# NMO stack by inverse of forward modeling
#
subroutine invstack(  nt,model,nx,gather,rr,t0,x0,dt,dx,slow,niter)
integer it, ix, iter, nt,      nx,                           niter
real	t0,x0,dt,dx,slow, gather(nt,nx), rr(nt,nx), model(nt)
temporary real	dmodel(nt), smodel(nt), dr(nt,nx), sr(nt,nx)
do it= 1, nt
	model(it) = 0.0 
do it= 1, nt
	do ix= 1, nx
		rr(it,ix) = gather(it,ix)
do iter = 0, niter {
	call imospray( 1,0,slow,x0,dx,t0,dt,nx,nt,dmodel,rr)  	# nmo-stack
	call imospray( 0,0,slow,x0,dx,t0,dt,nx,nt,dmodel,dr)   	# modeling
	call cgstep(iter, nt, model, dmodel, smodel, nt*nx, rr, dr, sr)
	}
return; end