Low dispersive modeling of Rayleigh waves on partly staggered grids     

O. Rojas  B. Otero  J. E. Castillo  S. M. Day 《Computational Geosciences》2014,18(1):29-43

In elastic media, finite-difference (FD) implementations of free-surface (FS) boundary conditions on partly staggered grid (PSG) use the highly dispersive vacuum formulation (VPSG). The FS boundary is embedded into a “vacuum” grid layer (null Lame’s constants and negligible density values) where the discretized equations of motion allow computing surface displacements. We place a new set of compound (stress-displacement) nodes along a planar FS and use unilateral mimetic FD discretization of the zero-traction conditions for displacement computation (MPSG). At interior nodes, MPSG reduces to standard VPSG methods and applies fourth-order centered FD along cell diagonals for staggered differentiation combined with nodal second-order FD in time. We perform a dispersion analysis of these methods on a Lamb’s problem and estimate dispersion curves from the phase difference of windowed numerical Rayleigh pulses at two FS receivers. For a given grid sampling criterion (e.g., six or ten nodes per reference S wavelength λ ^S), MPSG dispersion errors are only a quarter of the VPSG method. We also quantify root-mean-square (RMS) misfits of numerical time series relative to analytical waveforms. MPSG RMS misfits barely exceed 10 % when nine nodes sample the minimum S wavelength $\lambda _{\text {MIN}}^{\mathrm {S}}$ in transit (along distances $\sim $ 145 $\lambda _{\text {MIN}}^{\mathrm {S}}$ ). In same tests, VPSG RMS misfits exceed 70 %. We additionally compare MPSG to a consistently fourth-order mimetic method designed on a standard staggered grid. The latter equates the former’s dispersion errors on grids twice denser and shows higher RMS precision only on grids with six or less nodes per $\lambda _{\text {MIN}}^{\mathrm {S}}$ .  相似文献   

Modeling the onset of photosynthesis after the Chicxulub asteroid impact     

Noel Perez  Rolando Cardenas  Osmel Martin  Reinaldo Rojas 《Astrophysics and Space Science》2013,343(1):7-10

We do a preliminary modelling of the photosynthetic rates of phytoplankton at the very beginning of the Paleogene, just after the impact of the Chicxulub asteroid, which decisively contributed to the last known mass extinction of the Phanerozoic eon. We assume the worst possible scenario from the photobiological point of view: an already clear atmosphere with no ozone, as the timescale for soot and dust settling (years) is smaller than that of the full ozone regeneration (decades). Even in these conditions we show that most phytoplankton species would have had reasonable potential for photosynthesis in all the three main optical ocean water types. This modelling could help explain why the recovery of phytoplankton was relatively rapid after the huge environmental stress of that asteroid impact. In a more general scope, it also reminds us of the great resilience of the unicellular biosphere against huge environmental perturbations.  相似文献   

Saturn's cloud structure and temporal evolution from ten years of Hubble Space Telescope images (1994-2003)     

S. Pérez-Hoyos  A. Sánchez-Lavega  J.F. Rojas 《Icarus》2005,176(1):155-174

We present a study of the vertical structure of clouds and hazes in the upper atmosphere of Saturn's Southern Hemisphere during 1994-2003, about one third of a Saturn year, based on Hubble Space Telescope images. The photometrically calibrated WFPC2 images cover the spectral region between the near-UV (218-255 nm) and the near-IR (953-1042 nm), including the 890 nm methane band. Using a radiative transfer code, we have reproduced the observed center-to-limb variations in absolute reflectivity at selected latitudes which allowed us to characterize the vertical structure of the entire hemisphere during this period. A model atmosphere with two haze layers has been used to study the variation of hazes with latitude and to characterize their temporal changes. Both hazes are located above a thick cloud, putatively composed of ammonia ice. An upper thin haze in the stratosphere (between 1 and 10 mbar) is found to be persistent and formed by small particles (radii ∼0.2 μm). The lower thicker haze close to the tropopause level shows a strong latitudinal dependence in its optical thickness (typically τ∼20-40 at the equator but τ∼5 at the pole, at 814 nm). This tropospheric haze is blue-absorbent and extends from 50 to 100 mbar to about ∼400 mbar. Both hazes show temporal variability, but at different time-scales. First, there is a tendency for the optical thickness of the stratospheric haze to increase at all latitudes as insolation increases. Second, the tropospheric haze shows mid-term changes (over time scales from months to 1-2 years) in its optical thickness (typically by a factor of 2). Such changes always occur within a rather narrow latitude band (width ∼5-10°), affecting almost all latitudes but at different times. Third, we detected a long-term (∼10 year) decrease in the blue single-scattering albedo of the tropospheric haze particles, most intense in the equatorial and polar areas. Long-term changes follow seasonal insolation variations smoothly without any apparent delay, suggesting photochemical processes that affect the particles optical properties as well as their size. In contrast, mid-term changes are sudden and show various time-scales, pointing to a dynamical origin.  相似文献   

