Borehole logging results. From left to right: natural gamma signals detected in the two boreholes BDR-D1 (dashed line) and BDR-D2 (solid line) (API), gamma density (GD), neutron backscattering (NN counts) and visual categorization of the bore-core from BDR-D2 as a grey-scale bar (D2, see also Fig. 10 and Appendix). Along borehole parallel trajectories (see also Fig. 12), V_P(0) and the absolute value of the velocity gradient are shown for comparison of the seismic tomography result with the logging data.