Eustatic and hydrodynamic controls on the architecture of a deep shelf sand bank (Celtic Sea)

The architecture of a tidal sand bank in the south-eastern Celtic Sea was examined using very high-resolution seismic surveys. The bank comprises four depositional units. The lowest unit 1 is characterized by gently dipping (1–8°) strata that strike parallel to the length of the bank. Unit 1 is erosionally overlain by unit 2, which forms the bulk of the bank. This unit consists of stacked sets of downcurrent-dipping (7–12°) master bedding formed by climbing, sinuous-crested tidal dunes that are up to 20 m high. These deposits are locally incised by an anastomosed channel network (unit 3) that may represent a buried swatchway system. The upper part of the bank comprises wave-related deposits that are mainly preserved on the bank flanks (unit 4). The outer bank surface is erosional. The bank is believed to have formed during the last post-glacial sea-level rise. The facies evolution from unit 1 to unit 3 indicates an upward increase in tidal energy, mainly characterized by the thickening of dune cross-bed sets in unit 2. The majority of bank growth is inferred to have occurred in water depths of the order of 60 m. This evolution was controlled by relative sea-level rise, which is likely to have caused an episode of tidal resonance with associated strong tidal currents that were responsible for the incision of the deep, cross-cutting channels of unit 3. The transition to wave-dominated sedimentation in unit 4 is related to the decay of resonance with continued sea-level rise.     

Description and first results of an explicit electrical scheme in a 3D cloud resolving model

The three-dimensional non-hydrostatic mesoscale model MésoNH of the French community offers the numerical environment to develop a cloud electrification scheme in a consistent way with the original mixed phase microphysical scheme. The charge separation mechanisms are entirely due to non-inductive processes and result from elastic ice–snow, ice–graupel and snow–graupel collisions. The electric charges carried by each of the five hydrometeor categories are transported along the airflow and are exchanged according to the various microphysical mass transfer rates but assuming a power law distribution of the individual charges as a function of the particle size. The electric field is diagnosed at each time step after integrating the electric potential induced by a net charge density in the Poisson equation. Finally, a lightning ash is triggered when the electric field locally steps over a given threshold. It propagates in two opposite directions until the magnitude of the electric field falls below a prescribed value. A fractal branching algorithm is then activated to extend lightning streamers away from the main channel and toward cloudy regions where substantial charge densities are present. Charges are neutralized along the tortuous lightning path with a simple procedure that preserves total charge conservation.The complete electrification scheme tested for an ideal case of vigorous supercellular storm shows an intense electrical activity all along its lifecycle. We show that the model is able to produce a direct tripolar structure of the charges as the result of a temperature charge reversal of − 10 °C and of the different sedimentation rates of the hydrometeors.     

Textural evolution of polyhedral olivine experiencing rapid cooling rates

Dynamic crystallization experiments in the CaO–MgO–Al₂O₃–SiO₂ (CMAS) system have been used to investigate the change in crystal shape when pre-existing polyhedral olivine crystals are cooled rapidly (1,639–2,182°C/h). Polyhedral olivines are crystallized initially in a first step using a slow cooling rate (2°C/h), then skeletal and dendritic overgrowths develop on the polyhedral crystals during a subsequent fast cooling event. During this second episode small dendritic olivines also nucleate within the liquid phase. Observation of the experimental sample by optical microscopy shows that the polyhedral olivine shape progressively changes to a skeletal and then to a dendritic morphology in the following sequence: polyhedral ⇒ hopper polyhedral ⇒ dendritic polyhedral. This evolutional sequence is discussed in terms of changes in the crystal growth conditions during cooling and a general relation between these olivine dynamic crystallization experiments and the integrated model of crystal growth by Sunagawa (Bull Minér 104:81–87, 1981, Morphology of crystals, Terra Scientific Publishing Company, 1987) is proposed.     

Weathering of sandstone lotus petals at the Angkor site: a 1,000-year stone durability trial

This article is based on field investigations carried out at Ta Keo and Banteay Srei temples (Angkor, Cambodia), which have similar environmental histories and display the same recurrent decorative pattern (lotus petal) in three different sandstone lithotypes. Based on two original scales of mechanical and chemical weathering, the degree of deterioration of c.2000 sandstone petals was visually assessed, resulting in the proposal of a durability scale. An explanatory scheme was provided, based on laboratory analyses of stone materials (ICP-AES, XRD, SEM-EDS, polarizing microscopy and mercury injection). The key drivers of sandstone durability are both mineralogical and petrophysical. The remarkable preservation of the pink sandstone is due to the quartzose nature of its clasts, the non-swelling clay content of its abundant matrix (kaolinite) and its porosity structure (large connected pores and probable non-connected porosity). The overall good preservation of the green sandstone is due to its extremely low porosity and specific surface area, associated with its strong cementation. The severe deterioration of the grey sandstone is due to its laminated structure (oriented biotites), the susceptibility to hygric swelling and solution of its binding agents (chlorite/smectite and calcite) and its porosity structure, which evolves during a two-stage weathering sequence. The first stage is characterized by a bimodal porosity due to the partial clogging of macropores by swelling clays, and the second stage by a substantial increase in porosity and the penetration of salts (barite and anhydrite). Conversely, oxidation phenomena associated with weathering can lead to a drastic reduction of porosity.     

