Free-surface flow in porous media and periodic solution of the shallow-flow approximation

The equations describing the flow of liquid in a porous medium with a free surface are expanded when the shallow-flow assumption holds. Second-order theory is used to describe the propagation of steady periodic motion in the medium, driven by the oscillating level of a reservoir in contact with it. A linearized solution of the second-order theory is compared with a numerical solution and is found adequate even when the amplitude of the motion is comparable to the mean depth of the liquid. The predictions of the analysis are found in good agreement with two laboratory experiments.     

Impact of explosive eruption scenarios at Vesuvius

In the paper the first attempt at the definition of a model to assess the impact of a range of different volcanic hazards on the building structures is presented. This theoretical approach has been achieved within the activities of the EXPLORIS Project supported by the EU. A time history for Sub-Plinian I eruptive scenario of the Vesuvius is assumed by taking advantage of interpretation of historical reports of volcanic crises of the past [Carafa, G. 1632. In opusculum de novissima Vesuvij conflagratione, epistola isagogica, 2^a ed. Napoli, Naples; Mascolo, G.B., 1634. De incendio Vesuvii excitato xvij. Kal. Ianuar. anno trigesimo primo sæculi Decimiseptimi libri X. Cum Chronologia superiorum incendiorum; & Ephemeride ultimi. Napoli; Varrone, S., 1634. Vesuviani incendii historiae libri tres. Napoli], numerical simulations [Neri, A., Esposti Ongaro, T., Macedonio, G., Gidaspow, D., 2003. Multiparticle simulation of collapsing volcanic columns and pyroclastic flows. J. Geophys. Res. Lett. 108, 2202. doi:10.1029/2001 JB000508; Macedonio, G., Costa, A., Longo, A., 2005. HAZMAP: a computer model for volcanic ash fallout and assessment of subsequent hazard. Comput. Geosci. 31,837–845; Costa, A., Macedonio, G., Folch, A., 2006. A three-dimensional Eulerian model for transport and deposition of volcanic ashes. Earth Planet. Sci. Lett. 241,634–647] and experts' elicitations [Aspinall, W.P., 2006. Structured elicitation of expert judgment for probabilistic hazard and risk assessment in volcanic eruptions. In: Mader, H.M. Coles, S.G. Connor, C.B. Connor, L.J. (Eds), Statistics in Volcanology. Geological Society of London on behalf of IAVCEI, pp.15–30; Woo, G., 1999. The Mathematics of Natural Catastrophes. Imperial College Press, London] from which the impact on the building structures is derived. This is achieved by an original definition of vulnerability functions for multi-hazard input and a dynamic cumulative damage model. Factors affecting the variability of the final scenario are highlighted. The results show the high sensitivity of hazard combinations in time and space distribution and address how to mitigate building vulnerability to subsequent eruptive phenomena [Baxter, P., Spence, R., Zuccaro, G., 2008-this issue. Risk mitigation and emergency measures at Vesuvius].     

Growth of large sulfide structures on the endeavour segment of the Juan de Fuca ridge

Mapping and sampling with DSRV “Alvin” has established that sulfide blocks 0.5 m across, dredged from the axial valley of the Endeavour Segment at 47°57′N, are samples of unusually large sulfide structures. The steep-sided structures, up to 30 m in length, 20 m in height, and 10–15 m across, are localized by venting along normal faults at the base of the western axial valley wall, and are distributed for about 200 m along strike paralleling the 020 trend of the ridge crest. High-temperature fluids (350 to more than 400°C) pass through the massive sulfide structures and enter seawater through small, concentric “nozzle-like” features projecting from the top or the sides of the larger vent structures. Diffuse, low-temperature flow is pervasive in the vicinity of the active sulfide structures, exiting from basalt and sulfide surfaces alike. Evidence of recent volcanic activity is sparse.The two largest samples taken with the dredge would not have been recoverable using the submersible. These samples represent massive, complex portions of the sulfide structures which were not closely associated with rapid high-temperature fluid flow at the time of sampling; they contain textural evidence of sealed hydrothermal fluid exit channels. Mineralogy is dominated by Fe sulfides nnd amorphous silica. Pyrite, marcasite, wurtzite, chalcopyrite, and iss are the most common sulfide phases. Pyrrhotite, galena, and sphalerite are present in trace amounts. Barite, amorphous silica, and chalcedony are the only non-sulfide phases; anhydrite is not observed in any of the dredge samples, although it is common in the chimney-like samples recovered by “Alvin”.Specific mineralogical-textural zones within the dredge samples are anaoogous to individual layers in East Pacific Rise at 21°N and southern Juan de Fuca Ridge samples, with two exceptions: a coarse-grained, highly porous Fe sulfide-rich interior containing sulfidized tubeworm casts, and a 2–5 cm thick zone near the outer margin of the samples dominated by late stage amorphous silica. The porous interior may have formed by dendritic crystal growth from a slowly circulating fluid within a large enclosed chamber. The amorphous silica deposited from a seawater/hydrothermal fluid mixture percolating slowly through the walls of the enclosed chamber; conductive cooling of the fluid as it traversed the walls allowed amorphous silica to precipitate. These silica-rich zones are the densest, most durable portions of the structures and may be responsible for the lasting stability of the large sulfide features.Observations in these samples are consistent with two distinct phases of development. Phase 1 is analogous to chimney growth and construction at 21°N and ends when flow channels become sealed to rapid flow of through-going fluid. The flow is evidently redirected within the structure. Phase 2 includes dissolution of anhydrite and precipitation of amorphous silica during conductive cooling of sluggishly circulating hydrothermal fluid or seawater/hydrothermal fluid mixtures. Evolution of vent structures through phase 2 allows lateral and vertical growth of unusually large structures.     

