Influence of fluctuating supply on the emplacement dynamics of channelized lava flows

The evolution of lava flows emplaced on Mount Etna (Italy) in September 2004 is examined in detail through the analysis of morphometric measurements of flow units. The growth of the main channelized flow is consistent with a layering of lava blankets, which maintains the initial geometry of the channel (although levees are widened and raised), and is here explicitly related to the repeated overflow of lava pulses. A simple analytical model is introduced describing the evolution of the flow level in a channelized flow unit fed by a fluctuating supply. The model, named FLOWPULSE, shows that a fluctuation in the velocity of lava extrusion at the vent triggers the formation of pulses, which become increasingly high the farther they are from the vent, and are invariably destined to overflow within a given distance. The FLOWPULSE simulations are in accordance with the observed morphology, characterized by a very flat initial profile followed by a massive increase in flow unit cross-section area between 600 and 700 m downflow. The modeled emplacement dynamics provides also an explanation for the observed substantial “loss” of the original flowing mass with increasing distance from the vent.     

Reconstructing eruptive source parameters from tephra deposit: a numerical study of medium-sized explosive eruptions at Etna volcano

Since the 1970s, multiple reconstruction techniques have been proposed and are currently used, to extrapolate and quantify eruptive parameters from sampled tephra fall deposit datasets. Atmospheric transport and deposition processes strongly control the spatial distribution of tephra deposit; therefore, a large uncertainty affects mass derived estimations especially for fall layer that are not well exposed. This paper has two main aims: the first is to analyse the sensitivity to the deposit sampling strategy of reconstruction techniques. The second is to assess whether there are differences between the modelled values for emitted mass and grainsize, versus values estimated from the deposits. We find significant differences and propose a new correction strategy. A numerical approach is demonstrated by simulating with a dispersal code a mild explosive event occurring at Mt. Etna on 24 November 2006. Eruptive parameters are reconstructed by an inversion information collected after the eruption. A full synthetic deposit is created by integrating the deposited mass computed by the model over the computational domain (i.e., an area of 7.5 × 10⁴ km ²). A statistical analysis based on 2000 sampling tests of 50 sampling points shows a large variability, up to 50 % for all the reconstruction techniques. Moreover, for some test examples Power Law errors are larger than estimated uncertainty. A similar analysis, on simulated grain-size classes, shows how spatial sampling limitations strongly reduce the utility of available information on the total grain size distribution. For example, information on particles coarser than ?(?4) is completely lost when sampling at 1.5 km from the vent for all columns with heights less than 2000 m above the vent. To correct for this effect an optimal sampling strategy and a new reconstruction method are presented. A sensitivity study shows that our method can be extended to a wide range of eruptive scenarios including those in which aggregation processes are important. The new correction method allows an estimate of the deficiency for each simulated class in calculated mass deposited, providing reliable estimation of uncertainties in the reconstructed total (whole deposit) grainsize distribution.     

Comparing the performances of four stochastic optimisation methods using analytic objective functions, 1D elastic full‐waveform inversion,and residual static computation

We compare the performances of four stochastic optimisation methods using four analytic objective functions and two highly non‐linear geophysical optimisation problems: one‐dimensional elastic full‐waveform inversion and residual static computation. The four methods we consider, namely, adaptive simulated annealing, genetic algorithm, neighbourhood algorithm, and particle swarm optimisation, are frequently employed for solving geophysical inverse problems. Because geophysical optimisations typically involve many unknown model parameters, we are particularly interested in comparing the performances of these stochastic methods as the number of unknown parameters increases. The four analytic functions we choose simulate common types of objective functions encountered in solving geophysical optimisations: a convex function, two multi‐minima functions that differ in the distribution of minima, and a nearly flat function. Similar to the analytic tests, the two seismic optimisation problems we analyse are characterised by very different objective functions. The first problem is a one‐dimensional elastic full‐waveform inversion, which is strongly ill‐conditioned and exhibits a nearly flat objective function, with a valley of minima extended along the density direction. The second problem is the residual static computation, which is characterised by a multi‐minima objective function produced by the so‐called cycle‐skipping phenomenon. According to the tests on the analytic functions and on the seismic data, genetic algorithm generally displays the best scaling with the number of parameters. It encounters problems only in the case of irregular distribution of minima, that is, when the global minimum is at the border of the search space and a number of important local minima are distant from the global minimum. The adaptive simulated annealing method is often the best‐performing method for low‐dimensional model spaces, but its performance worsens as the number of unknowns increases. The particle swarm optimisation is effective in finding the global minimum in the case of low‐dimensional model spaces with few local minima or in the case of a narrow flat valley. Finally, the neighbourhood algorithm method is competitive with the other methods only for low‐dimensional model spaces; its performance sensibly worsens in the case of multi‐minima objective functions.     

