Started: 2008-10-07 20:35:54
Last activity: 2008-10-08 07:19:21
Topics:
Val Zimmer
2008-10-07 20:35:54
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).

Valerie Zimmer

• Frederik Tilmann
2008-10-08 07:19:21
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).

Valerie Zimmer
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--
===============
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