Combining Geotechnical and Geophysical Data to Build a 3D Model of the Var Valley, Nice (France)

Date: 4/26/2019

Time: 09:30 AM

Room: Vashon

In seismic risk studies, the assessment of lithologic site effect is based on an accurate knowledge of mechanical properties and geometry of superficial geological formations. Therefore, we currently work on improving a 3D subsurface model of the lower Var valley, an economically developing zone of the city of Nice, southeastern France.

Using punctual and ambient vibration array measurements, the alluvial deposits of the lower Var valley close to Nice (France) are investigated. In particular, the S waves velocity profile and the bedrock depth are estimated. These data are included together with geotechnical observations (more than 200 boreholes though only few of them reaching the bedrock) in a commercial software (GDM, BRGM) allowing to model, represent and visualize geoscientific data and geological models in 3D. From the geophysical data, the fundamental resonance frequency pointed on H/V curves indicates the basin/bedrock interface with the help of the well known fo=Vs/4H relationship . The contribution of these constraints in the 3D model are analyzed by comparing the model derived exclusively from the geophysical data analysis and the one derived from the borehole database. The result shows that the nature of the bedrock, found between 20 m and 80 m depth underneath the recent alluvial sediments is laterally varying. Furthermore, the average S wave velocity in these deposits is found to be close to 410 m/s.

The 3D model of the lower Var valley will be used to simulate strong ground motions in order to evaluate and prevent the seismic risk in the area in the frame of the Ritmica project supported by Université Côte d'Azur through one of its JEDI transdisciplinary programs.

Presenting Author: Ophélie Rohmer

Authors