Evidence of a shallow magma intrusion beneath the NE Rift system of Mt. Etna during 2013

Continuous GPS (CGPS) data, collected at Mt. Etna between April 2012 and October 2013, clearly define inflation/deflation processes typically observed before/after an eruption onset. During the inflationary process from May to October 2013, a particular deformation pattern localised in the upper North Eastern sector of the volcano suggests that a magma intrusion had occurred a few km away from the axis of the summit craters, beneath the NE Rift system. This is the first time that this pattern has been recorded by CGPS data at Mt. Etna. We believe that this inflation process might have taken place periodically at Mt. Etna and might be associated with the intrusion of batches of magma that are separate from the main feeding system. We provide a model to explain this unusual behaviour and the eruptive regime of this rift zone, which is characterised by long periods of quiescence followed by often dangerous eruptions in which vents can open at low elevation and thus threaten the villages in this sector of the volcano.     

Integration of ground and satellite data to model Mediterranean forest processes

The current work presents the testing of a modeling strategy that has been recently developed to simulate the gross and net carbon fluxes of Mediterranean forest ecosystems. The strategy is based on the use of a NDVI-driven parametric model, C-Fix, and of a biogeochemical model, BIOME-BGC, whose outputs are combined to simulate the behavior of forest ecosystems at different development stages. The performances of the modeling strategy are evaluated in three Italian study sites (San Rossore, Lecceto and Pianosa), where carbon fluxes are being measured through the eddy correlation technique. These sites are characterized by variable Mediterranean climates and are covered by different types of forest vegetation (pine wood, Holm oak forest and Macchia, respectively). The results of the tests indicate that the modeling strategy is generally capable of reproducing monthly GPP and NEE patterns in all three study sites. The highest accuracy is obtained in the most mature, homogenous pine wood of San Rossore, while the worst results are found in the Lecceto forest, where there are the most heterogeneous terrain, soil and vegetation conditions. The main error sources are identified in the inaccurate definition of the model inputs, particularly those regulating the site water budgets, which exert a strong control on forest productivity during the Mediterranean summer dry season. In general, the incorporation of NDVI-derived fAPAR estimates corrects for most of these errors and renders the forest flux simulations more stable and accurate.     

A feasibility study of borehole radar as a permanent downhole sensor

Permanent downhole sensors provide the eyes and ears to the reservoir and enable monitoring the reservoir conditions on a real‐time basis. In particular, the use of sensors and remotely controlled valves in wells and on the surface, in combination with reservoir flow models provide enormous benefits to reservoir management and oil production. We suggest borehole radar measurements as a promising technique capable to monitor the arrival of undesired fluids in the proximity of production wells. We use 1D modelling to investigate the expected signal magnitude and depth of investigation of a borehole radar sensor operating in an oilfield environment. We restrict the radar applicability to environments where the radar investigation depth can fit the reservoir size necessary to be monitored. Potential applications are steam chamber monitoring in steam assisted gravity drainage processes and water front monitoring in thin oil rim environments. A more sophisticated analysis of the limits of a radar system is carried out through 2D finite‐difference time‐domain simulations. The metal components of the wellbore casing can cause destructive interference with the emitted signal. A high dielectric medium surrounding the production well increases the amplitude of the signal and so the radar performance. Other reservoir constraints are given by the complexity of the reservoir and the dynamic of the fluids. Time‐lapse changes in the heterogeneity of the background formation strongly affect the retrieval of the target reflections and gradual fluid saturation changes reduce the amplitudes of the reflections.     

The SISMA prototype system: integrating Geophysical Modeling and Earth Observation for time-dependent seismic hazard assessment

