Abstract In June 2023, we measured ten near‐surface seismic profiles to image the critical zone of dolerite (diabase) dykes that cut the sandstone at Cap Fréhel in Brittany (France). Seismic ray tracing and inversion are used to determine the thickness of the regolith layer from first‐arrival P‐wave traveltimes. The 2D subsurface models show strong lateral velocity contrasts between weathered dolerite and sandstone. The weathered dolerite layer is between 6 m and 15 m thick at locations several hundred metres apart. The weathered sandstone is often only 1–2 m thick, sometimes up to 4 m. Seismic P‐waves propagate with in the weathered dolerite, depending on the degree of alteration, and with or more at the top of the unweathered dolerite. In weathered sandstone, P‐wave velocities increase from at the surface to at a depth of 1 m. The unweathered sandstone at a depth of 3–4 m has high P‐wave velocities of over , mostly exceeding , depending on the degree of fracturing. We have introduced smoothness constraints for the model parameters of the ray tracing and inversion software Rayinvr , so that the regularization parameter is replaced by the prior covariance of the constraints. The resolution matrix shows that the interface depth is always coupled with the velocity above the interface and sometimes also with the velocity of the head wave below the interface. The head wave velocities in the unweathered layer are not well resolved, which we attribute to a limited source–receiver distance and strong lateral velocity inhomogeneities when a seismic profile intersects both dolerite and sandstone. Seismic tomography based on wave propagation models without first‐order discontinuities does not allow to accurately determine the regolith thickness. The Akaike information criterion favours the Rayinvr models with fewer model parameters over seismic tomography models calculated by solving the eikonal equation or the shortest path method.
Zillmer et al. (Thu,) studied this question.