For shallow subsoil, the estimates of in-situ porosity and water saturation are important, but until now it has been difficult to estimate them reliably. We relate seismic and GPR reflection coefficients to porosity and water saturation using a shared earth model. Using this model, we propose a method to integrate quantitatively seismic and GPR angle-dependent reflection coefficients. The new approach has been tested through numerical simulations. The results clearly show that from either seismic or GPR data alone it is impossible to obtain unique estimates for porosity and water saturation; however, a correct integration of those two data types leads to unique and stable estimates at a subsoil layer boundary. Potential applications of this approach exist in hydrogeology and environmental, agricultural and geotechnical engineering.