Fig. 7

a Initial real-world situation at the northern tunnel portal. The 3D topography is interpolated at mesh nodes from the swissALTI3D DEM. The topographic mesh coloured by elevation is associated with a single unit as indicated by the 2D GBT geological profile, while the neighbouring units are shown with coloured mesh edges only. The mesh resolution is higher in the vicinity of the surface gravity measurement points (red circles) and in the vicinity of the reference model line (red line), which is by definition at y = 0 m along the whole study area. The topography sampled along the reference model line corresponds to the reference model topographic profile, and it is the same profile to which the geological profile was adapted, and which is used for the synthetic gravity computation. b Final, adapted reference model situation of the same area at the northern tunnel portal. View of the 2.5D topography as laterally continued from the 2D reference model line topographic profile (red line). This 2.5D structure is correctly accounted for by our forward gravity modelling routine, which neglects any topography variation along the profile-perpendicular coordinate (Won & Bevis, 1987). The red line following the topographic profile at y = 0 m represents the 2D reference model line, to which the observed gravity measurements are translated (green circles) prior to the comparison with the synthetic values. The newly introduced terrain-adaptation correction (TAC, see Sect. 4.2.3) takes care of all changes from a to b. It should be noted that the spatial extent of each unit block guarantees the consideration of topographical masses for a minimum radius of 167 km around each gravity measurement, and that horizontal axes in panels a and b were limited for displaying purposes