HVSR passive seismic stratigraphy for the investigation of planetary volcanic analogues

This paper presents new results of the application of passive seismic surveys for the stratigraphic investigation of planetary volcanic analogues. We tested HVSR (Horizontal-to-Vertical Spectral Ratio) surveys that seem particularly suitable for planetary subsurface exploration. In order to exploit the potential of these surveys in stratigraphic applications, HVSR surveys were undertaken at the Tinguaton volcanic region (Lanzarote, Canary Islands). Seismic noise measurements were collected during the PANGAEA-X 2018 testing campaign which formed part of the European Space Agency's (ESA) astronaut training program. Single-station, free-field seismic noise data were collected along two orthogonal profiles, one crossing the Tinguaton volcano, the other passing alongside the Tinguaton caldera. Seismic stratigraphic sections obtained from the shear wave velocity constrained inversion of the main H/V peaks provided the main impedance contrasts between layers and a bulk estimate of the shear wave velocity in the layers. Contour maps of the HVSR patterns were used to recognize and identify the main volcanic units, such as scoria deposits and different basaltic lava flows, geological structures, such as a regional fault related to the volcanic vent source, and morphological characteristics, such as the Tinguaton caldera. This study also identified several pitfalls that may limit the accuracy of such investigations. These include the multilayer setting of scoria deposits and, partly, of underlying basalts, as well as the presence of very thin horizons or a very gradual increase in shear wave velocity. Our study also shows the topographic effect due to the volcano's sharp topography along crater rim is significant and results in severe artifacts in the data. From our work, it is clear that HVSR method is an effective method to support the investigation of planetary volcanic terrains on the Moon and Mars with similarly volcanic complex geological settings, even where horizontal layering and the isotropic site response are not fully verified.