The depth and geometry of the bedrock/regolith interface influence many near-surface processes. Geophysics offer means to image them non-invasively since they correspond to discontinuities in physical properties. For instance, refraction seismic applications can leverage the strong correlation between seismic velocity and porosity, while DC resistivity measurements with their sensitivity to electrical conductivity are indirectly sensitive to porosity and clay content. Combining these two data types is beneficial since their spatial resolution patterns and sensitivities to relevant target properties differ. Here, we propose a probabilistic joint inversion framework that uses DC resistivity and first-arrival seismic travel times to infer a common interface geometry separating two heterogeneous sub-domains. A synthetic test case demonstrates that the method is more accurate in sampling the target interface than inversion results obtained using single datasets. We then applied the joint inversion method to field data from the Calhoun Critical Zone Observatory in South Carolina, where it resolves apparent contradictions in the individual inversion results. It produces a bedrock topography that mirrors surface topography and offers clear indications of a more fractured bedrock below the surface topography lows.