The ‘small-world’ nature of fracture/conduit networks: Possible implications for disequilibrium transport of magmas beneath mid-ocean ridges

In this contribution we show that natural fracture/conduit networks can be studied by using a new method based on Graph Theory. A number of natural networks at different length scales (from the meter to the millimeter) are analysed and results show that they have typical attributes of ‘small-world’ networks, a special class of networks characterized by high global and local transport efficiency. To our knowledge, this topological feature of natural fracture networks is recognized here for the first time. By starting from results on natural fracture/conduit networks, the possible implications are discussed by focusing on disequilibrium transport of magmas in the upper mantle beneath mid-ocean ridges. Results indicate that the ‘small-world’ topology of natural fracture/conduit networks is an important characteristic to ensure disequilibrium delivery of melts through the upper mantle, thus offering a good explanation of geochemical features of magmas. The remarkable point here is that the modelling of melt migration has been constrained by using real fracture network systems. The results presented in this work may contribute to a better understanding of melt migration in fracture network systems and of the way geochemical features of magmas may be influenced by their transport history.     

Connectivity and single/dual domain transport models: tests on a point-bar/channel aquifer analogue

In porous aquifers, groundwater flow and solute transport strongly depend on the sedimentary facies distribution at fine scale, which determines the heterogeneity of the conductivity field; in particular, connected permeable sediments could form preferential flow paths. Therefore, properly defined statistics, e.g. total and intrinsic facies connectivity, should be correlated with transport features. In order to improve the assessment of the relevance of this relationship, some tests are conducted on two ensembles of equiprobable realizations, obtained with two different geostatistical simulation methods—sequential indicator simulation and multiple point simulation (MPS)—from the same dataset, which refers to an aquifer analogue of sediments deposited in a fluvial point-bar/channel association. The ensembles show different features; simulations with MPS are more structured and characterised by preferential flow paths. This is confirmed by the analysis of transport connectivities and by the interpretation of data from numerical experiments of conservative solute transport with single and dual domain models. The use of two ensembles permits (1) previous results obtained for single realizations to be consolidated on a more firm statistical basis and (2) the application of principal component analysis to assess which quantities are statistically the most relevant for the relationship between connectivity indicators and flow and transport properties.     

Numerical modelling of centrifuge dynamic tests of circular tunnels in dry sand

This paper describes the numerical simulation of two dynamic centrifuge tests on reduced scale models of shallow tunnels in dry sand, carried out using both an advanced bounding surface plasticity constitutive soil model and a simple Mohr–Coulomb elastic-perfectly plastic model with embedded nonlinear and hysteretic behaviour. The predictive capabilities of the two constitutive models are assessed by comparing numerical predictions and experimental data in terms of accelerations at several positions in the model, and bending moment and hoop forces in the lining. Computed and recorded accelerations match well, and a quite good agreement is achieved also in terms of dynamic bending moments in the lining, while numerical and experimental values of the hoop force differ significantly with one another. The influence of the contact assumption between the tunnel and the soil is investigated by comparing the experimental data and the numerical results obtained with different interface conditions with the analytical solutions. The overall performance of the two models is very similar indicating that at least for dry sand, where shear-volumetric coupling is less relevant, even a simple model can provide an adequate representation of soil behaviour under dynamic conditions.     

The MED-SUV virtual research environment for enabling the GEO Geohazard supersites in Italy

This paper presents the Virtual Research Environment (VRE) enabling two European GEO Geohazard Supersites in Italy. According to GEO (Group on Earth Observation) vision, Geohazard Supersites provide access to spaceborne and in-situ geophysical data and models for selected sites prone to natural hazards –noticeably, earthquakes and volcano eruptions. The VRE was implemented in the framework of the Mediterranean Supersite Volcanoes (MED-SUV) project, funded by the European Commission. MED-SUV realized one of the European supersite demonstrators covering the two Permanent Supersites selected in Italy: Mt. Etna and Campi Flegrei/Vesuvius. The MED-SUV VRE provides advanced services for heterogeneous data and information management and sharing. MED-SUV started identifying the main supersite requirements including: the interoperability with existing data/information supply systems, the support of policy-based access control, the access to processing capabilities provided by external platforms, the management resources for publishing and sharing new products, the integration with significant global systems such as GEOSS and EPOS. MED-SUV adopted a System of Systems (SoS) approach to address interoperability with the identified heterogeneous systems supplying data and information. The SoS approach is based on a brokering architecture, where a specialized component (i.e the MED-SUV Broker: MSB) connects the existing and next-coming data sources leaving them autonomous. MSB carries out all the necessary mediation and harmonization tasks exposing standard interfaces enabling the interconnection with external systems like GEOSS and EPOS. In addition, MSB is accessible via a JavaScript library implementing Web APIs to facilitate the development of Web and mobile applications.     

Debris flow hazard assessment by means of numerical simulations: implications for the Rotolon creek valley (Northern Italy)

On 4^th November 2010, a debris flow detached from a large debris cover accumulated above the lowermost portion of the Rotolon landslide (Vicentine Pre-Alps, NE Italy) and channelized in the valley below within the Rotolon Creek riverbed. Such event evolved into a highly mobile and sudden debris flow, damaging some hydraulic works and putting at high risk four villages located along the creek banks. A monitoring campaign was carried out by means of a ground based radar interferometer (GB-InSAR) to evaluate any residual displacement risk in the affected area and in the undisturbed neighbouring materials. Moreover, starting from the current slope condition, a landslide runout numerical modelling was performed by means of DAN-3D code to assess the impacted areas, flow velocity, and deposit distribution of the simulated events. The rheological parameters necessary for an accurate modelling were obtained through the back analysis of the 2010 debris flow event. Back analysis was calibrated with all of the available terrain data coming from field surveys and ancillary documents, such as topographic, geomorphological and geological maps, with pre- and post-event LiDAR derived DTMs, and with orthophotos. Finally, to identify new possible future debris flow source areas as input data for the new modelling, all the obtained terrain data were reanalysed and integrated with the GB-InSAR displacement maps; consequently, new simulations were made to forecast future events. The results show that the integration of the selected modelling technique with ancillary data and radar displacement maps can be a very useful tool for managing problems related to debris flow events in the examined area.     

Brick-like texture and radial rays in Triassic pisolites of Lombardy,Italy: A clue to distinguish ancient aragonitic pisolites

Triassic pisolites from the Calcare Rosso, Lombardy, Italy, were formed in a hypersaline vadose environment and now show alternating dolomite and calcite laminae. The calcite consists either of microsparite laminae with a brick-like fabric, or of a mass of mosaic crystals with the external form of square-ended rays.These features suggest that the original laminae and rays were aragonitic, like those of the Holocene supratidal pisolites of the Persian Gulf, which consist of alternating laminae of unoriented nannomicrite with Mg-rich mucilaginous material, and aragonitic fibers with radial orientation separated by mucilaginous films. It is suggested that the transformation to brick-like and ray textures passed through the following diagenetic path: (1) original formation of fibrous aragonite laminae; (2) local aggrading recrystallization of aragonite fibers to large square-ended rays during hypersaline phases; (3) dolomitization of Mg-rich mucilaginous nannomicrite laminae during hyposaline phases; (4) inversion of the aragonite fibers and rays to calcite on a piece-by piece basis that preserved the original textural details, when the pH or Mg/Ca ratio dropped.The brick-like and ray fabrics have not been found in laminae of continental freshwater pisolites because these were deposited as equant and stable crystals of low-Mg calcite. These textures consequently make it possible to establish the chemistry of the depositional and early diagenetic milieu for some ancient pisolitic rocks.