R. Mourgues

Sandbox modelling of thrust wedges with fluid-assisted detachments

Abstract We have used granular materials to model the development of thrust wedges, where migrating pore fluids assisted in the formation of detachments. The governing equations yield practical scales for linear dimensions, stresses and time. Using compressed air as a pore fluid, models a few centimetres thick were deformed in about half an hour. Model materials were sands of three different grain sizes and a loess. They had suitable values of density, permeability, cohesion and internal friction. Fluid flow obeyed Darcys law. At yield, the materials satisfied a Coulomb criterion for effective stresses. Models with various sequences of layers were submitted to horizontal shortening in a rectangular box. Compressed air entered through a sieve at the base. The fluid pressure was uniform over the basal boundary. In models made from a single material, the style of deformation depended on the fluid pressure. For no fluid flow, the thrust wedge was short and high, the surface slope attained large angles (30°) and internal structures were mainly forethrusts. For fluid pressures approaching lithostatic values, thrust wedges were longer and lower and surface slopes attained smaller angles. In models containing basal layers of small permeability, detachments formed beneath them and the structural style was dominated by interacting forethrusts and backthrusts. In multilayered models, thin-skinned detachments formed beneath less permeable layers in the sequence. To understand how fluid flow controlled the first stages of detachment, we calculated ideal vertical profiles of fluid pressure, vertical normal stress, effective stress and horizontal shear stress, for multilayered models in the undeformed state. The profiles are segmented, because material properties vary from layer to layer. Sharp drops in shear strength occur at positions where detachments were observed in the sandbox models. We infer that detachments resulted from large fluid pressures beneath relatively impermeable layers.

Experimental modeling of pressurized subglacial water flow: Implications for tunnel valley formation

Tunnel valleys are elongated hollows commonly found in formerly glaciated areas and interpreted as resulting from subglacial meltwater erosion beneath ice sheets. Over the past two decades, the number of studies of terrestrial tunnel valleys has continuously increased and their existence has been hypothesized also on Mars, but their formation mechanisms remain poorly understood. We introduce here, an innovative experimental approach to examine erosion by circulation of pressurized meltwater within the substratum and at the silicon-substratum interface. We used a permeable substratum (sand) partially covered by a viscous, impermeable and transparent lid (silicon putty), below which we applied a central injection of pure water. Low water pressures led to groundwater circulation in the substratum only, while water pressures exceeding the sum of the glaciostatic and lithostatic pressures led to additional water circulation and formation of drainage landforms at the cap-substratum interface. The formation of these drainage landforms was monitored through time and their shapes were analyzed from digital elevation model obtained by stereo-photogrammetry. The experimental landforms include valleys that are similar to natural tunnel valleys in their spatial organization and in a number of diagnostic morphological criteria, such as undulating longitudinal profiles and “tunnel” shapes. These results are consistent with the hypothesis that overpressurized subglacial water circulation controls the formation of tunnel valleys.

Laser-Doppler acoustic probing of granular media with in-depth property gradient and varying pore pressures

Non-contacting ultrasonic techniques recently proved to be efficient in the physical modeling of seismic-wave propagation at various application scales, as for instance in the context of geological analogue and seismic modeling. An innovative experimental set-up is proposed here to perform laser-Doppler acoustic probing of unconsolidated granular media with varying pore pressures. The preliminary experiments presented here provide reproducible results and exploitable data, thus validating both the proposed medium preparation and pressure gradient generation procedure.

Contribution of Seismic Methods to Hydrogeophysics

The characterisation and monitoring of aquifer systems mainly rely on piezometric and log data. Delineating spatial variations of lithology between piezometers is a delicate task, which inevitably generates errors possibly propagating into hydrogeological models. Seismic methods have been proposed to: (i) improve the low spatial resolution of borehole data, (ii) provide a characterisation of the subsurface geometry, and (iii) estimate the physical parameters of the medium influenced by the presence of water and the associated flow and transport processes. The joint study of pressure (P-) and shear (S-) wave seismic velocities (VP and VS, respectively), whose evolution is strongly decoupled in the presence of fluid, has been proposed through the estimation of the VP/VS ratio and Poissons ratio. A specific methodology has been developed for the combined exploitation of P- and surface waves present on single seismic records. The use of this methodology in several geological and hydrogeological contexts allowed for estimating VP/VS ratio lateral and temporal variations in good agreement with a priori geological information and existing geophysical and piezometric data. Laser-based ultrasonic techniques were also proposed to put these processing techniques in practice on perfectly controlled physical models and study elastic wave propagation in partially saturated porous media.

Physical Modelling of Seismic-wave Propagation over a Two Dimensional Granular Medium

Laboratory small-scale physical models and non-contacting ultrasonic techniques are used to tackle theoretical or methodological issues of seismic wave propagation and seismic methods. Literature shows a wide range of experiments, both in terms of materials used for the production of the physical models but also regarding the models geometry and the recording techniques, which are chosen according to the issue that is being addressed. With a proper choice of granulometries and deposition processes, we managed here to create a two-layer granular physical model with a relatively complex geometry and characterized by 2D structures, property contrast and velocity gradients within layers. We performed several small scale seismic acquisitions using a mechanical source and a laser vibrometer. The acquired seismograms were interpreted by applying the surface-wave method and by extracting P-wave refraction data. We managed to correctly reconstruct the geometry of the model and estimated the parameters controlling the velocity gradients of P and S waves for both layers. The results we got are coherent with the different compaction degree we obtained for the two layers and with previous studies conducted over similar media.

Modelled subglacial floods and tunnel valleys control the lifecycle of transitory ice streams

Ice streams are corridors of fast-flowing ice that control mass transfers from continental ice sheets to oceans. Their flow speeds are known to accelerate and decelerate, their activity can switch on and off, and even their locations can shift entirely. Our analogue physical experiments reveal that a life cycle incorporating evolving subglacial meltwater routing and bed erosion can govern this complex transitory behaviour. The modelled ice streams switch on and accelerate when subglacial water pockets drain as marginal outburst floods (basal decoupling). Then they decelerate when the lubricating water drainage system spontaneously organizes itself into channels that create tunnel valleys (partial basal recoupling). The ice streams surge or jump in location when these water drainage systems maintain low discharge but they ultimately switch off when tunnel valleys have expanded to develop efficient drainage systems. Beyond reconciling previously disconnected observations of modern and ancient ice streams into a single life cycle, the modelling suggests that tunnel valley development may be crucial in stabilizing portions of ice sheets during periods of climate change.

Surface-wave Analyses in Unconsolidated Granular Models with Increasing Degrees of Complexity

Using micrometric glass beads, we build small scale physical models with increasing degrees of complexity in order to address theoretical and methodological issues of seismic methods (velocity gradients, lateral variations, pore overpressure, etc.). We simulate seismic records at the surface of the laboratory models thanks to a mechanical source and a laser-Doppler vibrometer. From recorded seismograms, we are able to invert surface-wave dispersion for one or two-dimensional velocity structures. These experiments are for instance used as benchmarks for processing and inversion techniques, enable the validation of numerical methods, or make it possible to study issues related to pore fluids.