Coupled damage and plasticity modelling in transient dynamic analysis of concrete

In a concrete structure subjected to an explosion, for example a concrete slab, the material is subjected to various states of stress which lead to many modes of rupture. Closer to the explosive, a state of strong hydrostatic compression is observed. This state of stress produces an irreversible compaction of the material. Away from the zone of explosion, confinement decreases and the material undergoes compression with a state of stress, which is slightly triaxial. Finally, the compression wave can be reflected on a free surface and becomes a tensile wave, which by interaction with the compression wave, produces scabbing. We present, in this paper, a model aimed at describing these three failure modes. It is based on visco‐plasticity and rate dependent damage in which a homogenization method is used in order to include the variation of the material porosity due to compaction. The model predictions are compared with several experiments performed on the same concrete. Computations of split Hopkinson tests on confined concrete, a tensile test with scabbing, and an explosion on a concrete slab are presented. Copyright © 2001 John Wiley & Sons, Ltd.     

The large volume multi-anvil press as a highP-T deformation apparatus

The rheological properties of mantle materials are being investigated up to pressures of 16 GPa and temperatures of 1600°C for times up to 24 h, using a new sample assembly for the 6–8 multi-anvil apparatus. Al₂O₃ pistons, together with a liquid confining medium, are used to generate deviatoric stress in the specimen. Strain rates are estimated by monitoring the relative displacement of the guide blocks of the multi-anvil apparatus, scaled to the total axial strain of the sample. The applied stress on the sample is estimated using grain size piezometry. Strain rates and flow stresses of approximately 10^–4 to 10^–6 s^–1 and 50 to 250 MPa respectively, are presently attainable.Preliminary results on San Carlos olivine single crystals, partially dynamically recrystallized to a grain size of 10 to 300 m, indicate that the effective viscosity of polycrystalline olivine is consistent with values obtained from olivine single crystal creep laws. Assuming a dislocation creep mechanism (n3.5) with (010)[001] as the dominant slip system, the data are best fit using a creep activation volume of 5 to 10×10^–6 m³ mol^–1.     

Frequency–wavenumber modelling and migration of 2D GPR data in moderately heterogeneous dispersive media

An algorithm for modelling and migrating ground penetrating radar (GPR) data in moderately heterogeneous dispersive media is presented. The method is based on wavefield extrapolation in the frequency–wavenumber (f–k) domain, from the solution of the 2D Maxwell's equations. The wavefield is extrapolated by a phase-shift technique using a constant relative permittivity K and a quality factor Q. It is then modified by a correction term to handle the lateral K and Q variations. The spatial distribution of the K and Q-factor values, representing the given model parameters, is introduced into the algorithm by a regular grid parametrization. The radar wave dispersion and attenuation, induced by relaxation processes, are taken into account by a linear frequency-dependent Q model, and expressed by a complex wavenumber in the propagation equation. A synthetic case and a field data set illustrate the potential of the method for frequencies of 300, 500 and 900 MHz. In the first case, a typical civil engineering problem is considered. The frequency dependence of the wave velocity and attenuation is well illustrated. The synthetic data are afterwards migrated using the initial model parameters. The results show the importance of using spatially varying model parameters in the migration processes. The second case concerns an application of the method to a real data set. In order to adjust the model parameters, a forward modelling sequence is performed until the best match between the measured and the synthetic data is achieved. A depth migration is then applied to the data, and the result is compared with the initial model parameters. In conclusion, we assess the contributions of the method to industrial applications, by discussing the performance of the algorithm compared with its limitations.     

Symplectic Mapping for Satellites and Space Debris Including Nongravitational Forces

The paper presents an efficient algorithm for the study of satellite and space debris orbits on long time intervals. The averaged equations of motion are integrated by means of the implicit midpoint method. This approach is known as a symplectic mapping technique. The perturbing forces included in the mapping are: the geopotential, the atmospheric drag, lunisolar perturbations and the direct radiation pressure (without shadow effects). The influence of the atmosphere is approximated by simple methods for the estimation of integrals. The described mapping is valid for the wide range of orbits including the resonant and the eccentric ones; it can be helpful in practical and theoretical problems. The lifetime of GPS transfer orbits is discussed as an exemplary application.