A Comparison of Random Field Models Beyond Bivariate Distributions

In order to determine to what extent a spatial random field can be characterized by its low-order distributions, we consider four models (specifically, random spatial tessellations) with exactly the same univariate and bivariate distributions and we compare the statistics associated with various multiple-point configurations and the responses to specific transfer functions. The three- and four-point statistics are found to be the same or experimentally hardly distinguishable because of ergodic fluctuations, whereas change of support and flow simulation produce very different outcomes. This example indicates that low-order distributions may not discriminate between contending random field models, that simulation algorithms based on such distributions may not reproduce the spatial properties of a given model or training image, and that the inference of high-order distribution may require very large training images.     

Salt tectonics at passive margins: Geology versus models

Salt tectonics at passive margins is currently interpreted as a gravity-driven process but according to two different types of models: i) pure spreading only driven by differential sedimentary loading and ii) dominant gliding primarily due to margin tilt (slope instability). A comparative analysis of pure spreading and pure spreading is made using simple mechanics as well as available laboratory experiments and numerical models that consider salt tectonic processes at the whole basin scale. To be effective, pure spreading driven by sedimentary loading requires large differential overburden thicknesses and therefore significant water depths, high sediment density, low frictional angles of the sediments (high fluid pore pressure) and a seaward free boundary of the salt basin (salt not covered by sediments). Dominant gliding does not require any specific condition to be effective apart from the dip on the upper surface of the salt. It can occur for margin tilt angles lower than 1° for basin widths in the range of 200-600 km and initial sedimentary cover thickness up to 1 km, even in the absence of abnormal fluid pressure. In pure spreading, salt resists and sediments drive whereas in dominant gliding both salt and sediments drive. In pure spreading, extension is located inside the prograding sedimentary wedge and contraction at the tip. Both extension and contraction migrate seaward with the sedimentary progradation. Migration of the deformation can create an extensional inversion of previously contractional structures. In pure spreading, extension is located updip and contraction downdip. Extension migrates downdip and contraction updip. Migration of the deformation leads to a contractional inversion of previously extensional structures (e.g. squeezed diapirs). Mechanical analysis and modelling, either analogue or numerical, and comparison with margin-scale examples, such as the south Atlantic margins or northern Gulf of Mexico, indicate that salt tectonics at passive margins is dominated by dominant gliding down the margin dip. On the contrary, salt tectonics driven only by differential sedimentary loading is a process difficult to reconcile with geological evidence.     

Zenisu Ridge: a deep intraoceanic thrust related to subduction, off southwest Japan

During the French-Japanese Kaiko project, Seabeam, seismic and submersible observations were made in the eastern part of the Nankai subduction zone, close to the area of collision between the Izu-Bonin island arc and the Japan margin. The most prominent feature is the Zenisu Ridge, an elongated relief of the Philippine Sea plate running parallel to the Trench. Magnetic anomalies indicate that the crust of the Zenisu Ridge is a part of the Shikoku oceanic basin formed in the Early Miocene, 23 Ma ago and presumably uplifted at a later stage. Structural analysis of seismic data and diving observations lead us to interpret the superficial structure as being due to compressive tectonics. Mapping the acoustic basement reveals that the southeastern flank of the ridge is bounded by a double thrust, both segments being of equal magnitude (vertical offset about 1 to 1.5 km). Geophysical data support the hypothesis of a main thrust cutting through most of the lithosphere and flattening at depth. The overall structure of the surrounding area reveals a compressive deformation zone widening toward the east, the magnitude of the compressive deformation decreasing westward as well as southward of the Zenisu Ridge.     

Geological study of sedimentary basins using SPOT data

This paper illustrates the benefits of DTMs created from SPOT images for the exploration of sedimentary basins. We chose an example located in the Ebro sedimentary basin in Spain, characterized by good outcropping conditions and slight deformations. The data used consist of a pair of SPOT panchromatic images and a SPOT XS image. The work consists of making up a 3D database, followed by interpretation of stereo pairs computed from orthoimages and the DTM. This interpretation is made on a stereoscopic desk and results in a digital file containing in the form of vectors all the observed faults and lithology. These vectors are then used to make calculations on the geometry of the objects they represent: we demonstrate that we can accurately measure layer directions and dips, sediment thicknesses and fault throws. Synthesis of perspective views made at the same time provide a good understanding of the structures and help to test their geometric consistency. Thus knowledge about relief given by DTMs helps to interpret remote detection images in 3D space, and particularly to accurately quantify the results of this interpretation.     

Modeling the Uncertainty in the Layout of Geological Units by Implicit Boundary Simulation Accounting for a Preexisting Interpretive Geological Model

Natural Resources Research - An implicit simulation algorithm that uses signed distance functions is proposed to represent subsurface geological configuration while accounting for two sources of...     

