Structure of a hypovolcanic acid complex inferred from magnetic susceptibility anisotropy measurements: the Western Red Hills granites (Skye,Scotland, Thulean Igneous Province)

 The geochemistry and the injection mechanism of hypovolcanic ring dykes have been extensively studied, but such is not the case for their internal fabric. The Tertiary Western Red Hills epigranites of the Isle of Skye are a classic example of such intrusions. Using anisotropy of magnetic susceptibility measurements, we present the first structural data of their internal magmatic fabric. The magnetic foliations, equated with the magmatic flow planes, have strikes which roughly follow the walls of the different intrusions. They dip steeply toward the convex wall of each intrusion. The lineations, or maximal magnetic susceptibility axes, generally have shallow plunges, except in the latest granite intrusion. These structures appear to be related to the compressional deformation of each intrusion toward the end of its crystallization. This shortening would be a consequence of a radial and compressive stress field acting after each injection of magma. This radial stress field is interpreted as the effect of high magma pressures originating from the acid magma chamber underlying the ring-dyke complex at a shallow depth. Received: 10 October 1995 / Accepted: 4 June 1997     

Exploring the hydrological effects of normal faults at the boundary of the Roer Valley Graben in Belgium using a catchment-scale groundwater flow model

Hydrogeology Journal - Faults may impact regional groundwater flow and transport, so it is important to include them during aquifer-system conceptualization and while constructing groundwater flow...     

Climatic oscillations in central Italy during the Last Glacial–Holocene transition: the record from Lake Accesa

This paper presents an event stratigraphy based on data documenting the history of vegetation cover, lake‐level changes and fire frequency, as well as volcanic eruptions, over the Last Glacial–early Holocene transition from a terrestrial sediment sequence recovered at Lake Accesa in Tuscany (north‐central Italy). On the basis of an age–depth model inferred from 13 radiocarbon dates and six tephra horizons, the Oldest Dryas–Bølling warming event was dated to ca. 14 560 cal. yr BP and the Younger Dryas event to ca. 12 700–11 650 cal. yr BP. Four sub‐millennial scale cooling phases were recognised from pollen data at ca. 14 300–14 200, 13 900–13 700, 13 400–13 100 and 11 350–11 150 cal. yr BP. The last three may be Mediterranean equivalents to the Older Dryas (GI‐1d), Intra‐Allerød (GI‐1b) and Preboreal Oscillation (PBO) cooling events defined from the GRIP ice‐core and indicate strong climatic linkages between the North Atlantic and Mediterranean areas during the last Termination. The first may correspond to Intra‐Bølling cold oscillations registered by various palaeoclimatic records in the North Atlantic region. The lake‐level record shows that the sub‐millennial scale climatic oscillations which punctuated the last deglaciation were associated in central Italy with different successive patterns of hydrological changes from the Bølling warming to the 8.2 ka cold reversal. Copyright © 2006 John Wiley & Sons, Ltd.     

The influence of lateral mixing on a phytoplankton bloom: Distribution in the Kerguelen Plateau region

A unique phytoplankton bloom appears every year during the austral spring/summer in the northern Kerguelen Plateau region. The Kerguelen Ocean and Plateau compared Study (KEOPS) showed that an increase in subsurface iron coming up from the seafloor through vertical mixing was responsible for the observed increase in chlorophyll-a above the plateau. We demonstrate that the bloom pattern is not a simple increase of biomass over shallow water: it is strongly influenced by the bathymetry and its spatial extent controlled by strong currents around the plateau. Here we focus on lateral mixing process to explain the particular shape of the bloom. We use the Smagorinsky [1963. General circulation experiments with the primitive equations. I. The basic experiment. Monthly Weather Review 91 (3), 99–164] formula to estimate and map fields of lateral mixing time scales (τ) due to barotropic tidal currents, barotropic atmospheric forced currents, Ekman velocities and geostrophic velocities. Results show that short time scale mixing is strongly influenced by the tides while the other processes have minor influences. Comparisons of τ and satellite chlorophyll-a images show that the spatial pattern of the bloom seems to be delimited by a barrier of high lateral mixing that is essentially due to tides. This emphasises the role played by the tides over the Kerguelen Plateau in supplying iron to the phytoplankton and containing the horizontal shape of the bloom. This is one of the first times such a link has been demonstrated, which has implications for the study of iron advection in the ocean.     

