Calculating the effective permeability of sandstone with multiscale lattice Boltzmann/finite element simulations

The lattice Boltzmann (LB) method is an efficient technique for simulating fluid flow through individual pores of complex porous media. The ease with which the LB method handles complex boundary conditions, combined with the algorithm’s inherent parallelism, makes it an elegant approach to solving flow problems at the sub-continuum scale. However, the realities of current computational resources can limit the size and resolution of these simulations. A major research focus is developing methodologies for upscaling microscale techniques for use in macroscale problems of engineering interest. In this paper, we propose a hybrid, multiscale framework for simulating diffusion through porous media. We use the finite element (FE) method to solve the continuum boundary-value problem at the macroscale. Each finite element is treated as a sub-cell and assigned permeabilities calculated from subcontinuum simulations using the LB method. This framework allows us to efficiently find a macroscale solution while still maintaining information about microscale heterogeneities. As input to these simulations, we use synchrotron-computed 3D microtomographic images of a sandstone, with sample resolution of 3.34 μm. We discuss the predictive ability of these simulations, as well as implementation issues. We also quantify the lower limit of the continuum (Darcy) scale, as well as identify the optimal representative elementary volume for the hybrid LB–FE simulations.     

Numerical simulation of bolt‐supported tunnels by means of a multiphase model conceived as an improved homogenization procedure

This paper examines the possibility of applying a homogenization procedure to analyze the convergence of a tunnel reinforced by bolts, regarded as periodically distributed linear inclusions. Owing to the fact that a classical homogenization method fails to account for the interactions prevailing between the bolts and the surrounding ground and thus tends to significantly overestimate the reinforcement effect in terms of convergence reduction, a so‐called multiphase model is presented and developed, aimed at improving the classical homogenization method. Indeed, according to this model, the bolt‐reinforced ground is represented at the macroscopic scale as the superposition of two mutually interacting continuous phases, describing the ground and the reinforcement network, respectively. It is shown that such a multiphase approach can be interpreted as an extension of the homogenization procedure, thus making it possible to capture the ground–reinforcement interaction in a proper way, provided the constitutive parameters of the model and notably those relating to the interaction law can be identified from the reinforced ground characteristics. The numerical implementation of this model in a finite element method‐based computer code is then carried out, and a first illustrative application is finally presented. Copyright © 2008 John Wiley & Sons, Ltd.     

Syn‐sedimentary salt diapirism as a control on fluvial‐system evolution: an example from the proximal Permian Cutler Group,SE Utah,USA

Loading of subsurface salt during accumulation of fluvial strata can result in halokinesis and the growth of salt pillows, walls and diapirs. Such movement may eventually result in the formation of salt‐walled mini‐basins, whose style of architectural infill may be used to infer both the relative rates of salt‐wall growth and sedimentation and the nature of the fluvial‐system response to salt movement. The Salt Anticline Region of the Paradox Basin of SE Utah comprises a series of elongate salt‐walled mini‐basins, arranged in a NW‐trending array. The bulk of salt movement occurred during deposition of the Permian Cutler Group, a wedge of predominantly quartzo‐feldspathic clastic strata comprising sediment derived from the Uncompahgre Uplift to the NE. The sedimentary architecture of selected mini‐basin fills has been determined at high resolution through outcrop study. Mini‐basin centres are characterized by multi‐storey fluvial channel elements arranged into stacked channel complexes, with only limited preservation of overbank elements. At mini‐basin margins, thick successions of fluvial overbank and sheet‐like elements dominate in rim‐syncline depocentres adjacent to salt walls; many such accumulations are unconformably overlain by single‐storey fluvial channel elements that accumulated during episodes of salt‐wall breaching. The absence of gypsum clasts suggests that sediment influx was high, preventing syn‐sedimentary surface exposure of salt. Instead, fluvial breaching of salt‐generated topography reworked previously deposited sediments of the Cutler Group atop growing salt walls. Palaeocurrent data indicate that fluvial palaeoflow to the SW early in the history of basin infill was subsequently diverted to the W and ultimately to the NW as the salt walls grew to form topographic barriers. Late‐stage retreat of the Cutler fluvial system coincided with construction and accumulation of an aeolian system, recording a period of heightened climatic aridity. Aeolian sediments are preserved in the lees of some salt walls, demonstrating that halokinesis played a complex role in the differential trapping of sediment.     

