c     Fortran file to NMO correct
c     the OBS data.  Treating depth units (Z)
c     as metres, and velocity units (V) as km/s
c     (or m/ms) so the resulting time shift
c     will be in ms.  Otherwise rounding is 
c     a significant problem.
      Z=1550
      V=1.490
      seis_header(seis_t_subweath_vel) =
     + ((Z**2 / V**2)
     + + seis_header(seis_t_offset)**2
     + / V**2)**0.5 - (Z / V)