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Journal articleHill J, Popova EE, Ham DA, et al., 2014,
Adapting to life: ocean biogeochemical modelling and adaptive remeshing
, Ocean Science, Vol: 10, Pages: 323-343An outstanding problem in biogeochemical modelling of the ocean is that many of the key processes occur intermittently at small scales, such as the sub-mesoscale, that are not well represented in global ocean models. This is partly due to their failure to resolve sub-mesoscale phenomena, which play a significant role in vertical nutrient supply. Simply increasing the resolution of the models may be an inefficient computational solution to this problem. An approach based on recent advances in adaptive mesh computational techniques may offer an alternative. Here the first steps in such an approach are described, using the example of a simple vertical column (quasi-1-D) ocean biogeochemical model. We present a novel method of simulating ocean biogeochemical behaviour on a vertically adaptive computational mesh, where the mesh changes in response to the biogeochemical and physical state of the system throughout the simulation. We show that the model reproduces the general physical and biological behaviour at three ocean stations (India, Papa and Bermuda) as compared to a high-resolution fixed mesh simulation and to observations. The use of an adaptive mesh does not increase the computational error, but reduces the number of mesh elements by a factor of 2–3. Unlike previous work the adaptivity metric used is flexible and we show that capturing the physical behaviour of the model is paramount to achieving a reasonable solution. Adding biological quantities to the adaptivity metric further refines the solution. We then show the potential of this method in two case studies where we change the adaptivity metric used to determine the varying mesh sizes in order to capture the dynamics of chlorophyll at Bermuda and sinking detritus at Papa. We therefore demonstrate that adaptive meshes may provide a suitable numerical technique for simulating seasonal or transient biogeochemical behaviour at high vertical resolution whilst minimising the number of elements in the mesh. M
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Journal articleMagee C, McDermott K, Stevenson CT, et al., 2014,
Influence of crystallised igneous intrusions on fault nucleation and reactivation during continental extension
, Journal of Structural Geology, Vol: 62, Pages: 183-193Continental rifting is commonly accommodated by the nucleation of normal faults, slip on pre-existing fault surfaces and/or magmatic intrusion. Because crystallised igneous intrusions are pervasive in many rift basins and are commonly more competent (i.e. higher shear strengths and Young's moduli) than the host rock, it is theoretically plausible that they locally intersect and modify the mechanical properties of pre-existing normal faults. We illustrate the influence that crystallised igneous intrusions may have on fault reactivation using a conceptual model and observations from field and subsurface datasets. Our results show that igneous rocks may initially resist failure, and promote the preferential reactivation of favourably-oriented, pre-existing faults that are not spatially associated with solidified intrusions. Fault segments situated along strike from laterally restricted fault-intrusion intersections may similarly be reactivated. This spatial and temporal control on strain distribution may generate: (1) supra-intrusion folds in the hanging wall; (2) new dip-slip faults adjacent to the igneous body; or (3) sub-vertical, oblique-slip faults oriented parallel to the extension direction. Importantly, stress accumulation within igneous intrusions may eventually initiate failure and further localise strain. The results of our study have important implications for the structural of sedimentary basins and the subsurface migration of hydrocarbons and mineral-bearing fluids.
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Journal articleJackson CAL, Carruthers TD, Mahlo S, et al., 2014,
Can polygonal faults help locate deep-water reservoirs?
, AAPG Bulletin, Vol: 98, Pages: 1717-1738Polygonal faults are compaction-related normal faults that develop in very fine-grained sedimentary successions. Despite their ubiquity, few studies have highlighted the application of polygonal fault mapping to identifying deepwater sandstone reservoirs. We use 3D seismic and borehole data from the Måløy Slope, offshore Norway to demonstrate that the distribution, cross-sectional geometry and throw characteristics of polygonal faults can be used to locate deepwater sandstone reservoirs. Two tiers of polygonal fault are identified in the Cretaceous to lower Palaeogene succession. The lowermost tier is stratigraphically restricted to the lower Barremian-to-lowermost Turonian succession and likely formed during the early Turonian. The uppermost tier is well-developed, spans the entire Cretaceous succession and likely formed during the Maastrichtian. An abrupt decrease in the thickness of the upper tier occurs where a 92 m thick, sandstone-rich slope fan is developed in the upper Turonian interval. Furthermore, the lower tips of faults in the upper tier, which are defined by anomalously high throw gradients, cluster at the top of the sandstone, resulting in decoupling of this tier from the underlying, early Turonian tier. We interpret that faults in the upper tier nucleated above the reservoir across the entire slope and that the slope fan sandstone acted as a mechanical barrier to downward fault propagation, resulting in abrupt thinning of the tier at the sandstone pinchout. Polygonal faults are not simply an academic curiosity; mapping of these enigmatic structures can have practical applications for the delineation of a variety of reservoir types in hydrocarbon-bearing sedimentary basins worldwide.
