NAME

     fkmod -  computes  seismograms  for  1D  elastic  model  via
     reflectivity method

SYNOPSIS

     fkmod -Nmodel -Ootap [ -V ]

DESCRIPTION

     fkmod -  computes  the  seismograms for a 1D model specified
     by the input flatfile (N). The USP output (O) gives the P-P,
     P-S, S-P, and S-S F-K spectral responses  in  an  amplitude-
     phase  format  suitable  for  input to USP program FFT2DA to
     obtain seismograms in the X-T domain.

  Command line arguments
     -N   Enter the filename of model  immediately  after  typing
          -N. Default: stdin.  (See below for model file format.)

     -O   Enter the filename of the  output  dataset  immediately
          after typing -O. Default: stdout.

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


  Input file format
     nlyr,nt,dt,nx,dx,alpha,cmin,smul,imul
     Vp(1),Vs(1),rho(1),thick(1)
     Vp(i),Vs(i),rho(i),thick(i)
     Vp(nlyr),Vs(nlyr),rho(nlyr)

     where

     nlyr = #layers incl. basal halfspace
     nt = number of time samples (pwr. of 2)
     dt = time sample interval
     nx = number of space samples (pwr. of 2)
     dx = space sample interval
     alpha = anti-wrap exp. time taper
     cmin = min. velocity to evaluate fk spectrum
     smul = 0 for no free surface multiples
     imul = 0 for no internal multiples
     (note: if all Vs() = 0, does only  acoustic  (P-P)  calcula-
     tion)

  Notes
     The lineheader entry SmpInt is set to 1 and the actual  time
     increment is written to entry UnitSc to allow other USP pro-
     grams to pick up the correct time increment.  After  fourier
     transforming  the output of FKMOD from frequency to time, an
     exponential gain must be applied to remove  the  exponential
     anti-wrap  taper.   This  can  be  done  using  USP  program
     TTOTHEN.  The appropriate gain exponent is  written  to  the
     FKMOD printfile when the verbose (V) option is enabled.  The
     F-K spectral output gives the upgoing  planewave  amplitudes
     at  the top of layer 1 due to unit downgoing planewave exci-
     tation.  In the X-T  domain  this  corresponds  to  unit  2D
     dipole  sources.   The  cmin  entry in the model file limits
     evaluation of the F-K spctrum to apparent  velocities  above
     cmin.   This  can  be used for example, to speed up calcula-
     tions when slow shear velocities are used to simulate acous-
     tic  layers.  Aside  from cmin, no shaping is applied to the
     F-K spectrum; the data may be bandlimited  using  other  USP
     programs  such as POLYMUTE or FILT.  The F-K output can also
     be used to create tau-p and 3D source responses.  This  code
     is  ideal for generating data to test surface related demul-
     tiple processing because the X-T output conforms to all sur-
     face related demultiple assumptions.


  Example
     fkmod -Nmodel -Ofkout -V
     fft2da -Nfkdat -R|ttothen -E -exp.9675 -Oxtdat
     model:
     5 1024 .008 1024 15 8 1000. 1 1
     1500.00   10.00  1.00  500.0
     2000.00  700.00  1.80  500.0
     2500.00 1200.00  2.10  500.0
     4000.00 2500.00  2.30 1000.0
     2300.00  900.00  2.30

BUGS

AUTHOR

     Tony Nekut EPTG/ATTC (x4106) after E.  Verschuur  of  DELPHI
     consortium.

COPYRIGHT

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