Imaging tectonic faults with small vertical offsets in argillites (clay rock) using geophysical methods is challenging. In the context of deep radioactive waste disposals, the presence of such faults has to be assessed because they can modify the rock-con- fining properties. In the Tournemire Experimental Platform (TEP, Aveyron, France), fault zones with small vertical offsets and complex shape have been identified from underground works. However, 3D high-resolution surface seismic methods have limitations in this context that led us to consider the de- tection and characterization of the faults directly from under- ground works. We investigated the potential of seismic full- waveform inversion (FWI) applied in a transmission configura- tion to image the clay rock medium in a horizontal plane be- tween galleries and compared it with first-arrival traveltime tomography (FATT). Our objective was to characterize seismic velocities of a block of argillites crossed by a subvertical fault zone with a small vertical offset. The specific measurement con- figuration allowed us to neglect the influence of the galleries on the wave propagation and to simplify the problem by consider- ing a 2D isotropic horizontal imaging domain. Our FWI scheme relied on a robust adaptation of early arrival waveform tomog- raphy. The results obtained with FATT and FWI were in accor- dance, and both correlated with the geologic observations from the gallery walls and boreholes. We found that even though vari- ous simplifications was done in the inversion scheme and only a part of the data was used, FWI allowed us to get higher reso- lution images than FATT, and it was especially less sensitive to the incomplete illumination because it also used diffracted en- ergy. Our results highlighted the complexity of the fault zone, showing a complex interaction of the main fault system with a secondary system composed of decimetric fractures associated with the presence of water.