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Journal articleSolano JMS, Jackson MD, Sparks RSJ, et al., 2014,
Evolution of major and trace element composition during melt migration through crystalline mush: Implications for chemical differentiation in the crust
, American Journal of Science, Vol: 314, Pages: 895-939, ISSN: 0002-9599We present the first quantitative model of heat, mass and both majorand trace element transport in a mush undergoing compaction that accounts forcomponent transport and chemical reaction during melt migration and which isapplicable to crustal systems. The model describes the phase behavior of binarysystems (both eutectic and solid solution), with melt and solid compositions determinedfrom phase diagrams using the local temperature and bulk composition. Traceelement concentration is also determined. The results demonstrate that componenttransport and chemical reaction generate compositional variation in both major andtrace elements that is not captured by existing geochemical models. In particular, wefind that, even for the simplest case of a homogenous, insulated column that isinstantaneously melted then allowed to compact, component transport and reactionleads to spatial variations in major element composition that, in this case, producesmelt that is more enriched in incompatible elements than predicted by batch melting.In deep crustal hot zones (DCHZ), created by the repeated intrusion of hot, mantlederivedmagmas, buoyant melt migrating upwards accumulates in high porosity layers,but has a composition corresponding to only a small fraction of batch melting, becauseit has locally equilibrated with mush at low temperature; moreover, melt migration andchemical reaction in a layered protolith may lead to the rapid formation of highporosity melt layers at the interface between different rock compositions. In both ofthese cases, the melt in the high porosity layer(s) is less enriched in incompatible traceelements than predicted if it is assumed that melt with the same major elementcomposition was produced by batch melting. This distinctive decoupling of major andtrace element fractionation may be characteristic of magmas that originate in DCHZ.Application of the model to a number of crustal systems, including the Ivrea-Verbanozone, the Rum layered intrusion, and the Hol
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Journal articleGarel F, Goes S, Davies DR, et al., 2014,
Interaction of subducted slabs with the mantle transition-zone: A regime diagram from 2-D thermo-mechanical models with a mobile trench and an overriding plate
, GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS, Vol: 15, Pages: 1739-1765- Author Web Link
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- Citations: 132
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Journal articleReusche M, Winsor K, Carlson AE, et al., 2014,
<sup>10</sup>Be surface exposure ages on the late-Pleistocene and Holocene history of Linnébreen on Svalbard
, Quaternary Science Reviews, Vol: 89, Pages: 5-12, ISSN: 0277-3791Arctic glaciers were sensitive to past changes in high-latitude winter precipitation and summer temperature. Here we develop a late-Pleistocene to Holocene history for Linnébreen (Linné Glacier) in western Svalbard using 10Be surface exposure ages on isolated erratic and moraine boulders. We show that Linnébreen had separated from the larger ice sheet over Svalbard and was retreating up valley around the start of the Younger Dryas cold period. We attribute this retreat during a cold period on Svalbard to moisture starvation of Linnébreen from advanced sea ice and/or elevated shortwave boreal summer insolation that overwhelmed any reduction in sensible heat. After an ice-free period during the early to middle Holocene, Linnébreen reformed sometime after 4.6±0.2ka, and was at a position roughly equivalent to its Little Ice Age (LIA) maximum extent before it began to retreat at 1.6±0.2ka. Comparison with calibrated 14C dates from three other glaciers could suggest that this period of ice retreat at ~1.6ka could be regional in extent. Linnébreen occupied the pre-LIA moraine when there was an increased ratio of cold Arctic-sourced relative to warm Atlantic-sourced waters around Svalbard and advanced sea ice. The retreat of Linnébreen at ~1.6ka was concurrent with the increased presence of warm Atlantic waters around Svalbard and attendant sea-ice retreat. These coincident changes in ocean temperatures, sea-ice extent, and Linnébreen moraine age could imply a climatic forcing of the pre-LIA advance and retreat of Linnébreen. Summer temperatures, rather than changes in precipitation, would then be dominant in driving ice retreat, although the possibility of stochastic glacier-margin variability cannot be excluded. Our data therefore suggest that Linnébreen may have responded differently to past changes in sea-ice extent that could depend on the background climate state (deglacial climate vs
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Journal articleStafford PJ, 2014,
Crossed and nested mixed‐effects approaches for enhanced model development and removal of the ergodic assumption in empirical ground‐motion models
