Joint structure-based inversion of electrical resistivity and seismic travel-time data for fault characterization.

Structure-based inversion offers a geologically informed alternative to conventional voxel-based approaches by explicitly representing subsurface interfaces. This enables inversion results to be interpreted in terms of meaningful geological structures rather than smoothed property distributions. Extending this concept to a joint inversion framework further allows multiple geophysical data sets to update a single shared geological model, exploiting complementary sensitivities to better constrain subsurface structure and reduce interpretational ambiguity.

Here, we investigate the effectiveness of structure-based joint inversion for imaging tectonic features in a faulted near-surface environment by jointly inverting electrical resistivity tomography (ERT) and travel-time tomography data. The inversion framework is built around an implicit geomodel in which fault-related interface points are included directly in the model vector (Balza Morales et al., 2025). Both geophysical methods contribute to a single objective function, enabling tectonic interface geometry and associated physical property distributions to evolve consistently during inversion.

The workflow is evaluated using (i) synthetic experiments in a crosshole setting and (ii) field data acquired in the Southern Erft block, a structurally complex tectonic setting in the Lower Rhein Embayment (Menzel et al., 2024). This work provides two contrasting environments: one in which ERT and SRT exhibit strongly complementary sensitivities, leading to improved interface recovery and increased stability of fault geometry updates in joint inversion; and a second in which limited coverage from one method restricts the degree of complementarity, so that while joint inversion can still be performed, it offers only minor improvements over the single-method structure-based inversion. The field case is used to assess how structure-based inversion improves fault interpretation relative to voxel-based inversion through explicit parameterization of geological interfaces derived from an initial conceptual geomodel, and to test whether joint inversion produces more robust and consistent updates to the shared geomodel under realistic acquisition conditions and noise.

By systematically contrasting voxel-based, single-method structure-based, and joint structure-based inversions, the analysis examines how increasing levels of geological coupling influence the stability, interpretability, and geological plausibility of inferred fault architecture while maintaining consistency with an optimized (joint) data misfit. While demonstrated here using ERT and seismic travel-times the proposed evaluation strategy and inversion framework are transferable to other geophysical methods and subsurface applications where structural complexity limits conventional interpretation.

References:

Menzel, N. and Klitzsch, N. and Altenbockum, M. and Müller, L. and Wagner, F.M. (2024): Prospection of faults on the Southern Erftscholle (Germany) with individually and jointly inverted refraction seismics and electrical resistivity tomography. Journal of Applied Geophysics. https://doi.org/10.1016/j.jappgeo.2024.105549

Balza-Morales, A., Förderer, A., Wellmann, F., Maurer, H., & Wagner, F. M. (2025). Structure-based geophysical inversion using implicit geological models. Geophysical Journal International, https://doi.org/10.1093/gji/ggaf445