Wavefield Migration plus Monte Carlo Imaging of 3D Prestack Seismic Data

Prestack wave‐equation migration has proved to be a very accurate shot‐by‐shot imaging tool. However, 3D imaging with this technique of a large field acquisition, especially one with hundreds of thousands of shots, is prohibitively costly. Simply adapting the technique to migrate many superposed shot‐gathers simultaneously would render 3D wavefield prestack migration cost‐effective but it introduces uncontrolled non‐physical interference among the shot‐gathers, making the final image useless. However, it has been observed that multishot signal interference can be kept under some control by averaging over many such images, if each multishot migration is modified by a random phase encoding of the frequency spectra of the seismic traces. In this article, we analyse this technique, giving a theoretical basis for its observed behaviour: that the error of the image produced by averaging over M phase encoded migrations decreases as M^?1 . Furthermore, we expand the technique and define a general class of Monte‐Carlo encoding methods for which the noise variance of the average imaging condition decreases as M^?1 ; these methods thus all converge asymptotically to the correct reflectivity map, without generating prohibitive costs. The theoretical asymptotic behaviour is illustrated for three such methods on a 2D test case. Numerical verification in 3D is then presented for one such method implemented with a 3D PSPI extrapolation kernel for two test cases: the SEG–EAGE salt model and a real test constructed from field data.     

Irregular Satellites of Jupiter: a study of the capture direction

The irregular satellites of Jupiter are believed to be captured asteroids or planetesimals. In the present work is studied the direction of capture of these objects as a function of their orbital inclination. We performed numerical simulations of the restricted three-body problem, Sun-Jupiter-particle, taking into account the growth of Jupiter. The integration was made backward in time. Initially, the particles have orbits as satellites of Jupiter, which has its present mass. Then, the system evolved with Jupiter losing mass and the satellites escaping from the planet. The reverse of the escape direction corresponds to the capture direction. The results show that the Lagrangian points L1 and L2 mainly guide the direction of capture. Prograde satellites are captured through these two gates with very narrow amplitude angles. In the case of retrograde satellites, these two gates are wider. The capture region increases as the orbital inclination increases. In the case of planar retrograde satellites the directions of capture cover the whole 360° around Jupiter. We also verified that prograde satellites are captured earlier in actual time than retrograde ones. This paper was presented at the Asteriods, Comets and Meteors meeting held at Búzios, Rio de Janeiro, Brazil in August 2005 and could not be included in the special issue related to that conference.     

New stacked central configurations for the planar 5-body problem

A stacked central configuration in the n-body problem is one that has a proper subset of the n-bodies forming a central configuration. In this paper we study the case where three bodies with masses m ₁, m ₂, m ₃ (bodies 1, 2, 3) form an equilateral central configuration, and the other two with masses m ₄, m ₅ are symmetric with respect to the mediatrix of the segment joining 1 and 2, and they are above the triangle generated by {1, 2, 3}. We show the existence and non-existence of this kind of stacked central configurations for the planar 5-body problem.     

Effect of atmospheric dust loading on martian albedo measurements

This work is devoted to the analysis of the variation of albedo measured by orbiting instruments with atmospheric opacity on Mars. The study has been conduced by analysing Mars Global Surveyor Thermal Emission Spectrometer (MGS-TES) data from martian regions with different surface albedo.In support of these data, synthetic spectra with different surface albedo and atmospheric opacities have been computed, so that a comparison has been performed. The synthetic spectra have been retrieved by using two different grain sizes for suspended dust (0.5 and 1.2 μm), allowing a comparison between the two models and the observations.Using the DCI, a parameter describing the quantity of dust deposited on the surface, the effectiveness of the single scattering approximation has been tested for low atmospheric opacity by analysing the quality of the linear fit up to different atmospheric opacity.For more opaque conditions two kinds of fits have been applied to the data, linear and second-order degree polynomial. In this case, we found that the polynomial fit better describes the observations.The analysis of these data made it possible to notice a peculiar trend, already reported by Christensen (1988), of the albedo over Syrtis Major after the occurrence of dust storms, but, differently from that work, now the study of DCI together with atmospheric opacity and albedo allowed us to robustly confirm the hypothesis made by Christensen.Finally, the comparison between observations and synthetic spectra computed with models with different particles grain sizes indicates that dust particles of 0.5 μm diameter are the most effective to change the aerosol atmospheric opacity on Mars.     

