Abstract: Modeling acoustic field scattered by an underwater assembly of gas bubbles or similar resonant monopole scatterers is of considerable theoretical and practical interest. Using effective medium theory to describe the phase speed and attenuation in bubble clouds and using the addition theorem for the spherical wave functions to relate the scattered sound fields of both the bubble cloud and sea surface to a common origin, we obtain a modified scattering cross-section of the cloud near the boundary. Numerical simulations reveal that, with increasing void fraction or increasing cloud radius,the natural frequencies of the cloud can extend to smaller frequencies. Periodic oscillatory perturbations exists in the modified scattering cross-section that is due to the effect of sea surface and such oscillation becomes more considerable with increasing depth. Besides, if the viscous damping dominate the damping term, another oscillation will exists near the resonance frequencies. The approach may serve to model acoustic scattering by fish schools near the sea surface,bubble plumes and methane leaks near the seafloor.