NAME
prepmig - read and analyze seismic trace headers in prepara-
tion for prestack depth migration algorithm sammig.
SYNOPSIS
prepmig [ -Nsg_file ] [ -Ofile_geometry ] [ -Tfile_topo ] [
-Wfile_water ] [ -dstadsta ] [ -azimazim ] [ -zdatumzdatum ]
[ -X ] [ -SGRDat ] [ -DphInd ] [ -S ] [ -vsp] [ -xvoxvo ] [
-zvozvo ] [ -dzdz ] [ -? ]
DESCRIPTION
prepmig is designed to read trace header shot locations,
shot/receiver distances, station locations,
elevations/topography and other geometry values from a usp
format seismic worktape and write them out as a flat file,
thereby divorcing the Kirchhoff ray trace migration program
kray and the prestack Kirchhoff depth migration program,
sammig, from AMOCO's ever evolving header formats.
In addition, the geometry information is processed to pro-
vide information such as maximum and minimum extents of
live traces in the data, leftmost and rightmost live trace
distances from each shot, and so on, to allow programs kray
and sammig to have efficient dynamic memory allocation as
well as to allow for easier implementation on parallel pro-
cessors.
If the -X option is not specified, the station coordinates
are assumed to be separated by a fixed value dsta. Specifi-
cally,
xr=RecInd*dsta
yr=0.
zr=GrpElv
xs=.1*SrcLoc*dsta
ys=0.
zs=SrPtEl-ShtDep
where
(xr,yr,zr) define receiver station location in 3 space,
(xs,ys,zs) define shot station location in 3 space,
RecInd is the trace header receiver station location,
GrpElv is the trace header receiver elevation,
SrcLoc is the trace header shot station location multi-
plied by 10,
SrPtEl is the trace header shot elevation,
ShtDep is the trace header shot depth, and
dsta is the distance in m (ft) between station loca-
tions.
prepmig gets both its data and its parameters from command
line arguments. These arguments specify the input, outputs,
and certain floating point values to override the current
restrictions to trace header integer values.
Command line arguments
-N file_sg
Enter the input seismic data file name containing com-
mon shot gathers immediately after typing -N unless the
input is from a pipe in which case the -N entry must be
omitted. This input file should include the complete
path name if the file resides in a different directory.
Example -N/export/data2/san_juan/shot_gathers tells the
program to look for file 'shot_gathers' in directory
'/export/data2/san_juan'.
-O file_geometry
Enter the output data set name immediately after typing
-O. Default = stdout This ascii output file contains
a summary of array lengths, data mins and maxs, and min
and max topography as well as a listing of all shot
receiver coordinates for every trace. This output data
set also requires the full path name (see above).
-T file_topo
Enter the output data set name immediately after typing
-T. This file will contain the topography extracted
from the seismic line headers in xsd format. Default is
to not generate this file.
-W file_water
Enter the output data set name immediately after typing
-W. This file will contain the water bottom depth
extracted from the seismic line headers in xsd format.
Default is to not generate this file.
-dsta dsta
Enter the station location increment. No default is
allowed, since current usp lineheaders cannot express
fractional increments (eg. 13.33333m).
-azim azim
If present, reproject the data onto a (x',y') dip axis
that goes through the first live shot at an angle of
azim from the North. The position of the first live
shot is considered to be (x',y')=(.1*SrcLoc*dsta,0.).
In the case of walkaway vsps, the source indexing will
of course be independent of the receiver indexing
(which runs down the hole). (Default: azim is deter-
mined to be the azimuth of a straight line connecting
the first and last shot points).
-zdatum zdatum
If present, this option will override the source and
receiver elevation information lying in the trace
headers with the value of zdatum in the file following
the the -O option. A positive entry implies a positive
elevation. (Default: Use the trace header elevation
information).
-X If present, determine the x and y location of the
source and receivers from the trace header words
SrPtXC, SrPtYC, RcPtXC and RcPtYC. (Default: Assume
the source and receiver lie on a regular grid deter-
mined by the SrcLoc, RecInd and dsta).
-DphInd
If present, determine the SrcLoc (Source Location Index
*10) from the DphInd (CDP/Depth Index) by the formula
SrcLoc=10*(DphInd-RecInd).
-DphInd
Backward compatability flag for older version of maip.
Use only if you're dealing with a maip dataset created
before Sep 24, 2003.
-S If present, assume the input data consists of single
trace stacked records. prepmig will then generate a
geometry file consistent with coincident source and
receiver acquisition where the shot location =
DphInd*dsta.
-vsp If present, the input data are considered to be walka-
way vsp records. Since the conventional (RecInd and
SrcLoc) indices are now orthogonal the user must make
sure that the trace position and elevation headers dis-
cussed in the -X option are complete and correct.
Remember that ShtElv and GrpElv have the positive axis
point up such that typically, down vsp locations will
have a negative value. Since the DphInd is also not
simply defined, the water bottom depth trace header
WDepDP are assumed to be measured halfway between
SrPtXC and RcPtXC.
velocity worktape. This variable is used only in generating the
xsd topography or water bottom plot segment files invoked by the
-T and -W options for use in graphics program xsd (default
dz=10.)
-dz dz Enter the depth increment between samples on the seismic
-xvo xvo
Enter the x position of the first trace of the seismic
velocity worktape file. If the first trace of the
velocity model corresponds to seismic station location
(RecInd) 500 and the distance between station locations
(dsta) is 26.6667m, then xvo = 13333.35m. This vari-
able is used only in generating the xsd topography or
water bottom plot segment files invoked by the -T and
-W options for use in graphics program xsd.
-zvo zvo
Enter the z position of the of the first sample of the
seismic velocity worktape in m (ft). This will be the
elevation of the top of the velocity model. Folk tradi-
tions require that zvo be positive for a value lying
above sealevel. This variable is used only in generat-
ing the xsd topography or water bottom plot segment
files invoked by the -T and -W options for use in
graphics program xsd.
-? Enter the command line argument '-?' to get online
help. The program terminates after the help screen is
printed.
NOTE 1: The non-expert user is highly encouraged to use the
job submission xwindows program xmig to ensure efficient,
relatively error free construction of a complete Kirchhoff
mbs flow using the sammig algorithm. NOTE 2: For extremely
long lines the value of SrcLoc (source location *10) will
exceed the maximum integer*2 value allowed of 32767. If
SrcLoc is set to 32767 by program maip, prepmig will
attempt to read the source location *10 from trace header
word SGRDat. NOTE 3: For data coming from Disco formats,
the SrcLoc will usually be missing but the DphInd will be
present. The -DphInd option will allow one to calculate the
SrcLoc index without running a ufh shellscript.
BUGS
No bugs known at present. This program will need to be
rewritten each time we change header formats.
SEE ALSO
vsm, vsamp, kray, sammig, xmig, maip
AUTHOR
K. J. Marfurt (APR, 1993).
COPYRIGHT
copyright 2001, Amoco Production Company
All Rights Reserved
an affiliate of BP America Inc.
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