (I think that Antelope's definition of
    "vertical" is different, e.g. Up = 180 and Down = 0, although, I cannot
    confirm it).

    Thanks for your time!
    Valerie Zimmer
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    http://www.iris.washington.edu/mailman/listinfo/sac-help


    --
    ===============
    Frederik Tilmann
    Bullard Laboratories Tel. +44 1223 765545
    Department of Earth Sciences Fax. +44 1223 360779
    University of Cambridge email: tilmann<at>esc.cam.ac.uk
    Madingley Road
    Cambridge CB3 0EZ
    UK





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