Combining borehole log-stratigraphies and ambient vibration data to build a 3D Model of the Lower Var Valley, Nice (France)

• Published in: Engineering geology 2020-06, Vol.270, p.105588, Article 105588
• Main Authors: Rohmer, O., Bertrand, E., Mercerat, E.D., Régnier, J., Pernoud, M., Langlaude, P., Alvarez, M.
• Format: Article
• Language: English
• Subjects: Ambient vibrations; Earth Sciences; Geophysics; Geotechnical boreholes; Lithostratigraphic model; Lower Var Valley; Mechanics; Physics; Sciences of the Universe; Vibrations
• ISSN: 0013-7952; 1872-6917
• DOI: 10.1016/j.enggeo.2020.105588

In seismic hazard studies, site effects assessment is based on an accurate knowledge of mechanical properties and geometry of superficial geological formations. The Lower Var Valley (Nice, southeastern France) is a 2 km wide fluvial sedimentary basin where site effects have been observed on earthquake recordings. In addition, this area is a significant economically developing zone including the second largest airport of France. Therefore, it is highly important to define a 3D sub-surface model of the basin to understand the ground motion amplifications. Based on the compilation of several years of geotechnical and geophysical studies, the Quaternary alluvial deposits of the Lower Var Valley were investigated using 331 borehole log-stratigraphies and 439 ambient vibration measurements. These data were combined in a commercial software that models and allows to visualize geoscientific data and geological models in 3D. The borehole log-stratigraphies helped to define a representative lithostratigraphic soil profile and the geometry of the different sedimentary layers. Only few boreholes are reaching the engineering bedrock (Pliocene conglomerates characterized by a S-wave velocity larger than 800 m.s−1). From the ambient vibration data, the fundamental resonance frequency identified from H/V curves indicates the bedrock depth using the well-known f0 = Vs/4H relationship. These H/V curves were interpreted using geological knowledge of the studied zone. Our results show that the bedrock depth varies, in the center of the valley, between 50 m in the northern part of the studied zone and more than 100 m on the river estuary. First, the average S-wave velocity in the alluvial deposits is estimated according to combined analysis of the borehole log-stratigraphies and H/V calculations on ambient vibration measurements. The spatial variability of this velocity has been translated into a velocity zonation of the valley. Second, this zonation allows us to extrapolate the Quaternary-Pliocene limit in the whole valley. The 3D model of the Lower Var Valley will be useful in future numerical simulations of seismic ground motions in order to evaluate and mitigate seismic risk in the area. •New 3D geotechnical model of the Lower Var Valley, France•Local seismic hazard knowledge•Major economic zone of Nice•Comprehensive geological-geotechnical and geophysical (seismology) database