, Bulletin of the Seismological Society of America, Vol: 104, Pages: 702-719, ISSN: 0037-1106Limitations in the size of strong‐motion databases that are used for the development of empirical ground‐motion models has necessitated the use of the ergodic assumption. Several recent efforts, using different databases from around the world, have been made to estimate the single‐station standard deviation of spectral accelerations. The computed estimates have been found to be very stable globally, despite the various researchers using quite different approaches. This paper demonstrates that the multistage procedures that have been adopted by previous researchers can be replaced by the use of more elaborate mixed‐effects regression analyses. Additionally, the traditional use of additive random effects to capture source, path, and site effects is shown to have conceptual shortcomings that are addressed through the use of a more complex treatment of mixed‐effects models. A model for the residual variance (within‐event single‐station variance) is presented using these advanced approaches.
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Journal articleDeveugle PEK, Jackson MD, Hampson GJ, et al., 2014,
A comparative study of reservoir modeling techniques and their impact on predicted performance of fluvial-dominated deltaic reservoirs
, AAPG BULLETIN, Vol: 98, Pages: 729-763, ISSN: 0149-1423- Author Web Link
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- Citations: 26
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Journal articleJardine RJ, 2014,
Advanced laboratory testing in research and practice: The 2nd Bishop Lecture
, Geotechnical Research, Vol: 1, Pages: 2-31This paper demonstrates the special capabilities and practical value of advanced laboratory testing, focusing on its application in advancing the understanding and prediction of how driven piles function and perform in sand. Emphasis is placed on integrating laboratory research with analysis and field observations, drawing principally on work by the author, his colleagues and research group. The laboratory studies include highly instrumented static and cyclic stress-path triaxial experiments, hollow cylinder and ring-shear interface tests and micro-mechanical research. Soil element testing is combined with model studies in large laboratory calibration chambers, full-scale field investigations and numerical simulations to help advance fundamental methods for predicting pile behaviour that have important implications and applications, particularly in offshore engineering.
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- Citations: 15
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Journal articleBray VJ, Collins GS, Morgan JV, et al., 2014,
Hydrocode simulation of Ganymede and Europa cratering trends - How thick is Europa's crust?
, ICARUS, Vol: 231, Pages: 394-406, ISSN: 0019-1035- Author Web Link
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- Citations: 42
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Journal articleYang ZX, Jardine RJ, Zhu BT, et al., 2014,
Stresses Developed around Displacement Piles Penetration in Sand
, JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING, Vol: 140, ISSN: 1090-0241- Author Web Link
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- Citations: 38
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Journal articleCamara A, Nguyen K, Ruiz-Teran AM, et al., 2014,
Serviceability limit state of vibrations in under-deck cable-stayed bridges accounting for vehicle-structure interaction
, Engineering Structures, Vol: 61, Pages: 61-72, ISSN: 0141-0296Verification of the serviceability limit state of vibrations due to traffic live loads can be neglected in conventional types of concrete road bridges but becomes critical in the design of slender structures like under-deck cable-stayed bridges. The novelty of the work presented in this article is that an innovative vehicle-bridge interaction model is employed, in which realistic wheel dimensions of heavy trucks, road roughness profiles and the cross slope of the road are considered in nonlinear dynamic analyses of detailed three-dimensional finite element models. An extensive parametric study is conducted to explore the influence of the bridge parameters such as the longitudinal and transverse cable arrangement and the support conditions, in addition to the load modelling, road quality, the wheel size, the transverse road slope and the vehicle position and speed on the response of under-deck cable-stayed bridges. It has been observed that the vibrations perceived by pedestrians can be effectively reduced by concentrating the cable-system below the deck at the bridge centreline. The Fourier amplitude spectrum of the acceleration at critical positions along the deck proved that the response of under-deck cable-stayed bridges is not dominated only by contributions at the fundamental mode and, consequently, the conventional deflection-based methods are not valid to assess the users comfort. Instead, Vehicle-Bridge Interaction analyses are recommended for detailed design, considering the wheel dimensions if the pavement quality is bad and/or if the wheel radius is large. Finally, we verify through multiple approaches that the comfort of pedestrian users is more critical than that of vehicle users. However, the comfort of vehicle users is shown to be significantly affected when the road quality is poor.
