Collapse and explosion in a rotating star

The collapse, bounce, shock wave and expansion of the envelope of a rotating star have been analysed in the adiabatic approximation using the particle-in-cell method. The bounce takes place first in the equatorial plane and a shock wave arises there which shortly afterwards crosses the surface of the star. In the envelope, and to a less extent in the remainder of the star, there is a fast and lasting meridional motion the direction of which changes. As a consequence of the fast meridional motion in the envelope, mass and angular momentum are transported towards the axis of rotation. If the initial star rotates fast enough this will cause a secondary radial expansion in the polar region and a mass ejection. These motions reduce the strong anisotropy caused originally by the equatorial expansion. Strong whirls may arise along the axis of rotation. In the remainder of the star the meridional motion becomes supersonic. The temperature in the envelope depends to a high degree on the choice of the equation of state. Massloss is proportional to the energy initially added. The final loss of angular momentum and of energy is quite large, both losses being about 25%.     

Paleomagnetism of early cretaceous arapey formation (Northern Uruguay)

We report detailed rock-magnetic and paleomagnetic directional data from 35 lava flows (302 standard paleomagnetic cores) sampled in the Central-Northern region of Uruguay in order to contribute to the study of the paleosecular variation of the Earth’s magnetic field during early Cretaceous and to obtain precise Cretaceous paleomagnetic pole positions for stable South America. The average unit direction is rather precisely determined from 29 out of 35 sites. All A₉₅ confidence angles are less than 8°, which points to small within-site dispersion and high directional stability. Normal polarity magnetizations are revealed for 19 sites and 10 are reversely magnetized. Two other sites yield well defined intermediate polarities. The mean direction, supported by a positive reversal test is in reasonably good agreement with the expected paleodirection for Early Cretaceous stable South America and in disagreement with a 10° clockwise rotation found in the previous studies. On the other hand, paleomagnetic poles are significantly different from the pole position suggested by hotspot reconstructions, which may be due to true polar wander or the hotspot motion. Our data suggest a different style of secular variation during (and just before) the Cretaceous Normal Superchron and the last 5 Ma, supporting a link between paleosecular variation and reversal frequency.     

A front-tracking method for the simulation of three-phase flow in porous media

Under certain physically reasonable assumptions, three-phase flow of immiscible, incompressible fluids can be described by a 2×2 nongenuinely nonlinear, hyperbolic system. We combine analytical solutions to the corresponding Riemann problem with an efficient front-tracking method to study Cauchy and initial-boundary value problems. Unlike finite difference methods, the front-tracking method treats all waves as discontinuities by evolving shocks exactly and approximating rarefactions by small entropy-violating discontinuities. This way, the method can track individual waves and give very accurate (or even exact) resolution of discontinuities. We demonstrate the applicability of the method through several numerical examples, including a streamline simulation of a water-alternating-gas (WAG) injection process in a three-dimensional, heterogeneous, shallow-marine formation.     

Geostatistical seismic inversion for non‐stationary patterns using direct sequential simulation and co‐simulation

Geostatistical seismic inversion methods are routinely used in reservoir characterisation studies because of their potential to infer the spatial distribution of the petro‐elastic properties of interest (e.g., density, elastic, and acoustic impedance) along with the associated spatial uncertainty. Within the geostatistical seismic inversion framework, the retrieved inverse elastic models are conditioned by a global probability distribution function and a global spatial continuity model as estimated from the available well‐log data for the entire inversion grid. However, the spatial distribution of the real subsurface elastic properties is complex, heterogeneous, and, in many cases, non‐stationary since they directly depend on the subsurface geology, i.e., the spatial distribution of the facies of interest. In these complex geological settings, the application of a single distribution function and a spatial continuity model is not enough to properly model the natural variability of the elastic properties of interest. In this study, we propose a three‐dimensional geostatistical inversion technique that is able to incorporate the reservoir's heterogeneities. This method uses a traditional geostatistical seismic inversion conditioned by local multi‐distribution functions and spatial continuity models under non‐stationary conditions. The procedure of the proposed methodology is based on a zonation criterion along the vertical direction of the reservoir grid. Each zone can be defined by conventional seismic interpretation, with the identification of the main seismic units and significant variations of seismic amplitudes. The proposed method was applied to a highly non‐stationary synthetic seismic dataset with different levels of noise. The results of this work clearly show the advantages of the proposed method against conventional geostatistical seismic inversion procedures. It is important to highlight the impact of this technique in terms of higher convergence between real and inverted reflection seismic data and the more realistic approximation towards the real subsurface geology comparing with traditional techniques.     

Local coordination of Zn in hydroxy-interlayered minerals and implications for Zn retention in soils

The objective of this study was to determine the local coordination of Zn in hydroxy-interlayered smectite (HIS) as a function of Zn loading and synthesis conditions and to assess the importance of hydroxy-interlayered minerals (HIM) for Zn retention in contaminated soils. Published and newly collected extended X-ray absorption fine structure (EXAFS) spectra of HIS reacted with Zn at molar Zn/hydroxy-Al ratios from 0.013 to 0.087 (corresponding to final Zn contents of 1615-8600 mg/kg Zn) were evaluated by shell fitting. In Zn-HIS, Zn was octahedrally coordinated to oxygen at 2.06-2.08 Å and surrounded by Al atoms at 3.03-3.06 Å in the second-shell. With increasing molar Zn/hydroxy-Al ratio, the coordination number of second-shell Al decreased from 6.6 to 2.1. These results were interpreted as a progressive shift from Zn incorporation in the vacancies of gibbsitic Al-polymers to Zn adsorption to incomplete Al-polymers and finally uptake by cation exchange in the polymer-free interlayer space of HIS with increasing Zn loadings. In a second part, we determined the speciation of Zn in eight contaminated soils (251-1039 mg/kg Zn) with acidic to neutral pH (pH 4.1-6.9) using EXAFS spectroscopy. All soils contained hydroxy-Al interlayered vermiculite (HIV). The analysis of EXAFS spectra by linear combination fitting (LCF) showed that a substantial fraction of total Zn (29-84%) was contained in HIM with high Zn loading. The remaining Zn was adsorbed to organic and inorganic soil components and incorporated into phyllosilicates. In sequential extractions of Zn-HIS spiked into quartz powder and the Zn contaminated soils, Zn was mainly released in the two most resistant fractions, in qualitative agreement with the findings from LCF. Our results suggest that formation of Zn-HIM may strongly retain Zn in pristine and moderately contaminated acidic to neutral soils. Due to their limited sorption capacity, however, HIM do not allow for the accumulation of high levels of Zn in response to continued Zn input into soils.