An innovative approach to seismic hazard assessment is illustrated that, based on the available knowledge of the physical properties of the Earth structure and of seismic sources, on geodetic observations, as well as on the geophysical forward modeling, allows for a time-dependent definition of the seismic input. According to the proposed approach, a fully formalized system integrating Earth Observation data and new advanced methods in seismological and geophysical data analysis is currently under development in the framework of the Pilot Project SISMA, funded by the Italian Space Agency. The synergic use of geodetic Earth Observation data (EO) and Geophysical Forward Modeling deformation maps at the national scale complements the space- and time-dependent information provided by real-time monitoring of seismic flow (performed by means of the earthquake prediction algorithms CN and M8S) and permits the identification and routine updating of alerted areas. At the local spatial scale (tens of km) of the seismogenic nodes identified by pattern-recognition analysis, both GNSS (Global Navigation Satellite System) and SAR (Synthetic Aperture Radar) techniques, coupled with expressly developed models for interseismic phase, allow us to retrieve the deformation style and stress evolution within the seismogenic areas. The displacement fields obtained from EO data provide the input for the geophysical modeling, which eventually permits to indicate whether a specific fault is in a “critical state.” The scenarios of expected ground motion (shakemaps) associated with the alerted areas are then defined by means of full waveforms modeling, based on the possibility to compute synthetic seismograms by the modal summation technique (neo-deterministic hazard assessment). In this way, a set of deterministic scenarios of ground motion, which refer to the time interval when a strong event is likely to occur within the alerted area, can be defined both at national and at local scale. The considered integrated approach opens new routes in understanding the dynamics of fault zones as well as in modeling the expected ground motion. The SISMA system, in fact, provides tools for establishing warning criteria based on deterministic and rigorous forward geophysical models and hence allows for a well-controlled real-time prospective testing and validation of the proposed methodology over the Italian territory. The proposed approach complements the traditional probabilistic approach for seismic hazard estimates, since it supplies routinely updated information useful in assigning priorities for timely mitigation actions and hence it is particularly relevant to Civil Defense purposes.     

Database of geo-hydrological disasters for civil protection purposes

This paper presents the results of a research concerning available historical information about natural hazards (landslides and floods) and consequent disasters in the Consortium of Mountain Municipalities of Valtellina di Tirano, in Northern Italy. A geo-referenced database, collecting information till 2008, was designed with the aim of using available data of historical events for hazard estimation and the definition of risk scenarios as a basis for Civil Protection planning and emergency management purposes. This database and related statistics about landslides and floods are shown, and a brief overview of historical disasters caused by natural hazards in the study area is presented. A case study showing how useful the database can be to define a simple but realistic scenario is described. Information availability and reliability is discussed and possible uncertainties are underlined. The study shows that collecting and making use of historical information for the definition of hypothetical scenarios and the evaluation of territorial threats is a fundamental source of knowledge to deal with future emergencies.     

Aquatic macrophyte diversity assessment: Validation of a new sampling method for circular-shaped lakes

We propose and validate a new sampling method to assess the presence, abundance and distribution of macrophytes in circular-shaped lakes according to the requirements of the Water Framework Directive (WFD2000/60/EC). The results of the macrophyte survey, and in particular of macrophyte diversity, obtained using this method are also discussed.The sampling is based on randomly selected transects homogeneously distributed around the perimeter of the lake. The number of transects is proportional to the lake's size. The method was validated on six Italian volcanic lakes using computational resampling procedures on a total of 126 transects.Using resampling procedures, we show that the proposed approach identifies more than 75% of the overall species richness through a moderate sampling effort. According to our results, Charophytes dominate aquatic vegetation in Italian volcanic lakes. Species diversity is highest at shallow depths, whereas the most abundant species, such as Chara polyacantha, are located at an intermediate depth between the shoreline and the maximum growing depth.     

Two‐dimensional nonlinear diffusive numerical simulation of geomorphic modifications to cinder cones

The temporal evolution of simple landforms such as cinder cones by nonlinear diffusive processes is studied through the use of a new 2D numerical model using well‐established and accurate numerical mathematics and high‐resolution digital elevation models (DEMs). Extending 1D (profile) nonlinear diffusion analyses used in cinder cone, hillslope and fault scarp evolution studies, we have implemented a 2D numerical model with a spatially and temporally varying sediment transport rate coefficient scaled nonlinearly by the ratio of local slope to critical slope. The high accuracy and efficient numerical implementation are documented in the paper and the MATLAB toolkit developed is used to solve for the developmentof an initial 2D cone form. First, we examine the nonlinear transport rule and suggest a refinement that accounts explicitly for flux at threshold slopes. We find that the maximum diffusion (necessarily introduced in the numerical model to avoid infinite rates) at the critical slope controls the final morphology, especially approaching steady state. Secondly, solving the landscape evolution problem in 2D enables a natural accounting for sediment flux convergence or divergence in the profile. Thirdly, the boundary behavior of a given landscape element controls much of what happens in that domain and so we allow for arbitrary flux magnitude or elevation boundary conditions. Fourthly, landscapes are heterogeneous in their surface cover and so we allow for spatially and temporally varying transport rate k and we permit an arbitrary vertical displacement field within the model domain. To test the new formulation for the nonlinear term, the effect of variable diffusivity k and the numerical schemes implemented, we apply the model to cinder cones built on the flanks of Mount Etna in 2001 and 2002–2003. We explore the effects of DEM resolution with data from the 2001 cone and the utility of spatially variable diffusivity to explain the variation in erosion measured by differencing repeat light detection and ranging (LIDAR) surveys gathered in 2004 and 2007 over the 2002–2003 cone complex. Copyright © 2013 John Wiley & Sons, Ltd.