NAME

     vmodvavg - Velocity Perturbation Model Generation

SYNOPSIS

     vmodvavg [-Nntap] [-Ootap] [-Ccardfile] [-V]

DESCRIPTION

     vmodvavg generates a V(t) suite which can  be  stretched  to
     depth  by program vttd and used for prestack depth migration
     in program cubevz or poststack V(z) migration on the Connec-
     tion  Machine.  In the typical vmodvavg flow a V(z) trace is
     extracted from an input V(x,z) dataset using  program  editt
     and  converted  to  time using program vttd.  vmodvavg reads
     this V(t) trace and converts  it  to  average  velocity  vs.
     time, perturbs in average velocity as directed by user input
     cards, converts the suite  back  to  V(t)  and  outputs  it.
     Another  vttd  converts  the V(t) suite to a V(z) suite.  To
     setup a canned vmodvavg flow type:

     catpat vmodvavg > vmodvavg.flow

     Edit the flow with a  text  editor,  filling  in  the  input
     V(x,z) dataset name, the trace number to extract and an out-
     put dataset name, save it, and type:

     startjob vmodvavg.flow &

     When using the canned flow, all command line arguments  will
     be  substituted  into  the vmodvavg step for you, but you do
     need to fill out the card deck.

  Command line arguments
     -N ntap
          Enter the name of the input V(t) dataset  after  typing
          -N.

     -O otap
          Enter the name of the output V(t) suite  dataset  after
          typing -O.

     -V   Enter -V for a verbose printout.

     -Ccardfile
          If the parameters are in a separate card file enter the
          name of that file after typing -C, or by using the cat-
          pat command a script file will be  created  which  will
          contain the command line arguments and the other neces-
          sary parameters for  executing  vmodvavg.   The  script
          file  may  then be editted to enter the correct values.
          The script file may be executed  using  the  "startjob"
          command.

          Script file format:

          vmodvavg -Nntap -Ootap \
          -vmodvavg.crd"\
          VEL MODEL   # FUNCTS    VBLOCK       T/D    LOW/HI
                         nfunc    vblock    timdep     lowhi
          VMULT      TIME/DPTH     VMULT
          VMULT              0      1.05
          VMULT           3000       1.1
          VMULT          20000       1.2
          "

     # FUNCTS nfunc
          Number of  V(t)  functions  to  produce.   Assume  that
          LOW/HI  flag is 3 for a symmetrical suite.  Actual out-
          put functions can be limited by setting LOW/HI to 1  or
          2.

     VBLOCK vblock
          Allowable error  (in  velocity  units).   The  velocity
          functions will be blocked into constant velocity layers
          such that the  error  in  the  velocity  values  is  no
          greater  than  VBLOCK.   Velocity  blocking is used for
          greater efficiency in the migrations.

     T/D timdep
          1 -- input velocities are a function of time
          2 -- input velocities are a function of depth

      LOW/HI lowhi
          A flag that selects functions to be output.
          1 -- input function and the lower half of the suite
          2 -- input function and the higher half of the suite
          3 -- (default) symmetrical suite about input function
          NOTE: do NOT adjust #FUNCTS to compensate for 1 or 2.

     TIME/DPTH timedpth
          Time value for velocity multipliers int the VMULT card.
          (if T/D is 2, then this is a depth value)

     VMULT vmult
          Ratio of maximum average velocity to the input  average
          velocity  at the current time/depth.  A symmetric func-
          tion is also produced that is slower than the input  by
          this  ratio.  The intervening functions are produced by
          linear  interpolation  in  average  velocity  vs.  time
          between the minimum, input and maximum functions.

SEE ALSO

     cubevz, vmod, fsi, cubevz.flow, apkr.flow, apkr, p2iv

BUGS

     None known.

AUTHOR

     David Nelson

COPYRIGHT

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