  We consider the linear problem for water-waves created by sources on the
bottom and the free surface in a 3-D basin having slowly varying profile
$z=-D(x)$. The fluid verifies Euler-Poisson equations. These (non-linear)
equations have been given a Hamiltonian form by Zakharov, involving canonical
variables $(\xi(x,t),\eta(x,t))$ describing the dynamics of the free surface;
variables $(\xi,\eta)$ are related by the free surface Dirichlet-to-Neumann
(DtN) operator. For a single variable $x\in{\bf R}$ and constant depth, DtN
operator was explicitely computed in terms of a convergent series. Here we
neglect quadratic terms in Zakharov equations, and consider the linear response
to a disturbance of $D(x)$ harmonic in time when the wave-lenght is small
compared to the depth of the basin. We solve the Green function problem for a
matrix-valued DtN operator, at the bottom and the free-surface.
