The knowledge of the distribution of leachate and gas in a municipal landfill is of vital importance to the landfill operators performing improved landfill treatments and for environmental protection and efficient biogas extraction. We have explored the potential of using the velocity fields of seismic S- and P-waves to delineate the wet and gassy (relatively dry, gas/air-filled) zones inside a landfill. We have analyzed shallow S- and P-wave reflection data and seismic surface-wave data acquired at a very heterogeneous landfill site, where biogas was extracted. A joint interpretation of the independently estimated velocity fields from these various approaches has allowed us to localize anomalously low- and high-velocity zones in the landfill. From the complementary information provided by P- and S-wave velocity fields, we have inferred the leachate-bearing wet zones and the gassy zones inside the landfill. Independent measurements of gas flow and mechanical tip resistance to waste deformation validate our seismic interpretations.