Theoretical and experimental developments in acoustics have always been helpful to geophysicists investigating the propagation of mechanical waves through the Earth, whatever the scale of interest. Acoustics, non-destructive evaluation, exploration seismic and seismology obviously share common issues that can be addressed simultaneously thanks to the development of measurement devices and laboratory physical experiments. Laser-based ultrasonic techniques have been for instance providing appropriate tools for studying seismic-wave propagation in near-surface geophysics, thanks to small-scale physical modelling using homogeneous and consolidated materials (such as metal and thermoplastics that can be easily manufactured into various shapes and provide a wide range of mechanical parameters). There is however a need to study the propagation of seismic waves in more complex and realistic media, more particularly when the problematic of unconsolidated and/or porous materials have to be considered.

We thus addressed the ability of laser-based experiments for the characterisation of dry granular materials. An experimental set-up has been developed and a methodology validated on multi-layered glass beads models. A mechanical source and a laser-Doppler vibrometer were used to record small-scale seismic lines at the surface of the granular medium. When guided surface acoustic mode theory should prevail in such unconsolidated granular packed structure under gravity, we only considered elastic-wave propagation in stratified media to interpret recorded data. Thanks to basic seismic processing and inversion methods, we were able to correctly retrieve the gradients of pressure- and shear-wave velocities in our models. 3D elastic finite difference simulations of the experiments offered, despite significant differences in terms of amplitudes, a supplementary validation of our approximations, as far as elastic properties of the medium were concerned.

We took advantages of this experimental set-up to simulate a seismic surface-wave survey over a laterally varying granular medium. We were able to construct a physical model with two layers presenting distinct in-depth velocity gradients, separated by a dipping interface. We used this physical model to address the efficiency of an innovative surface-wave processing technique developed to retrieve 2D structures from a limited number of receivers. Similar experiments were performed in order to monitor granular media with varying pore pressures, in the context of geological analogue and seismic modelling studies. A compressed air reservoir was then used at the base of a model to generate a pore pressure gradient in the medium. Despite the noise generated by the air injection, recorded seismograms presented coherent and exploitable wavefields, showing the influence of decreasing differential pressure on the medium wave propagation velocities. In the context of hydrogeophysics, we adapted this experimental set-up to investigate the influence of varying water levels on seismic prospecting tools used to study hydrosystems. The differences in travel time and phase velocity observed between the dry and wet models showed patterns that interestingly matched the observed water level and depth of the capillary fringe, thus offering attractive perspectives for studying water content variations in soils for instance.

Suggested references :
Bodet, L., A. Dhemaied, R. Martin, R. Mourgues, F. Rejiba and V. Tournat, 2014, Small-scale physical modelling of seismic-wave propagation using unconsolidated granular media, Geophysics, 79, 6.

Pasquet, S., L., Bodet, P., Bergamo, R., Guérin, R., Martin, R., Mourgues, V., Tournat, 2016, Small-scale seismic monitoring of varying water levels in granular media. Vadose Zone J., 15, 7.