Cretaceous deformation of the southern Central Andes: synorogenic growth strata in the Neuquén Group (35° 30′–37° S)          下载免费PDF全文

Lucas M. Fennell  Andrés Folguera  Maximiliano Naipauer  Guido Gianni  Emilio A. Rojas Vera  Germán Bottesi  Victor A. Ramos 《Basin Research》2017,29(Z1):51-72

The Neuquén Group is an Upper Cretaceous continental sedimentary unit exhumed during the latest Miocene contractional phase occurred in the southern Central Andes, allowing a direct field observation and study of the depositional geometries. The identification of growth strata on these units surrounding the structures of the frontal parts of the Andes, sedimentological analyses and U–Pb dating of detrital components, allowed the definition of a synorogenic unit that coexisted with the uplift of the early Andean orogen since ca. 100 Ma, maximum age obtained in this work, compatible with previous assignments and constrained in the top by the deposition of the Malargüe Group, in the Maastrichtian (ca. 72 Ma). The definition of a wedge top area in this foreland basin system, where growth strata were described, permitted to identify a Late Cretaceous orogenic front and foredeep area, whose location and amplitude contrast with previous hypotheses. This wedge top area was mostly fed from the paleo‐Andes with small populations coming from sources in the cratonic area that are interpreted as a recycling in Jurassic and Lower Cretaceous sections, which contrasts with other analyses performed at the foredeep zone that have mixed sources. In particular, Permian sources are interpreted as coming directly from the cores of the basement structures, where Neopaleozoic sections are exposed, next to the synorogenic sedimentation, implying a strong incision in Late Cretaceous times with an exhumation structural level similar to the present. The maximum recognised advance for this Late Cretaceous deformation in the study area is approximately 500 km east of the Pacific trench, which constitutes an anomaly compared with neighbour segments where Late Cretaceous deformations were found considerably retracted. The geodynamic context of the sedimentation of this unit is interpreted as produced under the westward fast moving of South America, colliding with two consecutive mid‐ocean ridges during a period of important plate reorganisation. The subduction of young, anhydrous, buoyant lithosphere would have produced changes in the subduction geometry, reflected first by an arc waning/gap and subsequently by an arc migration that coexisted with synorogenic sedimentation. These magmatic and deformational processes would be the product of a shallow subduction regime, following previous proposals, which occurred in Late Cretaceous times, synchronous to the sedimentation of the Neuquén Group.  相似文献   

Multi-hazard assessment in Europe under climate change   总被引：2，自引：0，他引：2  

Giovanni Forzieri  Luc Feyen  Simone Russo  Michalis Vousdoukas  Lorenzo Alfieri  Stephen Outten  Mirco Migliavacca  Alessandra Bianchi  Rodrigo Rojas  Alba Cid 《Climatic change》2016,137(1-2):105-119

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TiN PRODUCTION IN A GLOW DISCHARGE     

P.J. Arango  A. Devia  A.F. Rojas  C.A. Peña  J.A. Ortiz  E. Restrepo 《Astrophysics and Space Science》1997,256(1-2):517-521

We have been studying direct current glow discharges aiming at the production of different paint of coatings on steel substrates. In a set of preliminary experiments, long lasting (hours) discharges performed in a Nitrogen atmosphere between a stell anode (holding the substrates) and a titanium cathode separated by a small gap (few mm) produced very thin coatings of the substrates whose colour depend on the parameter of (interelectrodes gap, gas pressure, discharge current and time duration). In some conditions, the coatings showed a golden colour, typical of the Nti compound. A discussion of the results is given.  相似文献   

A method to predict the group fissuring and faulting caused by regional groundwater decline     

E. Rojas  J. Arzate  M. Arroyo   《Engineering Geology》2002,65(4):20-260

Ground fissuring is a recurrent problem in many countries where water extraction surpasses the natural recharge of aquifers. Due to differential settlement, the soil layer undergoes deformation and cracks with serious consequences for civil infrastructure. Here, we propose an approximate analysis of the fissuring process that can be used to predict the location of cracks, which increasingly affect some middle- and large-sized cities in the world. For that purpose, the ground loss theory is applied to sediments overlying a sinusoidal-shaped graben. This analysis shows the existence of a tensile zone at the border of the graben with maximal values on its shoulder where tension cracks are more likely to appear. It also shows that soil deformation under differential settlement may evolve into ground faulting if water withdrawal continues. Finally, when a crack has completely developed, the tensile zone shifts towards the center of the graben, creating a new area for potential cracking and faulting.  相似文献   

Montecarlo DLA-type simulations of wetting effects in fluid displacement in porous media     