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Journal articleMichael NA, Carter A, Whittaker AC, et al., 2014,
Erosion rates in the source region of an ancient sediment routing system: comparison of depositional volumes with thermochronometric estimates
, Journal of the Geological Society, Vol: 171, Pages: 401-412 -
Journal articleFunke SW, Farrell PE, Piggott MD, 2014,
Tidal turbine array optimisation using the adjoint approach
, Renewable Energy, Vol: 63, Pages: 658-673, ISSN: 0960-1481Oceanic tides have the potential to yield a vast amount of renewable energy. Tidal stream generators are one of the key technologies for extracting and harnessing this potential. In order to extract an economically useful amount of power, hundreds of tidal turbines must typically be deployed in an array. This naturally leads to the question of how these turbines should be configured to extract the maximum possible power: the positioning and the individual tuning of the turbines could significantly influence the extracted power, and hence is of major economic interest. However, manual optimisation is difficult due to legal site constraints, nonlinear interactions of the turbine wakes, and the cubic dependence of the power on the flow speed. The novel contribution of this paper is the formulation of this problem as an optimisation problem constrained by a physical model, which is then solved using an efficient gradient-based optimisation algorithm. In each optimisation iteration, a two-dimensional finite element shallow water model predicts the flow and the performance of the current array configuration. The gradient of the power extracted with respect to the turbine positions and their tuning parameters is then computed in a fraction of the time taken for a flow solution by solving the associated adjoint equations. These equations propagate causality backwards through the computation, from the power extracted back to the turbine positions and the tuning parameters. This yields the gradient at a cost almost independent of the number of turbines, which is crucial for any practical application. The utility of the approach is demonstrated by optimising turbine arrays in four idealised scenarios and a more realistic case with up to 256 turbines in the Inner Sound of the Pentland Firth, Scotland.
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Journal articleJohnson JS, Bentley MJ, Smith JA, et al., 2014,
Rapid thinning of pine island glacier in the early holocene
, Science, Vol: 343, Pages: 999-1001, ISSN: 0036-8075Pine Island Glacier, a major outlet of the West Antarctic Ice Sheet, has been undergoing rapid thinning and retreat for the past two decades. We demonstrate, using glacial-geological and geochronological data, that Pine Island Glacier (PIG) also experienced rapid thinning during the early Holocene, around 8000 years ago. Cosmogenic (10)Be concentrations in glacially transported rocks show that this thinning was sustained for decades to centuries at an average rate of more than 100 centimeters per year, which is comparable with contemporary thinning rates. The most likely mechanism was a reduction in ice shelf buttressing. Our findings reveal that PIG has experienced rapid thinning at least once in the past and that, once set in motion, rapid ice sheet changes in this region can persist for centuries.
