Standard geological modelling based on boreholes and geological maps can be strengthened using geophysical data. Constrains such as gravity, magnetic and seismic data have already been used. We propose a novel method combining boreholes and the resistivity model resulting from inversion of airborne time domain electromagnetic data. First, the "geophysical" top of the chalk has been identified in the resistivity model after a detailed crossanalysis of resistivities versus boreholes. Then, we jointly interpolated slopes extracted from this geophysical surface together with the top of the chalk in boreholes. Comparison of uncertainties between this model together with pure geological and geophysical models shows that the joint modelling yields the most accurate top of the chalk. A cross-section, intersecting five boreholes (two used as control boreholes) and displaying each of the three surfaces, highlights the usefulness to take into account "geophysical slopes" when modelling. The proposed joint modelling improves what is commonly obtained with geological or geophysical data. This makes the method very attractive for detailed 3D geological modelling.