Rafael Rangel  Sergio Rojas 《Computational Geosciences》2009,13(2):215-225

In this work, we report diffusion-limited aggregation (DLA)-type Montecarlo computations of a stochastic model of displacement of a viscous fluid by another that preferentially wets a porous medium, for the case when both fluids are immiscible in the absence of buoyancy forces. The model has the aim to simulate cooperative invasion processes found in experiments of immiscible wetting displacement. The model considers the nonlocal effects of the Laplacian pressure field and the capillary forces via hydrodynamic equations in the Darcy regime with a boundary condition for the pressure at the interface. The boundary condition contains two different types of disorder: the capillary term, which constitutes an additive random disorder, and a term containing an effective random surface tension, which couples to a curvature (it constitutes a multiplicative random term that carries nonlocal information of the whole pressure). We generate different displacement patterns for different setting of the parameters of the model. We analyze these patterns by studying the scaling properties of the interface that separate the two fluids and calculating the fractal dimension of the interface. The results show the existence of three distinct regimes of scaling. One regime at the smallest-length scales is due to the multiplicative random disorder together with the nonlocal coupling; it reveals itself in a roughness exponent α ≈ 0.80. Additionally, we find a DLA-type scaling regime with a roughness exponent α ≈ 0.60 at the largest scales and intermediate scaling regime with α ≈ 0.70 corresponding to invasion percolation with trapping. Each regime has definite scaling ranges that depend on the capillary number and the relative wetting tendency of the fluids. The behavior of the fractal dimensions of the interfaces of the aggregates constitutes a further confirmation of the existence of three scaling regimes and the multi-self-affinity of the perimeter of the interface boundaries.  相似文献   

In Situ U‐Th‐Pb Dating and Sr‐Nd Isotope Analysis of Bastnäsite by LA‐(MC)‐ICP‐MS     

Yue‐Heng Yang  Fu‐Yuan Wu  Qiu‐Li Li  Yamirka Rojas‐Agramonte  Jin‐Hui Yang  Yang Li  Qian Ma  Lie‐Wen Xie  Chao Huang  Hong‐Rui Fan  Zi‐Fu Zhao  Cheng Xu 《Geostandards and Geoanalytical Research》2019,43(4):543-565

Bastnäsite is the end member of a large group of carbonate–fluoride minerals with the common formula (REE) CO₃F·CaCO₃. This group is generally widespread and, despite never occurring in large quantities, represents the major economic light rare earth element (LREE) mineral in deposits related to carbonatite and alkaline intrusions. Since bastnäsite is easily altered and commonly contains inclusions of earlier‐crystallised minerals, in situ analysis is considered the most suitable method to measure its U‐Th‐Pb and Sr‐Nd isotopic compositions. Electron probe microanalysis and laser ablation (multi‐collector) inductively coupled plasma‐mass spectrometry of forty‐six bastnäsite samples from LREE deposits in China, Pakistan, Sweden, Mongolia, USA, Malawi and Madagascar indicate that this mineral typically has high Th and LREE and moderate U and Sr contents. Analysis of an in‐house bastnäsite reference material (K‐9) demonstrated that precise and accurate U‐Th‐Pb ages could be obtained after common Pb correction. Moreover, the Th‐Pb age with its high precision is preferable to the U‐Pb age because most bastnäsites have relatively high Th rather than U contents. These results will have significant implications for understanding the genesis of endogenous ore deposits and formation processes related to metallogenic geochronology research.  相似文献   

Effects of protandric sex change on radula,pedal morphology,and mobility in Crepidula fecunda (Gastropoda: Calyptraeidae)     

O. R. Chaparro  S. V. Pereda  I. Bahamondes‐Rojas 《新西兰海洋与淡水研究杂志》2013,47(5):881-890

Protandrism was confirmed in Crepidula fecunda Gallardo 1979 by histological analysis of the gonad. Individuals were classified as immature (12 mm or less in shell length), male (13–26 mm), intersex (25–28 mm), and female (over 26 mm). There was an evident loss of mobility with growth— large females became completely sessile. The presence of epithelial glands in mobile individuals was associated with the need for lubrication during crawling, whereas the proliferation of subepithelial glands in sessile females was associated with adhesion. Mobility of immature and male individuals was related to grazing activities on biofilms, and in males was also associated with copulation. Grazing activity of motile individuals was carried out so that the radula teeth did not come into direct contact with the substratum, but rather harvested the biofilm surface. Sessile females used the radula to rasp the area under their shells, thus cleaning the substratum in preparation for oviposition. Since this process required that the radula enter into direct contact with the substratum, it suffered abrasion of radular teeth and loss of radular denticles, producing important alterations in its appearance in mature females. A complex interaction was observed between size, mobility, histological changes in the foot, activity, and tooth‐wear of the radula, which have been related to the sessile habit, sex change, and the oviposition process in this species.  相似文献