Fig. 6

Graphical representation of the numerical mesh used for the density-dependent gravity correction computation. The digital elevation model (either swissALTI3D or SRTM) depending on the required resolution) is sampled at each mesh node and subsequently used for the computations. The resolution of this static mesh is higher (10 m) in the vicinity of the gravity measurement points (red circles) and along the reference model line (including the original GBT track), while it decreases with increasing distance from this central zone. The two insets show the varying spatial resolution for two particular areas, along the reference model line and at around the northern tunnel portal. The thin horizontal black lines in the two insets mark the boundaries between geological units at the surface digitised from the GBT geological profile, which were included as a line of nodes in the mesh. It is of fundamental importance that the mesh respects these boundaries, allowing for a correct and consistent density mapping from the 3D structures to the 2D profile, for the computation of the terrain-adaptation corrections