Estimation of near surface soil moisture in a sloping terrain of a Himalayan watershed using ENVISAT ASAR multi‐incidence angle alternate polarisation data

Soil moisture is one of the important input variables in hydrological and water erosion models. The extraction of information on near surface soil moisture from synthetic aperture radar (SAR) is well established mostly for flat terrain and using low incidence angle single polarisation data. The ENVISAT advanced SAR (ASAR) data available in multiple incidence angles and alternate polarisation modes were investigated in this study for soil moisture estimation in sloping terrain. The test site was Sitla Rao watershed in the Lesser Himalayas of northern India. Empirical models were developed to estimate near surface soil moisture in bare agricultural fields using alternate polarisation ASAR data. Both soil moisture and surface roughness field measurements were performed during the satellite passes. Backscatter from medium incidence angle (IS‐4) and vertical‐vertical (VV) polarisation signal is correlated better with volumetric soil moisture content compared to other incidence angles. The model parameters were further improved, and soil moisture estimation was refined by combining medium incidence angle (IS4) vertical‐horizontal polarisation response as another variable along with VV polarisation response. The effect of slope on the radar backscatter was minimized by incorporating local incidence angles derived from an ASTER DEM. Copyright © 2012 John Wiley & Sons, Ltd.     

Mean and eddy motion in the Skagerrak/northern North Sea: insight from a numerical model

Insight regarding the mean and eddy motion in the Skagerrak/northern North Sea area is gained through an analysis of model-simulated currents, hydrography, kinetic energy and relative vorticity for the 2 years 2000 and 2001. In this a -coordinate ocean model is used. Since the tidal currents are generally strong in the area, care is exercised to distinguish the mesoscale (eddy) motion from higher-frequency motion such as tides, before computing the mean and eddy kinetic energy. The model-simulated response is first compared with available knowledge of the circulation in the area, and when available, also with sea-surface temperature obtained from satellite imagery. It is concluded that the model appears to faithfully reproduce most of what is known, in particularly the upper mixed layer circulation. An analysis of the mean and eddy kinetic energy reveals that many of the mesoscale structures found in the area are recurrent. This is particularly true for the structures off the southern tip of Norway. Also in general, areas of strong mean and eddy kinetic energy are co-located. The exception is the area off the southern tip of Norway, where the eddy kinetic energy is much larger than its mean counterpart. An analysis of the relative vorticity reveals that the variability found is due to the occurrence of recurrent anticyclonic eddies. It is hypothesized that these eddies are generated due to an offshore veering of the Norwegian coastal current (NCC) as it reaches the eastern end of the Norwegian Trench plateau. Here it becomes a free jet, which is then vulnerable to either barotropic instability caused by the horizontal shear in the jet-like structure of the NCC at this point, or a baroclinic (frontal) instability. The latter may come into play when the NCC veers offshore and its relatively fresh water meets the inflowing saline water of Atlantic origin, a frontogenesis that may become strong enough for cyclogenesis to take place. Due to the depth-independent nature of the model-generated eddies, the barotropic instability is the most likely candidate. It remains to resolve the reason for the offshore veering of the NCC. The most likely candidate mechanisms are vortex squeezing or simply that the coastline curvature is large enough for the NCC to separate from the coast in a hydraulic sense.Responsible Editor: Phil Dyke     

Specific features of ice conditions in the Sea of Azov and the Kerch Strait in the winter season of 2005/06

Peculiarities of the ice situation in the Sea of Azov and the Kerch Strait during the cold winter of 2005/06 are considered using a set of satellite images in the visible range, accumulated at the Southern Research Institute of Marine Fisheries and Oceanography (in Kerch). The ice conditions of the water area over the last years are examined and compared to the climatic data.     

The Oeschinensee rock avalanche,Bernese Alps,Switzerland: a co-seismic failure 2300 years ago?

Landslide deposits dam Lake Oeschinen (Oeschinensee), located above Kandersteg, Switzerland. However, past confusion differentiating deposits of multiple landslide events has confounded efforts to quantify the volume, age, and failure dynamics of the Oeschinensee rock avalanche. Here we combine field and remote mapping, topographic reconstruction, cosmogenic surface exposure dating, and numerical runout modeling to quantify salient parameters of the event. Differences in boulder lithology and deposit morphology reveal that the landslide body damming Oeschinensee consists of debris from both an older rock avalanche, possibly Kandertal, as well as the Oeschinensee rock avalanche. We distinguish a source volume for the Oeschinensee event of 37 Mm³, resulting in an estimated deposit volume of 46 Mm³, smaller than previous estimates that included portions of the Kandertal mass. Runout modeling revealed peak and average rock avalanche velocities of 65 and 45 m/s, respectively, and support a single-event failure scenario. ³⁶Cl surface exposure dating of deposited boulders indicates a mean age for the rock avalanche of 2.3 ± 0.2 kyr. This age coincides with the timing of a paleo-seismic event identified from lacustrine sediments in Swiss lakes, suggesting an earthquake trigger. Our results help clarify the hazard and geomorphic effects of rare, large rock avalanches in alpine settings.     

