CLASSIFICATION OF TWENTY-SIX CHONDRITES FROM ROOSEVELT COUNTY,NEW MEXICO

Twenty-six of the fifty-seven stone meteorites listed in Huss (1979) from Roosevelt County, New Mexico, have been classified in the present study. Microprobe analyses indicate 15 H type, 9 L type and 2 LL type chondrites. Based on compositional, textural, and locational comparisons, as many as 10 chondrites may be paired to three distinct falls.     

Numerical simulation of fracture flow with a mixed-hybrid FEM stochastic discrete fracture network model

We introduce a discrete fracture network model of stationary Darcy flow in fractured rocks. We approximate the fractures by a network of planar circle disks, which is generated on the basis of statistical data obtained from field measurements. We then discretize this network into a mesh consisting of triangular elements placed in three-dimensional space. We use geometrical approximations in fracture planes, which allow for a significant simplification of the final triangular meshes. We consider two-dimensional Darcy flow in each fracture. In order to accurately simulate the channeling effect, we assign to each triangle an aperture defining its hydraulic permeability. For the discretization we use the lowest order Raviart-Thomas mixed finite element method. This method gives quite an accurate velocity field, which is computed directly and which satisfies the mass balance on each triangular element. We demonstrate the use of this method on a model problem with a known analytical solution and describe the generation and triangulation of the fracture network and the computation of fracture flow for a particular real situation.     

Quaternary sediment dynamics in the Belgica mound province,Porcupine Seabight: ice-rafting events and contour current processes

The Belgica cold-water coral banks on the eastern slope of the Porcupine Seabight are closely associated with bottom currents. In order to better understand the local temporal and spatial characteristics, as well as the palaeoclimatologic influences, a 26 m long core, taken on a small contourite drift, was studied. This sediment record of approximately 100 ka BP reveals new insights into the regional glacial and sedimentary processes, which are intrinsically linked to several geological, climatological, biological and hydrodynamic variables. The glacial sequences in the core contain six ice-rafting events (IRE). They are comparable with the North Atlantic Heinrich Events, although their characteristics show dominant influences from the proximal British–Irish Ice Sheet (BIIS). These IRE have a low magnetic susceptibility and are deposited during two or three ice-rafting pulses. The record of ice-rafting suggests a millennial-scaled BIIS destabilisation and confirms the start of a final retreat about 25 ka ago. Additionally, the glacial sequence corresponds to a muddy contourite, influenced by bottom-current strength variations during interstadials, possibly triggered by sporadic reintroductions of Mediterranean Outflow Water in a glacial North Atlantic Ocean. The interglacial sequence features an 11-m thick deep-water massive sand unit, probably deposited under a high-energy bottom-current regime.     

Diffusion-creep modelling of fibrous pressure-shadows

Grain-scale deformation of a bimineralic composite by normal-stress-driven grain-boundary diffusive mass-transfer has been modelled using diffusion-creep software that, for the first time, allows different diffusion coefficients for interphase boundaries and grain-boundaries as well as for different components. By selecting suitable coefficients we have obtained a fully coupled mechanical and mass-transfer numerical solution for the formation of fibrous pressure-shadows or strain fringes at the interface between a rigid object and a deforming rock matrix of different compositions. The model is two-dimensional and run in pure-shear velocity mode to a stretch ratio of 1.4. The first results are reported, which show dissolution and face-controlled antitaxial growth of a soluble, polycrystalline, calcite matrix at the interface with a circular and relatively insoluble pyrite object.The ratio of CaCO₃ diffusion along calcite–pyrite versus calcite–calcite boundaries strongly influences the extent of pressure-shadow growth and the pattern of strains in the matrix. A low ratio produces little growth, significant perturbation, and deflection of total-strain axes around the pyrite. An intermediate ratio produces appreciable growth, little perturbation, and near-parallel total-strain axes. A high ratio produces larger pressure-shadows, and total-strain axes that bend in towards the pyrite as if it were a weak object. Sliding and rotation of matrix grains are also affected.These results differ fundamentally from purely physical models of ductile flow around a rigid object and demonstrate the influence of interface diffusional properties on the resulting behaviour. In particular, assumptions such as perturbation of the flow by the object, deflection of external foliation around pre-deformational porphyroblasts and relationships of pressure-shadow strain to bulk strain may need to be re-examined in the light of this finding.     

A parallel ensemble-based framework for reservoir history matching and uncertainty characterization

We present a parallel framework for history matching and uncertainty characterization based on the Kalman filter update equation for the application of reservoir simulation. The main advantages of ensemble-based data assimilation methods are that they can handle large-scale numerical models with a high degree of nonlinearity and large amount of data, making them perfectly suited for coupling with a reservoir simulator. However, the sequential implementation is computationally expensive as the methods require relatively high number of reservoir simulation runs. Therefore, the main focus of this work is to develop a parallel data assimilation framework with minimum changes into the reservoir simulator source code. In this framework, multiple concurrent realizations are computed on several partitions of a parallel machine. These realizations are further subdivided among different processors, and communication is performed at data assimilation times. Although this parallel framework is general and can be used for different ensemble techniques, we discuss the methodology and compare results of two algorithms, the ensemble Kalman filter (EnKF) and the ensemble smoother (ES). Computational results show that the absolute runtime is greatly reduced using a parallel implementation versus a serial one. In particular, a parallel efficiency of about 35 % is obtained for the EnKF, and an efficiency of more than 50 % is obtained for the ES.