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Journal articleMichael NA, Whittaker AC, Carter A, et al., 2014,
Volumetric budget and grain-size fractionation of a geological sediment routing system: Eocene Escanilla Formation
, Geological Society of America Bulletin, Vol: 126, Pages: 585-599 -
Journal articleLe Voci G, Davies DR, Goes S, et al., 2014,
A systematic 2-D investigation into the mantle wedge's transient flow regime and thermal structure: Complexities arising from a hydrated rheology and thermal buoyancy
, GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS, Vol: 15, Pages: 28-51, ISSN: 1525-2027 -
Journal articleBarrott JJ, Dudeney AWL, Mason PJ, 2014,
Spatial and temporal relationships between Eocene sand horizons and iron contamination in stream water in the Thames Basin west of London, UK
, Geochemistry-Exploration Environment Analysis, Vol: 14, Pages: 33-44, ISSN: 1467-7873Analyses of stream water samples associated with Tertiary (Eocene) sands of the Bracklesham Formation 30–60 km west of London showed numerous occurrences of groundwater containing elevated iron(II) concentrations (1–15 mg/l Fe), which oxidise to red-brown ferrihydrite in localised wetlands and streams and join onward flows towards the River Thames. GIS mapping of iron concentrations in relation to topographical and geological data within a 20×30 km project area indicated some 168 km of iron-contaminated water.The results are consistent with downward permeation of groundwater through contained Camberley, Windlesham and Bagshot sand horizons and predominant W-E groundwater flow of up to 35 km in the Bagshot horizon, which rests on impermeable London Clay. Glauconite, comprising up to 70% of the sand near the base of the Windlesham horizon, apparently provides the main source of soluble iron in the near-anoxic, near-neutral, conditions normally prevailing. Pyrite is also a possible source, especially where quarrying or other excavation releases water from previously isolated volumes, simultaneously causing elevated sulphate and acidity levels via relatively rapid sulphide oxidation. Mechanisms proposed are underpinned through comparison with reports on analogous systems in the UK and abroad.Streams and lakes in the vicinity of seepages are highly visible and of decreased biodiversity. However, because of their natural origin, they are not subject to the same public concern and environmental control as compositionally similar near-neutral water from many former coal mines elsewhere - where iron derives primarily from pyrite. Also, while comparatively persistent, they appear destined for rapid depletion in geological terms: the unusual isolation of the sands until the Quaternary indicates a short leaching period and suggests a reason why iron-rich water from natural ferruginous sands is seldom of sufficient significanc
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Journal articleFitch PJR, Jackson MD, Hampson GJ, et al., 2014,
Interaction of stratigraphic and sedimentological heterogeneities with flow in carbonate ramp reservoirs: impact of fluid properties and production strategy
, Petroleum Geoscience, Vol: 20, Pages: 7-26, ISSN: 1354-0793It is well known that heterogeneities in carbonate reservoirs impact fluid flow during production. However, few studies have examined the impact of the same heterogeneities on flow behaviour with different fluid properties and production scenarios. We use integrated flow simulation and experimental design techniques to investigate the relative, first-order impact of stratigraphic and sedimentological heterogeneities on simulated recovery in carbonate ramp reservoirs. Two production strategies are compared, which promote dominance of either horizontal or vertical flow.We find that the modelled geology is more important than the simulated fluid properties and production scenarios over the ranges tested. Of the heterogeneities modelled here, rock properties and stratigraphic heterogeneities that control reservoir architecture and the spatial distribution of environment of deposition (EOD) belts are important controls on recovery regardless of the production strategy. The presence of cemented hardground surfaces becomes the key control on oil recovery in displacements dominated by vertical flow. Permeability anisotropy is of low importance for all production strategies. The impacts of stratigraphic heterogeneities on recovery factor and water breakthrough are more strongly influenced by fluid properties and well spacing in displacements dominated by vertical flow. These results help to streamline the reservoir modelling process, by identifying key heterogeneities, and to optimize production strategies.
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Journal articleDavis M, Matmon A, Placzek CJ, et al., 2014,
Cosmogenic nuclides in buried sediments from the hyperarid Atacama Desert, Chile
, Quaternary Geochronology, Vol: 19, Pages: 117-126, ISSN: 1871-1014The evolution of Terrestrial Cosmogenic Nuclides (TCN) from an alluvial section in the Atacama Desert is examined. We reconstruct a burial history for the last ~10Ma using 40Ar/39Ar dating of volcanic ash layers interbedded with alluvial sediments; this independent dating allows us to distinguish between the effects of erosion, post-burial subsurface production, and radioactive decay during burial on TCN concentrations. Our TCN results show significant post-burial production, which is the result of the extremely slow sedimentation rate (~3m/Ma) and the old age of the sediments. Although distinct differences in TCN concentrations are apparent between the lower and upper parts of the sedimentary section, we show that these differences are most likely related to post-burial production and age, and not to changes in bedrock erosion rates or changes in elevation due to tectonic activity. Our approach provides a test to the applicability of the two-isotope cosmogenic burial dating system (26Al-10Be) in regions of extremely slow sedimentation rates. Our results reveal geomorphic stability in terms of erosion and sedimentation rates for the late Miocene-Pliocene in the Atacama Desert. © 2013 Elsevier B.V.
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