NAME
xwmig - do seismic cross-well++ migration
SYNOPSIS
xwmig [ -Nntap ] [ -Ootap ] [ [ -vmigcv (none)] [ -vmigsvs
(1500)] [ -vmigeve (6000)] [ -nvnv (100)] ] [ -xminxmin
(NONE)] [ -xmaxxmax (NONE)] [ -dxdx (NONE)] [ -zminzmin (0)]
[ -zmaxzmax (abs(xmax-xmin))] [ -dzdz (dx)] [ -chopchop (0)]
[ -delaydelay (0)] [ -nqnq (1000)] [ -D ] [ -V ] [ -? ]
DESCRIPTION
xwmig performs constant or variable V(z) velocity migration
in a 2-D plane for any number of traces in any sort/order.
The method is general in terms of source/receiver geometry
and can be used, in principle, for surface-to-surface, VSP,
reverse VSP, cross-well, single well (e.g., salt proximity
survey), etc. It's usefullness for surface seismic data, and
possibly VSP, can be somewhat limited if migrated constant
velocity. However, if one is interested in fucussing in on
a localized zone, say a salt face, then running several
using different velocitites can yield useful information
especially sine the constant velocity mode runs so fast.
The variable velocity mode builds up an image by summing the
images from many constant velocity migrations. This natur-
ally takes nv-times longer to run than the constant velocity
case but is completely general. The "goodness" of image is
dependent on summing enough constant velocity images withing
the correct velocity range.
Since this migration is based on constant velocity it is a
time-migration method. However, just to confuse the unwary
user, the output is an nx-by-nz depth image. The size of
the image (nx,nz) and the sample intervals (dx,dz) are
determined by user-supplied parameters described below.
VITAL: The program accesses the following header words:
SrPtXC -- source x-coordinate
SrPtEl -- source z-coordinate
RcPtXC -- receiver x-coordinate
GrpElv -- receiver z-coordinate
The header geometry must be correct or computer guano will
result. In verbose mode, the program prints source/receiver
coordinates for each processed trace. Since the actual
coordinates are used, the well(s) can be arbitrarily devi-
ated, or even horizontal, in the (x,z)-plane.
CAUTION: It is a good idea to have all coordinates positive.
The program has successfully migrated data with all postive
x-coords and all negative z-coords, but this may be fragile.
Mixed postive and negative coordinates in one direction
(i.e., z-coords go from -200 to +500) may be dangerous.
Xwmig works by spreading each input data amplitude along a
full elliptical impulse response with source and receiver at
the foci. The ellipse is built parametrically by pre-tabled
trig functions. The parameter nq determines how finely the
ellipse is sampled.
The output can either be in time (x-t) or depth (x-z). If in
time the output will have the same number of samples as the
input; if in depth the output will have a different number
of samples/trace [nz = (zmax-zmin)/dz].
xwmig gets both its data and its parameters from command
line arguments. These arguments specify the input, output,
the migration velocity (constant), the output image space,
some options affecting speed and avoidance of direct
arrivals, and verbose printout, if desired.
Command line arguments
-N ntap
Enter the input data set name or file 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/b/vsp/dummy tells the
program to look for file 'dummy' in directory '/b/vsp'.
-O otap
Enter the output data set name or file immediately
after typing -O. This output file is not required when
piping the output to another process. The output data
set also requires the full path name (see above).
-vmigc v
Mode 1: enter the constant migration velocity. The
default is to look for the range of velocities given
below.
-vmigs vs
Mode 2: enter the starting migration velocity. The
default is 1500m/s.
-vmige ve
Mode 2: enter the ending migration velocity. The
default is 6000m/s.
-nv nv
Mode 2: enter the number of velocities to use in range
vs -> ve. Default = 50
-xmin xmin
REQUIRED. Enter xmin.
The survet image space must be defined by the user.
This space is a rectangle with upper-left (x,z) coordi-
nate of (xmin,zmin) and lower-right coord of
(xmax,zmax). Further, the spatial step-size in x (dx)
and z (dz) are determined from user input.
-xmax xmax
REQUIRED. Enter xmax. (see discussion under xmin
parameter)
-dx dx
REQUIRED. Enter the step-size in x for the survey
image space. It is probably overkill to make dx much
less than one-quarter of a wavelength, but you can make
it 1 meter if you like. The size of dx, together with
(xmin,xmax) determines the number of output traces: nx
= ( xmax - xmin ) / dx + 1
-zmin zmin
Enter the z-coordinate of the top of the survey image
space. Default is 0.
-zmax zmax
Enter the z-coordinate of the bottom of the survey
image space. Default is abs(xmax-xmin).
-dz dz
REQUIRED. Enter the step-size in z for the survey
image space. It is probably overkill to make dz much
less than one-quarter of a wavelength, but you can make
it 1 meter if you like. The size of dx, together with
(xmin,xmax) determines the number of output traces: nz
= ( zmax - zmin ) / dz + 1
-chop chop
Enter the smallest abs(amplitude) to be processed.
Default is chop=0, which processes all the data values.
All values in the data less than chop, in an absolute
sense, will not be processed. This is useful for pro-
cessing synthetic data with lots of neligibly small
values (e.g., due to filtering). It can also be used
to speed up large jobs by ignoring, say, the smallest
10% of the data. In one trial, chopping at 10% of the
max amplitude resulted in a 40% time savings.
NOTE: The actual chop value is needed (not something
like -chop10%). This means that you have to know the
maximum value in the data. Example: Your data max is
12.33452, so -chop1.233452 will only process amplitudes
satisfying abs(amp)>1.233452 . Chop should always be
positive.
-delay delay
Enter the time delay (seconds) beyond direct arrival
time to begin processing input data values. Default is
delay=0, begin processing at the first time sample
beyond the calculated first arrival. This parameter
avoids processing the direct arrival, which we do not
want to migrate since it is not a reflection. But with
delay=0, some of the direct arrival waveform may be
interpreted as reflection energy and hence be migrated.
This just acts as a source of noise in the migrated
image. HELP: A good rule of thumb is to set delay to
the dominant period of the direct arrival, or twice
this amount to be safe. Example: Dominant period of
the direct arrival is .005 seconds, try -delay.015.
Note: it's always better to process out the downgoing
waves (in a VSP) or the direct arrival (in X-well data)
before doing the migration.
-nq nq
Enter angular step-size along impluse response ellipse.
Default is nq=2000. The impulse response ellipse is
divided into nq pieces. As the algorithm steps along
the ellise, it maps the migrated amplitude values onto
the rectangular grid of the output image space. If nq
is too small some output grid points will be skipped
and the image will look spotty. If nq is too large
there is no damage done, but does make the program run
slower.
It is unlikely that anyone will need to twiddle with
this parameter.
-oper oper
Enter operator type: -oper2 (double signed operator);
-oper1 (single signed operator: quick and dirty).
Default is 2
-D Enter the command line argument '-D' to get depth
domain output.
-V Enter the command line argument '-V' to get additional
printout.
-? Enter the command line argument '-?' to get online
help. The program terminates after the help screen is
printed.
BUGS
Beware of mixing positive and negative in the output image
space.
Bad example: -xmin100 -xmax-500 -dx-20 This is just ask-
ing for trouble!
The output data does not currently contain good trace header
info
AUTHOR
C. L. Liner, University of Tulsa, Geosciences Dept.
COPYRIGHT
copyright 2001, Amoco Production Company
All Rights Reserved
an affiliate of BP America Inc.
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