Magnetic susceptibility and electrical conductivity as a proxy for evaluating soil contaminated with arsenic,cadmium and lead in a metallurgical area in the San Luis Potosi State,Mexico

A total of 113 samples of waste and soil were collected from a site in the state of San Luis Potosi, Mexico, that was occupied for several years by the metallurgical industry. Specific magnetic susceptibility (MS), electrical conductivity (EC) and pH were determined, as well as the total and available concentrations of potentially toxic elements (PTEs) such as As, Cd, Cu, Fe, Pb and Zn, which may cause a health risk for humans, animals and ecosystems, and the concentrations of major ions in aqueous extracts of soils and wastes. The solid phases of the samples were also characterized. The results revealed that the soils and wastes exhibited elevated values of PTEs, MS and EC. For soils these values decreased with increasing distance from the waste storage sites. The MS values were elevated primarily due to the presence of Fe-oxyhydroxides, such as magnetite, hematite and goethite, which contain PTEs in their structure leading to a high correlation between the value of MS and the As, Cd, Fe and Pb contents (r = 0.57–0.91) as well as between the PTEs values (r = 0.68–0.92). The elevated EC values measured in the metallurgical wastes were the result of presence of the sulfate minerals of Ca, Mg and Fe. The pollution index, which indicates the levels of simultaneous toxicity from elements such as As, Cd and Pb, was determined, with extreme hazard zones corresponding to areas that exhibit high MS values (0.91 correlation). In conclusion, MS measurements can be used as an indirect indicator to evaluate the PTE contamination in metallurgical areas, and EC measurements can aid in the identification of pollution sources.     

A software solution for point counting. Petrographic thin section analysis as a case study

In the field of geology, as in many other fields, a common problem is the analysis of 2D sections. An example of this is the petrographic study of rock samples, i.e., the quantification of its mineral components and the percentages of each phase present in it. Conventionally, this quantification has been determined by point counting performed on thin sections. Although point counting is very time consuming, it is of common use in several domains including geology, biology, medicine, and materials sciences, among others. Point counting in thin sections is normally conducted through mechanical or electromechanical devices attached to a microscope; such devices are very expensive, offer limited functionality, and are very time consuming. In this contribution, we introduce an interactive visualization application called Rock.AR that reduces the amount of time required to apply this technique and simplify its work flow. It provides visual tools like distortion techniques, overview + detail, and statistics to assist the technique. A comparison between this application and similar tools through users' experiences is provided. We compare the time it takes to complete a session of point counting and the tools' usability.     

Mouvements voisins de collision quadruple dans le probleme trapezoidal des 4 corps

Celestial Mechanics and Dynamical Astronomy - On reprend ici l'étude du problème trapezoïdal des 4 corps, qui a été commencée par l'auteur dans [2]. En...     

A compact model for the planar rhomboidal 4-body problem

We obtain a compact model for the global study of the planar rhomboidal 4-body problem in a level of constant negative energy. This model is a variation of the non compact model obtained through a McGehee blow up transformation. but compactness permits to obtain results which are not clear in the other case.     

Pyrolisis of phosphorylated molecules and survivability limits during the atmospheric passage in earth-like planets

There is evidence that space energy sources could give place to the appearance of phosphorylated nucleosides outside of Earth. These compounds may have been delivered mainly by interplanetary dust particles due to the lower temperatures experienced during atmospheric deceleration and impacts to the terrestrial surface. In this report, we communicate the results of pyrolytic studies to simulate atmospheric survivability of adenosine-5′-diphosphates (ADP) (and adenosine-5′-monophosphate, adenosine and adenine as degradation products) at temperatures <500°C and at various time intervals. Our results revealed that phosphorylated and non-phosphorylated nucleosides transported by IDPs having sizes of 10⁻⁶- could resist temperatures up to 500°C generated during atmospheric entry. However, atmospheric passage should not exceed a time due to the thermal lability of these molecules. Because of the high half-life showed by ADP in the presence of meteoritic powder, it is thought that extraterrestrial delivery of very complex biomolecules is more suitable under such protected conditions. These data indicate that the formation of a Fe²⁺- and/or Ca²⁺-complex could increase the stability of the molecules in the presence of meteoritic matter. Therefore, if the synthesis of nucleosides, nucleotides or oligonucleotides could take place in icy bodies, then micron-sized dust could have contributed significantly to the availability of phosphorylated nucleosides in the early Earth or in extrasolar early Earth-like planets, and thereby could have allowed the arising of an early biological activity.     

Climate change and the incidence of a forest pest in Mediterranean ecosystems: can the North Atlantic Oscillation be used as a predictor?

Many forest pest species strongly depend on temperature in their population dynamics, so that rising temperatures worldwide as a consequence of climatic change are leading to increased frequencies and intensities of insect-pest outbreaks. In the Mediterranean area, the climatic conditions are strongly linked to the effects of the North Atlantic Oscillation (NAO). The aim of this work is to analyze the dynamics of the pine processionary moth (Thaumetopoea pityocampa), a severe pest of Pinus species in the Circunmediterranean, throughout a region of southern Spain, in relation to NAO indices. We related the percentage of forest plots with high defoliation by pine processionary moth each year with NAO values for the present and the three previous winters, using generalized linear models with a binomial error distribution. The time series is 16-year long, and we performed analyses for the whole database and for the five main pine species separately. We found a consistent relationship between the response variable and the NAO index. The relationship is stronger with pine species living at medium-high altitudes, such as Aleppo (P. halepensis), black (P. nigra), and Scots (Pinus sylvestris) pine, which show the higher defoliation intensities up to 3?years after a negative NAO phase. The results highlight, for the first time, the usefulness of using global drivers in order to understand the dynamics of pest outbreaks at a regional scale, and they open the window to the development of NAO-based predictive models as an early-warning signal of severe pest outbreaks.