The Characterization of a Former World War I Battlefield by Integrating Multiple Signals from a Multireceiver EMI Soil Sensor

World War I (WWI) battlefields contain a large diversity of buried features that are situated just below the plough layer. A noninvasive interdisciplinary approach is preferred to characterize the buried heritage embedded within its soil landscape. In this study, the potential of integrating both electrical conductivity and magnetic susceptibility measurements from a multireceiver electromagnetic induction (EMI) instrument at a heavily shelled WWI battlefield was evaluated. Primarily, both measurements allowed the attribution of approximate depth interval to the buried trench remains. These remains proved dominantly present within the 0.4–0.8 m depth interval. Subsequently, both apparent electrical conductivity and magnetic susceptibility measurements were separately integrated to improve the contrast between buried features and their surrounding background. Trenches that were ditches filled with topsoil material could be distinguished from trenches that were continuously or partially filled with metal reinforcements or debris. To conclude, integrating multiple signals from a multireceiver EMI system allowed for a more straightforward discrimination of the buried WWI relicts.     

Corrections to: Interpreting lacustrine bulk sediment δ15N values using metagenomics in a tropical hypersaline lake system

Journal of Paleolimnology -     

Dynamics of giant mass transport in deep submarine environments: the Matakaoa Debris Flow,New Zealand

The Matakaoa Debris Flow (MDF) is a 200‐km‐long mass‐transport deposit resulting from the failure of the Matakaoa continental margin, northeast New Zealand, ca. 38–100 ky ago. In this study, high‐quality bathymetric and seismic reflection data are used to identify the morpho‐structural characters that reflect the kinematics of the MDF, as well as its interactions with basin sediments. We demonstrate how the transport energy, together with the local topography led to the present geometry and complex structure of the MDF deposits. The remarkable transport energy of the MDF is demonstrated by its dynamic impact on adjacent sedimentary series, including erosion of the substratum, shearing and compressional deformation. In the proximal zone of transport, momentous substratum erosion, demonstrated by giant tool marks and truncated sediments at the base of the debrite, triggered the excavation of a large volume (>200 km³) of basin sediments. The size of transported blocks (up to 3‐km long) is used to estimate the matrix yield strength in an early stage of transport. In the distal zone of transport, 100 km north of the source, seismic profiles show the propagation of thrust structures from the MDF into adjacent basin sediments. This study highlights that the remarkable volume of 2000 km³ of deposits partly resulted from the propagation of compressive structures within the basin sedimentary series to the front of the debrite.     

Changes in atmospheric circulation and the Arctic Oscillation preserved within a millennial length reconstruction of summer cloud cover from northern Fennoscandia

Cloud cover currently represents the single greatest source of uncertainty in General Circulation Models. Stable carbon isotope ratios (δ¹³C) from tree-rings, in areas of low moisture stress, are likely to be primarily controlled by photosynthetically active radiation (PAR), and therefore should provide a proxy record for cloud cover or sunshine; indeed this association has previously been demonstrated experimentally for Scots pine in Fennoscandia, with sunlight explaining ca 90% of the variance in photosynthesis and temperature only ca 4%. We present a statistically verifiable 1011-year reconstruction of cloud cover from a well replicated, annually-resolved δ¹³C record from Forfjord in coastal northwestern Norway. This reconstruction exhibits considerable variability in cloud cover over the past millennium, including extended sunny periods during the cool seventeenth and eighteenth centuries and warm cloudy periods during the eleventh, early fifteenth and twentieth centuries. We find that while a generally positive relationship persists between sunshine and temperature at high-frequency, at lower (multi-decadal) frequencies the relationship is more often a negative one, with cool periods being sunny (most notably the Little Ice Age period from 1600 to 1750 CE) and warm periods more cloudy (e.g. the mediaeval and the twentieth century). We conclude that these long-term changes may be caused by changes in the dominant circulation mode, likely to be associated with the Arctic Oscillation. There is also strong circumstantial evidence that prolonged periods of high summer cloud cover, with low PAR and probably high precipitation, may be in part responsible for major European famines caused by crop failures.