Seismic images of magnetic activity on the far surface of the Sun are calculated using the seismic holography technique. The method is based on the comparison of waves going in and out of a particular point (focus) in the Sun and requires computation of the Green's function representing the disturbance measured on the front side (pupil) from a unit acoustic impulse originating at the focus. In this work we describe how to compute Green's functions for the Lagrangian pressure fluctuation from a spherical polar expansion of the adiabatic wave equations in the Cowling approximation. A comparison between the results obtained by using the ray approximation and the spherical polar expansion is shown. We use the acoustic-gravity wave equation in the plane-parallel limit in both cases and for the latter we take the asymptotic approximation for the radial part of the solution. We also show that the phase shift in the Green's functions due to incorrect modeling of the uppermost layers of the Sun can be estimated from the eigenfrequencies of the normal oscillation modes.