Localized and Cellular Patterns in a Vibrated Granular Layer

We propose a phenomenological model for pattern formation in a vertically vibrated layer of granular material. This model exhibits a variety of stable cellular patterns including standing rolls and squares as well as localized objects (oscillons and worms), similar to recent experimental observations(Umbanhowar et al., 1996). The model is an amplitude equation for the parametrical instability coupled to the mass conservation law. The structure and dynamics of the solutions resemble closely the properties of localized and cellular patterns observed in the experiments.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let

2D characterization of near-surface V P/V S: surface-wave dispersion inversion versus refraction tomography

International audienceThe joint study of pressure (P-) and shear (S-) wave velocities (Vp and Vs ), as well as their ratio (Vp /Vs), has been used for many years at large scales but remains marginal in near-surface applications. For these applications, and are generally retrieved with seismic refraction tomography combining P and SH (shear-horizontal) waves, thus requiring two separate acquisitions. Surface-wave prospecting methods are proposed here as an alternative to SH-wave tomography in order to retrieve pseudo-2D Vs sections from typical P-wave shot gathers and assess the applicability of combined P-wave refraction tomography and surface-wave dispersion analysis to estimate Vp/Vs ratio. We carried out a simultaneous P- and surface-wave survey on a well-characterized granite-micaschists contact at Ploemeur hydrological observatory (France), supplemented with an SH-wave acquisition along the same line in order to compare Vs results obtained from SH-wave refraction tomography and surface-wave profiling. Travel-time tomography was performed with P- and SH- wave first arrivals observed along the line to retrieve Vtomo p and Vtomo s models. Windowing and stacking techniques were then used to extract evenly spaced dispersion data from P-wave shot gathers along the line. Successive 1D Monte Carlo inversions of these dispersion data were performed using fixed Vp values extracted from Vtomo p the model and no lateral constraints between two adjacent 1D inversions. The resulting 1D Vsw s models were then assembled to create a pseudo-2D Vsw s section, which appears to be correctly matching the general features observed on the section. If the pseudo-section is characterized by strong velocity incertainties in the deepest layers, it provides a more detailed description of the lateral variations in the shallow layers. Theoretical dispersion curves were also computed along the line with both and models. While the dispersion curves computed from models provide results consistent with the coherent maxima observed on dispersion images, dispersion curves computed from models are generally not fitting the observed propagation modes at low frequency. Surface-wave analysis could therefore improve models both in terms of reliability and ability to describe lateral variations. Finally, we were able to compute / sections from both and models. The two sections present similar features, but the section obtained from shows a higher lateral resolution and is consistent with the features observed on electrical resistivity tomography, thus validating our approach for retrieving Vp/Vs ratio from combined P-wave tomography and surface-wave profiling

Behavior of seismic-acoustic parameters during deforming and failure of rock samples, large blocks and underground opening: base for monitoring

Abstract Construction of engineered projects, such as underground spaces, tunnels, machine halls etc. is connected with variations in stress-state and deformations of large volumes of rock mass. Latter can results in damage of rocks, their collapse into underground space, danger for equipment and risk for human life. To avoid such consequences different in situ geotechnical and geophysical monitoring is carried out during construction and exploitation of underground structures. Geophysical monitoring is based on observations for behavior of elastic shear- and longitudinal wave velocities (Vs and Vp, respectively) and microseismic activity. Behavior of the elastic velocities during deforming of the rock depends on type of the future failure that, in turn, is defined by structure and properties of medium and characteristics of stress state σ 3/σ 1 and hydrostatic pressure. These velocity variations are defined by difference in effective parameters of forming microfractures, whose geometry is distinguished at different modes of stress-state. At that character of interaction between microfractures determines type of the macrofailure. We studied behavior of longitudinal wave velocities during loading of rock samples, large blocks and underground opening orienting measurements along maximum (σ 1) and minimum (σ 3) stresses. It is shown that velocity variations along maximum stress is more informative at elastic phase of rock deformations (velocity increases), whereas velocity variations along axis of minimum stress is more informative at the stage of nonlinear deformation of rock (velocity begin decrease). These regularities are well appeared at deforming of large blocks and unloading of rock mass in underground openings. This knowledge should be used at planning and performing of monitoring of stability of underground structures

Sinkhole Field Above Karst Caves: Detection and Analysis Through Integrated Techniques

The present work illustrates the results of studies in a sinkhole field at Nociglia (Salento sub-region, Apulia, SE Italy) where the shallow phreatic speleogenesis operates close to the water table level with formation of karst caves, successive roof collapse, formation of wide caverns and sinkhole development. All of this creates serious problems to the nearby infrastructures, including a province road. Salento has a great number of active sinkholes, related to natural and anthropogenic cavities. Their presence is at the origin of several problems to the built-up environment, due to the increasing population growth and development pressures. In such a context, the detection of cavities, and the assessment of the sinkhole hazard presents numerous difficulties. A multidisciplinary approach, comprising geological, geomorphological and geophysical analyses, is necessary to obtain comprehensive knowledge of the complex phenomena in karstic areas. Geophysical methods can be of great help to identify and map the features related to the underground voids, likely evolving to sinkholes, by detecting contrasts in physical properties such as density and electrical resistivity, with the surrounding sediments. At the same time, recognition of the presence of sinkholes by geophysical methods has to adapt to the different geological conditions, and to take advantage of the integration among the several methodologies availabl

Seismic Surface-wave Prospecting Methods for Sinkhole Hazard Assessment along the Dead Sea Shoreline

International audienceThe Dead Sea (DS) coastal areas have been dramatically hit by sinkhole occurrences since around 1990. It has been shown that the sinkholes along both Israeli and Jordanian shorelines are linked to evaporate karst cavities that are formed by slow salt dissolution. Both the timing and location of sinkholes suggest that: 1) the salt weakens as the result of unsaturated water circulation, thus enhancing the karstification process; and 2) sinkholes appear to be related to the decompaction of the sediments above karstified zones. The location, depth, thickness and weakening of salt layers along the DS shorelines, as well as the thickness and mechanical properties of the upper sedimentary deposits, are thus considered as controlling factors of this on-going process. The knowledge of shear-wave velocities (Vs) should add valuable insights on mechanical properties of both the salt and its overburden. We have suggested Vs estimation using surface-wave prospecting methods, based on surface-wave dispersion measurements and inversion. Two approaches have been used. Along the Israeli shoreline, Vs mapping has been performed to discriminate weak and hard zones within salt layers, after calibration of inverted Vs near boreholes. It has been shown that there is a Vs increase in the DS direction. Initially examined weak zones, located near the salt edge, associated with karstified salt, are characterized by Vs values of 760–1,050 m/s, and extend 60–100 m from the salt edge in the DS direction. Hard salt zones with velocity Vs values greater than 1,500 m/s are located at distances of more than 100–220 m from the salt edge. Finally, transition zones (1,050 < Vs < 1,500 m/s) have a 40–160 m spread. On a Jordanian site, roll-along acquisition and dispersion stacking has been performed to achieve multi-modal dispersion measurements along linear profiles. Inverted pseudo-2-D Vs sections present low Vs anomalies in the vicinity of existing sinkholes and made it possible to detect decompacted sediments associated with potential sinkhole occurrences. Moreover, Vs profiles showed a high velocity unit at 40–50 m depth that can be interpreted as a salt layer