NAME

     vsm - smooth a velocity (or Q) worktape.

SYNOPSIS

     vsm [ -Nfile_in ] [ -Ofile_out ] [ -Tfile_topo ] [ -dxvdxv ]
     [  -dzvdzv ] [ -rsmrsm ] [ -hsmsmhsm ] [ -dsmdsm ] [ -zvozvo
     ] [ -GrpElv ] [ -threed ] [ -V ] [ -? ]

DESCRIPTION

vsm
is designed to read in a velocity (or Q) worktape and
smooth the slowness (or inverse Q) over radial, horizontal
or vertical distances as prescribed by the user. By smooth-
ing slowness en lieu of velocities, the two way travel times
along the smoothing directions are preserved. For instance,
if the data is smoothed in the vertical direction, z, we
wish to preserve :
$ tau (z) = sum from { zeta =z sub topo } to z d zeta /v( zeta )$ .

For radial smoothing, traveltimes are approximately preserved for all directions:

$ 1 / v sub smooth (x,z) = sum from { ksi =-r} to { ksi =+r}
sum from { zeta =-r} to { zeta =+r} w( ksi , zeta ) / {v(x+ ksi ,z+ zeta ) }$,

where

$w( ksi , zeta )=a*[1.- ( ksi sup 2 + zeta sup 2 ) sup 1/2 ] / r ~ if~ ( ksi sup 2 + zeta sup 2 ) < r sup 2 $,

= 0. otherwise,

and where 'a' is a normalization factor such that the sum of
all the weights, w, = 1.

Only the first velocity attribute (multi-trace record) will
be smoothed. Any subsequent velocity attributes (such as $
epsilon , delta , theta ,~ and~ vs/vp ) $ will be passed
unchanged. This will result in smoothing of an anisotropic
or elastic velocity wave field in a consistent way.

vsm 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 res-
trictions to trace header integer values.

Command line arguments
-N file_in
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 file_out
Enter the output data set name or file immediately
after typing -O unless output is to a pipe, in which
case the -O entry must be omitted. The output data set
also requires the full path name (see above). Piping
the flow 'vsm|vsamp|kray' performs the ray-building
step in prestack depth migration.

-T file_topo
Enter the (optional) input xsd format topography (with
elevations above sea level positive!) file name immedi-
ately after typing -T. The sample and trace units and
offsets must be world (m or ft) coordinates that are
consistent with, but not necessarily a one to one
correspondance to, the -xvo,zvo,dxv,dxz values provided
below. Providing the -T option will prevent smoothing
of the velocity across the topographic interface. The
velocity field above topography is unchanged and there-
fore unsmoothed. This part of the velocity field will
only be used if the floating datum plane used in pro-
grams vsamp (defined by header word FlDtEl) and
tstretch lies above topography. The interpolated topog-
raphy values will be stored in trace header GrpElv for
subsequent use by routine vsamp.

-dxv dxv
Enter the spatial increment between traces on the
seismic velocity worktape. Default is calculated by
multiplying trace header word Dx1000 * .001 where
Dx1000 has been filled in by one of the MBS velocity
model building programs.

-dzv dzv
Enter the depth increment between traces on the seismic
velocity worktape. Def ault is calculated by multiply-
ing trace header word Dz1000 * .001 where Dz1000 h as
been filled in by one of the MBS velocity model build-
ing programs.

-rsm rsm
Enter the radial smoothing distance in m (ft).
Slownesses are given weights of (1.-r/rsm) about the
point in question for r < rsm and 0 otherwise.. If the
-rsm option is not entered, the program will search for
the -hsm and/or -dsm options.

-hsm hsm
Enter the horizontal smoothing half width in m (ft).
All trace within nint(hsm/dxv) about the center trace
will be used in calculating the average slowness, where
nint denotes the nearest integer. A triangular weight
is applied in x, resulting in a pyramidal weight if
smoothing occurs in both x and z directions. If the
-dsm option is not entered, purely horizontal smoothing
will occur. The -hsm option ignored if -rsm option is
present.

-dsm dsm
Enter the vertical smoothing half width in m (ft). All
trace within nint(dsm/dzv) about the center trace will
be used in calculating the average slowness, where nint
denotes the nearest integer. A triangular weight is
applied in z, resulting in a pyramidal weight if
smoothing occurs in both x and z directions. If the
-hsm option is not entered, purely vertical smoothing
will occur.
The -dsm option ignored if -rsm option is present.
Enter the vertical smoothing distance in m (ft).
Slownesses are given weights of (1.-|d|/dsm) about the
point in question for |d| < dsm and 0 otherwise. If the
-hsm option is not entered, purely vertical smoothing
will occur. Otherwise, smoothing will be performed over
a rectangular aperture. -dsm option ignored if -rsm
option present.

-xvo xvo
Enter the x position of the first trace of the seismic
velocity worktape in m (ft). This value is required to
properly align the velocity model with the xsd pick
file following the -T option.

-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 (where elevations above sea level are posi-
tive!) of the top of the velocity model. For prestack
depth migration, this coordinate must be greater than
the seismic source and receiver elevations (header
words GrpElv and SrPtEl) extracted from routine prep-
mig. Folk traditions require that zvo be positive for a
value lying above sealevel. zvo will be saved in line-
header word MxRSEL. No default if -GrpElv or -T option
is invoked.

-GrpElv
Enter the command line argument '-GrpElv' to honor the
velocity discontinuity at the topographic surface
defined by trace header word GrpElv. The lineheader
word CrwNam is set to v_topo if this option is invoked.

-threed
Enter the command line argument '-threed' to smooth 3D
data. For 2D data sets that have more than a single
(multitrace) record only the first is smoothed, the
others being copied. This is to handle the case where
the other records are additional attributes. For the 3D
case the input data consist of multiple records (e.g.
inlines), and we must smooth each record as it is read
in.

-V Enter the command line argument '-V' to obtain verbose
output.

-? Enter the command line argument '-?' to get online
help. The program terminates after the help screen is
printed.

NOTE 1:

     In order to plot the xsd pick file on top  of  the  velocity
     data  set  it  is  imperative  that  the  units  and offsets
     correspond to the display units of the xsd window  in  which
     the picks will be displayed.

BUGS

     No bugs known at present.

AUTHOR

     K. J. Marfurt (APR)  extracted from S. H. Gray's   algorithm
     smallmig / smigA (1991). Option to honor topographic discon-
     tinuity option initiated  in  response  to  weathering  zone
     modeling  problems  studied  by  Mike  O'Brien  (1994). Same
     option toasted on 8/29/96 in honor of  Mary  Ann  Thornton's
     retirement.

COPYRIGHT

     copyright 2001, Amoco Production Company
               All Rights Reserved
          an affiliate of BP America Inc.

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