NAME

     level -  Elevation correction via upward wavefield  extrapo-
     lation

SYNOPSIS

     level [ -Nntap ] [ -Ootap ] [ -rsirs ] [ -reire ] [ -Velvel-
     mod  ]  [  -Refiref  ]  [ -trintgrpint ] [ -dipmaxdipmax ] [
     -gsgeom ] [ -taperitaper ] [ -V ] [ -S ] [ -POST ] [ -? ]

DESCRIPTION

     Note: THIS IS A NEW VERSION OF LEVEL WHICH DOESN'T NEED PRE-
     FILTERING WITH DIFFR.

     level corrects for  topography  by  wavefield  extrapolating
     from  applicable  elevations  through  a  constant  velocity
     replacement medium to a flat  reference  surface  above  the
     highest elevation point.

     A Kirchhoff wavefield extrapolator is used to move the  data
     from  the  topographic acquisition surface to the new refer-
     ence surface. Rays shot between the topographic  acquisition
     surface  and  the flat reference surface define travel times
     for each emergence angle of each trace. Input trace data  is
     then reconstructed as if acquired on the flat reference sur-
     face using, Huygen's principle, ray derived travel times and
     some amplitude and phase considerations.

     Surface  consistent  elevation  information  (ft  or  m)  is
     expected  in  correct  trace header positions; source eleva-
     tions in halfword 112 (SrPtEl), receiver elevations in half-
     word 120 (GrpElv), and, for the post-stack mode, correct CDP
     elevations in halfword 123 (DePtEl).

     For the pre-stack case, source and  receiver  locations  are
     extrapolated   separately,  with  cascaded  applications  of
     level.  Common source gathered input  data  is  required  to
     extrapolate  receiver locations. Common group gathered input
     data is required to extrapolate source locations. The  post-
     stack case requires only one application of level, with both
     source and receiver locations being extrapolated  simultane-
     ously.

     Thus, to accomplish total prestack redatuming of  a  surface
     seismic  line  requires  two  executions of the program in a
     cascaded fashion:

     Extrapolate Receiver Locations  --> Sort to Common Group -->
     Extrapolate Source Locations --> Continue Processing.

     level gets data and parameters from command line  arguments.
     These arguments specify the input and output trace data.

  Command line arguments
     -Nntap
          Enter input data set name  or  file  immediately  after
          typing  -N;  entering  nothing  specifies a stdin pipe.
          This 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 subdirectory 'vsp' stored in directory b.

     -Ootap
          Enter output data set name or  file  immediately  after
          typing  -O.  This output file is not required when pip-
          ing the output to another process.  The output data set
          also requires the full path name (see above).

     -rs irs
          first record to process.         (default = 1)

     -re ire
          last record to process.          (default = all)

     -ns ns
          first trace in record to process. (default = 1)

     -ne ne
          last trace in record to process.  (default = all)

     -Vel velmod
          constant replacement velocity  (no default)

     -Ref iref
          Reference Surface. Must be above the highest elevation.
          ( no default )

     -dipmax dipmax
          maximum  emergence  angle  to  extrapolate   (degrees).
          Angles greater than arcsin[ Vref/(2.0 * trint * fmax) ]
          where fmax is the maximum frequency content  (hz),  are
          aliased
           (default = 45)

     -trint grpint
          distance between traces  (no default)

     -gs geom
          specifies a gain curve (time ** geom)  applied  to  the
          data before extrapolation, and removed after extrapola-
          tion. This is a convenient way to  approximately  rein-
          troduce amplitude divergence.
           (default = 0)


     -taper itaper
          specifies the number of traces to taper the  start  and
          end of each input record.
           (default = 0)

     -S   Enter '-S' to extrapolate source locations. Input  data
          must  be  common  group  sorted.  If neither '-S' or '-
          POST' is specified, receiver locations are extrapolated
          and the input must be common source sorted data.

     -POST
          Enter '-POST'  to  extrapolate  post-stack  data.  Both
          source and receivers are extrapolated simultaneously in
          one pass with level.  The input stack  data  should  be
          one record with the appropriate number of traces.

     -V   Enter the command line argument '-V' to get  additional
          printout.

     -?   Enter this (or '-?' if you are running in  c-shell)  to
          get  online  help.   The  program  will terminate after
          printing a list of parameters.

EXAMPLE FLOW

       #Example flow - Wave Equation Elevation Correction
       #Pre-stack case  with  common  source  sorted  input  data
     (d.input).
       #Output (d.out) is common group sorted data, with source
       #and receivers extrapolated to the reference elevation.
        level   -Nd.input -Od.level -dipmaxN -VelN -RefN -trintN
        presort -Nd.level -Od.presort
        memsrt  -Nd.level -Od.sort -nd.presort -G
        level   -Nd.sort -Od.out -dipmaxN -VelN -RefN -trintN -S

BUGS

     - unknown

SEE ALSO

     datum presort sisort memsrt

AUTHOR

     W. P. May, D. E. Wagner and  S. H. Gray, 1991

COPYRIGHT

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