Lithologically constrained velocity–density relationships and vertical stress gradients in the North Alpine Foreland Basin, SE Germany

Abstract. We systematically analysed density and velocity data from 41 boreholes to establish velocity-density relationships for the main lithological units in the North Alpine Foreland Basin in SE Germany. We applied these relationships to velocity data and spliced the resulting density values with actual density data and a shallow density model to retrieve complete density profiles along 55 deep wellbores, which at least penetrated the Cenozoic section in the study area. We integrated density profiles to vertical stress to investigate the spatial distribution of vertical stress gradients. Thereby, we observed an eastward decrease of vertical stress gradients, which correlates well with the geological configuration of the North Alpine Foreland Basin in SE Germany. Thereby, vertical stress gradient profiles can be reasonably estimated as a function of true vertical depth below ground level TVD in the western, central, and eastern parts of the study area using a power law relationship:

West: 21 MPa/km + (TVD/325)^1/1.80, R² = 0.98

Central: 21 MPa/km + (TVD/410)^1/1.93, R² = 0.99

East: 21 MPa/km + (TVD/531)^1/1.95, R² = 1.00 

In addition, we also investigated the distribution of vertical stress gradients at the top of Upper Jurassic carbonates, an important aquifer for deep geothermal energy production. Our study, therefore, provides a valuable resource for future geophysical, geomechanical, and geological studies in the North Alpine Foreland Basin, both in a fundamental and applied research context.