The role of granites in volcanic-hosted massive sulphide ore-forming systems: an assessment of magmatic�Chydrothermal contributions

Assessment of geological, geochemical and isotopic data indicates that a significant subgroup of volcanic-hosted massive sulphide (VHMS) deposits has a major or dominant magmatic?Chydrothermal source of ore fluids and metals. This group, which is typically characterised by high Cu and Au grades, includes deposits such as those in the Neoarchean Doyon-Bousquet-LaRonde and Cambrian Mount Lyell districts. These deposits are distinguished by aluminous advanced argillic alteration assemblages or metamorphosed equivalents intimately associated with ore zones. In many of these deposits, ??³⁴S_sulphide is low, with a major population below ?3??; ??³⁴S_sulphate differs from coexisting seawater and ??³⁴S_{sulphate?Csulphide}????20?C30??. These characteristics are interpreted as the consequence of disproportionation of magmatic SO₂ as magmatic?Chydrothermal fluids ascended and cooled and as a definitive evidence for a significant magmatic?Chydrothermal contribution. Other characteristics that we consider diagnostic of significant magmatic?Chydrothermal input into VHMS ore fluids include uniformly high (>3 times modern seawater values) salinities or very ¹⁸O-enriched (??¹⁸O?>?5??) ore fluids. We do not consider other criteria [e.g. variable salinity, moderately high ??¹⁸O_fluid (2?C5??), ??³⁴S_sulphide near 0??, metal assemblages or a spatial association with porphyry Cu or other clearly magmatic-hydrothermal deposits] that have been used previously to advocate significant magmatic?Chydrothermal contributions to be diagnostic as they can be produced by non-magmatic processes known to occur in VHMS mineral systems. However, in general, a small magmatic?Chydrothermal contribution cannot be excluded in most VHMS systems considered. Conclusive data that imply minimal magmatic?Chydrothermal contributions are only available in the Paleoarchean Panorama district where coeval seawater-dominated and magmatic?Chydrothermal systems appear to have been physically separated. This district, which is characterised by chloritic and sericitic alteration assemblages and lacks aluminous advanced argillic alteration assemblages, is typical of many VHMS deposits around the world, suggesting that for ??garden variety?? VHMS deposits, a significant magmatic?Chydrothermal contribution is not required. Other than deposits associated with advanced argillic alteration assemblages, the only deposit for which we ascribe a major magmatic?Chydrothermal contribution is the Devonian Neves Corvo deposit. This deposit differs from other deposits in the Iberian Pyrite Belt and around the world in being extremely Sn-rich, with the Sn closely associated with Cu and in having formed from high ¹⁸O-rich fluids (??¹⁸O_fluid ??8.5??). We consider these characteristics, particularly the last, as diagnostic of a significant magmatic hydrothermal contribution. Our analysis indicates that two subgroups of VHMS deposits have a major magmatic?Chydrothermal contribution: Cu/Au-rich deposits with aluminous alteration assemblages and reduced, very Sn-rich deposits in which Sn was introduced in a high-temperature ore assemblage. Comparison with ??normal?? VHMS deposits suggests that these subgroups of VHMS deposits may form in specialised tectonic environments. The Cu/Au-rich deposits appear to form adjacent to magmatic arcs, an environment conducive to the generation of hydrous, oxidised melts by melting metasomatised mantle in the wedge above the subducting slab. This contrasts with the back-arc setting of ??normal?? VHMS deposits in which relatively dry granites (In this contribution, we use the term granite sensu latto) formed by decompression melting drive seawater-dominated hydrothermal circulation. The tectonic setting of highly Sn-rich VHMS deposits such as Neves Corvo is less clear; however, thick continental crust below the ore-hosting basin may be critical, as it is in other Sn deposits.     

Cuttability assessment of hard coal seams

Summary A number of field and laboratory tests have been carried out on more than 15 coal seams of compressive strengths ranging from 19 MPa to 44 MPa to evolve methods which would help in the selection of suitable coaling machines for hard coal seams. The effect of physico-mechanical properties on cuttability were studied in the laboratory for all these coal seams to identify the relevant parameters affecting the specific energy of coal cuttability. These data were subjected to regression analysis to find the best fit for estimation of laboratory specific energy of coal samples on the basis of simple laboratory and field tests for the strength parameters. Field studies were also conducted over a large number of active mechanized coal faces to study in situ cuttability along with the geo-mining conditions of the site. The field and the laboratory data so generated were correlated and an attempt is made to establish a relationship for estimating the field specific energy for a particular capacity of coaling machines by considering the geo-mining domain of the field in totality.