On the Utility of Horizontal-to-Vertical Spectral Ratios of Ambient Noise in Joint Inversion with Rayleigh Wave Dispersion Curves for the Large-N Maupasacq Experiment

Horizontal-to-Vertical Spectral Ratios (HVSR) and Rayleigh group velocity dispersion curves (DC) can be used to estimate the shallow S-wave velocity ([Formula: see text]) structure. Knowing the [Formula: see text] structure is important for geophysical data interpretation either in order to better constrain data inversions for P-wave velocity ([Formula: see text]) structures such as travel time tomography or full waveform inversions or to directly study the [Formula: see text] structure for geo-engineering purposes (e.g., ground motion prediction). The joint inversion of HVSR and dispersion data for 1D [Formula: see text] structure allows characterising the uppermost crust and near surface, where the HVSR data ([Formula: see text] to [Formula: see text]) are most sensitive while the dispersion data (1 to [Formula: see text]) constrain the deeper model which would, otherwise, add complexity to the HVSR data inversion and adversely affect its convergence. During a large-scale experiment, 197 three-component short-period stations, 41 broad band instruments and 190 geophones were continuously operated for 6 months (April to October 2017) covering an area of approximately [Formula: see text] with a site spacing of approximately 1 to [Formula: see text]. Joint inversion of HVSR and DC allowed estimating [Formula: see text] and, to some extent density, down to depths of around [Formula: see text]. Broadband and short period instruments performed statistically better than geophone nodes due to the latter’s gap in sensitivity between HVSR and DC. It may be possible to use HVSR data in a joint inversion with DC, increasing resolution for the shallower layers and/or alleviating the absence of short period DC data, which may be harder to obtain. By including HVSR to DC inversions, confidence improvements of two to three times for layers above [Formula: see text] were achieved. Furthermore, HVSR/DC joint inversion may be useful to generate initial models for 3D tomographic inversions in large scale deployments. Lastly, the joint inversion of HVSR and DC data can be sensitive to density but this sensitivity is situational and depends strongly on the other inversion parameters, namely [Formula: see text] and [Formula: see text]. Density estimates from a HVSR/DC joint inversion should be treated with care, while some subsurface structures may be sensitive, others are clearly not. Inclusion of gravity inversion to HVSR/DC joint inversion may be possible and prove useful.