A combined neural network/gradient‐based approach for the identification of constitutive model parameters using self‐boring pressuremeter tests

This paper presents a numerical procedure of material parameter identification for the coupled hydro‐mechanical boundary value problem (BVP) of the self‐boring pressuremeter test (SBPT) in clay. First, the neural network (NN) technique is applied to obtain an initial estimate of model parameters, taking into account the possible drainage conditions during the expansion test. This technique is used to avoid potential pitfalls related to the conventional gradient‐based optimization techniques, considered here as a corrector that improves predicted parameters. Parameter identification based on measurements obtained through the pressuremeter expansion test and two types of holding tests is illustrated on the Modified Cam clay model. NNs are trained using a set of test samples, which are generated by means of finite element simulations of SBPT. The measurements obtained through expansion and consolidation tests are normalized so that NN predictors operate independently of the testing depth. Examples of parameter determination are demonstrated on both numerical and field data. The efficiency of the combined parameter identification in terms of accuracy, effectiveness and computational effort is also discussed. Copyright © 2008 John Wiley & Sons, Ltd.     

Effect of the water/cement ratio on concrete behavior under extreme loading

This study focuses on identifying concrete behavior under severe triaxial loadings (near field detonation or ballistic impacts). In order to reproduce high stress levels with well‐controlled loading paths, static tests have been carried out on concrete samples by mean of a very high‐capacity triaxial press (stress levels on the order of 1 GPa). It is a longstanding fact that the water/cement ratio (W/C), upon entering the concrete composition, is a major parameter affecting the porosity and strength of the cement matrix of hardened concrete. The objective of this article is to quantify the effect of this ratio on concrete behavior under conditions of high confinement. From the composition of a reference ‘ordinary’ concrete (i.e. W/C=0.6), two other concretes have been produced with W/C ratios equal to 0.4 and 0.8, respectively. This article presents experimental results and their analysis regarding the effect of water/cement ratio (W/C) on concrete behavior under high confinement. It shows that when placed under high confinement, concrete behaves like a granular stacking composed of concrete without any influence from the level of cement matrix strength. Copyright © 2009 John Wiley & Sons, Ltd.     

Comparison of DC sensitivity patterns for anisotropic and isotropic media

Many rocks possess electrical properties with a clearly expressed anisotropy. The anisotropic character of the rocks is often overlooked in forming the Fréchet derivatives or sensitivity functions for parameter updating during the inversion of DC resistivity data. In this study we have compared the sensitivity patterns for an isotropic, homogeneous model with that for a transversely isotropic (i.e. anisotropic) model having a tilted axis of symmetry using a pole–pole array. The sensitivity functions are expressed in terms of the derivatives of the electric potential U with respect to the average conductivity σ_m (geometric mean of the longitudinal and transverse conductivities) and the coefficient of anisotropy λ. Results are plotted in both cross-section form and plan view for various dip and strike angles of the axis of symmetry. The derivative dU/dλ decreases more rapidly than the isotropic value dU/dσ, and shows pronounced asymmetry and weakening of magnitude with increasing dip of the plane of symmetry. The derivative dU/dσ_m also exhibits the asymmetric pattern (except for vertical and horizontal dip cases). The positive region between the electrodes only extends to a small depth compared to the isotropic derivative, even in the case of a vertical axis of symmetry (VTI medium). The ratio of this anisotropic derivative to the isotropic derivative, when plotted as a function of position and depth shows prominent differences in both the sign and the magnitude of the sensitivities, especially for steep dips and for strongly anisotropic rocks. The plot highlights the dangers of an isotropic assumption. Even for mildly anisotropic rocks (λ < 1.2) the possibility for error in interpretation is considerable. Combined borehole and surface measurements are needed to diagnose anisotropy. Further work is needed to design optimal electrode configurations in anisotropic situations.