Executive summary: The passive seismic survey of the Kaiserstuhl test site was initiated during discussion between partners of the PACIFIC and HiTech AlkCarb H2020 projects in February 2019. The final survey design is similar to an already existing geophysical profile crossing the Kaiserstuhl volcanic edifice surveyed by electrical techniques. A total of 66 3-component nodes were deployed along of the profile, and at the center, above a fault, a few additional nodes were deployed away from the profile to be used for earthquake detection and localization.

The nodes recorded during 25 days in October-November 2019. The raw data was of good quality with stable ambient noise records over the whole duration of acquisition. Cross-correlation showed Rayleigh and Love propagation with weak dispersion, testifying to a homogeneous medium in terms of seismic velocities. However individual correlations showed low signal to noise ratio.

We processed the correlations using a classical method of surface wave tomography, and we jointly inverted Rayleigh and Love wave dispersion curves in order to obtain a 3D S-wave velocity model.

The final Vs model showed three layers parallel to the surface with strong velocity contrasts. This is unexpected in such geological context and could be an edge effect of the inversion near the bottom of the model. Once removed, the Vs Anomaly model shows a homogeneous medium with only weak velocity changes (<5%). A positive anomaly dominates the model and coincides well with the location, size, and shape of the carbonatite pipe found in the existing geological and geophysical models.