Application of plurigaussian simulation to delineate the layout of alteration domains in Sungun copper deposit

An accurate estimation of mineral grades in ore deposits with heterogeneous spatial variations requires defining geological domains that differentiate the types of mineralogy, alteration and lithology. Deterministic models define the layout of the domains based on the interpretation of the drill holes and do not take into account the uncertainty in areas with fewer data. Plurigaussian simulation (PGS) can be an alternative to generate multiple numerical models of the ore body, with the aim of assessing the uncertainty in the domain boundaries and improving the geological controls in the characterization of quantitative attributes. This study addresses the application of PGS to Sungun porphyry copper deposit (Iran), in order to simulate the layout of four hypogene alteration zones: potassic, phyllic, propylitic and argillic. The aim of this study is to construct numerical models in which the alteration structures reflect the evolution observed in the geology.     

Modelling artificial night-sky brightness with a polarized multiple scattering radiative transfer computer code

As part of an ongoing investigation of radiative effects produced by hazy atmospheres, computational procedures have been developed for use in determining the brightening of the night sky as a result of urban illumination. The downwardly and upwardly directed radiances of multiply scattered light from an offending metropolitan source are computed by a straightforward Gauss–Seidel (G–S) iterative technique applied directly to the integrated form of Chandrasekhar's vectorized radiative transfer equation. Initial benchmark night-sky brightness tests of the present G–S model using fully consistent optical emission and extinction input parameters yield very encouraging results when compared with the double scattering treatment of Garstang, the only full-fledged previously available model.     

First-principles study of the electronic properties of A2B3 minerals, with A=Bi,Sb and B=S,Se

We determine the valence electron density and the electron band structure of stibnite, bismutinite, guanajuatite and antimonelite using the density functional theory. All the compounds present similar electronic properties and exhibit a quasi-1D character. We perform a detailed analysis of the charge topology, the atomic static charges and volumes.     

Chemical evolution models of local dwarf spheroidal galaxies

We calculate chemical evolution models for four dwarf spheroidal (dSph) satellites of the Milky Way (Carina, Ursa Minor, Leo I and Leo II) for which reliable non-parametric star formation histories have been derived. In this way, the independently-obtained star formation histories are used to constrain the evolution of the systems we are treating. This allows us to obtain robust inferences on the history of such crucial parameters of galactic evolution as gas infall, gas outflows and global metallicities for these systems. We can then trace the metallicity and abundance ratios of the stars formed, the gas present at any time within the systems and the details of gas ejection, of relevance to enrichment of the galaxies environment. We find that galaxies showing one single burst of star formation (Ursa Minor and Leo II) require a dark halo slightly larger that the current estimates for their tidal radii, or the presence of a metal-rich selective wind that might carry away much of the energy output of their supernovae before this might have interacted and heated the gas content, for the gas to be retained until the observed stellar populations have formed. Systems showing extended star formation histories (Carina and Leo I), however, are consistent with the idea that their tidally-limited dark haloes provide the necessary gravitational potential wells to retain their gas. The complex time structure of the star formation in these systems remains difficult to understand. Observations of detailed abundance ratios for Ursa Minor strongly suggest that the star formation history of this galaxy might in fact resemble the complex picture presented by Carina or Leo I, but localized at a very early epoch.     

A qualitative assessment of the influence of bioturbation in Lake Baikal sediments

The impact of bioturbation in Lake Baikal sediments, particularly on rhythmic layering and mixing, was assessed by studying the actual vertical distribution of benthic animals in continuous accumulation zones selected by seismic survey (Vydrino Shoulder, Posolskoe Bank, Continent Ridge). To assess the influence of the bioturbation, animals were extracted from short cores and identified at the relevant taxonomic level. The faunal distribution is examined in parallel with the bioturbation tracks observed in thin section. Oligochaeta, Nematoda, Ostracoda, Copepoda, Gammaridae, Chironomidae and Hydrachnidia were found inhabiting the sediment. Among them, only oligochaete worms were assumed to have a significant impact on sediment mixing because of their “conveyor belt” feeding. The other two most abundantly sampled groups, nematods and copepods, belong to the interstitial fauna that has no significant impact on the vertical displacement of sediment particles and do not ingest the sediment. The presence of a benthic fauna as deep as 15 cm in the sediment indicates that the possibility of sediment disturbance by invertebrate activity cannot be dismissed in Lake Baikal. The effect of biological mixing is more limited in the deepest stations because the number of potential bioturbators is reduced, qualitatively as well as quantitatively. Located in the abyssal zone, Continent and Vydrino (but outside turbidites) deep stations appear to be most promising sediment records for tracking climate signal at high resolution.