# Kirchhoff migration and diffraction.  (tutorial, slow)
#
subroutine kirchslow(   conj, add,  velhalf, t0,dt,dx, modl,nt,nx,  data)
integer ix,iy,it,iz,nz, conj, add,                          nt,nx
real x0,y0,dy,z0,dz,t,x,y,z,hs,     velhalf, t0,dt,dx, modl(nt,nx), data(nt,nx)
call conjnull(          conj, add,                     modl,nt*nx,  data,nt*nx)
x0=0.;  y0=0;  dy=dx;  z0=t0;  dz=dt; nz=nt
do ix= 1, nx {  x = x0 + dx * (ix-1)
do iy= 1, nx {  y = y0 + dy * (iy-1)
do iz= 1, nz {  z = z0 + dz * (iz-1)		# z = travel-time depth
	hs=      (x-y) / velhalf
	t = sqrt( z * z  +  hs * hs )
	it = 1.5 + (t-t0) / dt
	if( it <= nt )
		if( conj == 0 )
			data(it,iy) = data(it,iy) + modl(iz,ix)
		else
			modl(iz,ix) = modl(iz,ix) + data(it,iy)
		
	}}}
return; end