Effects of aperture on Marchenko focussing functions and their radiation behaviour at depth     

Yanadet Sripanich  Ivan Vasconcelos 《Geophysical Prospecting》2019,67(2):443-454

A focussing function is a specially constructed field that focusses on to a purely downgoing pulse at a specified subsurface position upon injection into the medium. Such focussing functions are key ingredients in the Marchenko method and in its applications such as retrieving Green's functions, redatuming, imaging with multiples and synthesizing the response of virtual sources/receiver arrays at depth. In this study, we show how the focussing function and its corresponding focussed response at a specified subsurface position are heavily influenced by the aperture of the source/receiver array at the surface. We describe such effects by considering focussing functions in the context of time-domain imaging, offering explicit connections between time processing and Marchenko focussing. In particular, we show that the focussed response radiates in the direction perpendicular to the line drawn from the centre of the surface data array aperture to the focussed position in the time-imaging domain, that is, in time-migration coordinates. The corresponding direction in the Cartesian domain follows from the sum (superposition) of the time-domain direction and the directional change due to time-to-depth conversion. Therefore, the result from this study provides a better understanding of focussing functions and has implications in applications such as the construction of amplitude-preserving redatuming and imaging, where the directional dependence of the focussed response plays a key role in controlling amplitude distortions.  相似文献   

A transparent aqueous-saturated sand surrogate for use in physical modeling   总被引：3，自引：0，他引：3  

Ivan L. Guzman  Magued Iskander  Eduardo Suescun-Florez  Mehdi Omidvar 《Acta Geotechnica》2014,9(2):187-206

This paper presents the geotechnical properties of a new family of synthetic transparent soils made of fused quartz, saturated with a matched refractive index water-based sucrose solution, suitable for modeling the behavior of sand in small-scale model tests. The dry density ranged between 1,134 and 1,358 kg/m³. The peak angle of friction was found to range from 46° to 57°. The average hydraulic conductivity was 1.7 × 10^?5 cm/s. The compressibility index (C _c) ranged from 0.34 to 0.57. The main advantage of fused quartz over available sand surrogates made of silica gel is that its solid structure better models the behavior of natural sand. The matching pore fluids are inert and non-toxic, which facilitates their use in educational settings. The availability of a safe and easy-to-use transparent sand permits measurement of three-dimensional deformation patterns and flow characteristics in controlled research experiments. The introduction of an aqueous solution permits the use of two immiscible pore fluids, one made of mineral oil and the other made of a sucrose solution, for modeling multiphase flow problems, as well as coupled flow-deformation problems.  相似文献   

Cave clastic sediments as a tool for refining the study of human occupation of prehistoric sites: insights from the cave site of La Cala (Cilento,southern Italy)          下载免费PDF全文

Ivan Martini  Annamaria Ronchitelli  Simona Arrighi  Giulia Capecchi  Stefano Ricci  Sem Scaramucci  Vincenzo Spagnolo  Paolo Gambassini  Adriana Moroni 《第四纪科学杂志》2018,33(5):586-596

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Determination of origin of NASA stratospheric particles     

I. Kapišinský  V. Figusch  A. Hajduk  J. Ivan  K. Iždinský 《Earth, Moon, and Planets》1996,73(1):7-14

Paper presents a reanalysis of four stratospheric particles registered in the Cosmic Dust Catalogs (CDC). Two particles have been classifed in the NASA Johnson Space Centre (JSC) in Houston as natural, terrestrial contaminants and the origin of the other two particles was found questionable, probably cosmic. Present study has confirmed these conclusions; however some new additional features in the chemical analysis have been found which can finally determine the classification of the particles with unclear origin.  相似文献   

Ejective and Sweeping Motions Above a Peatland and Their Role in Relaxed-Eddy-Accumulation Measurements and Turbulent Transport Modelling     

Gabriel Katul  Olli Peltola  Tiia Grönholm  Samuli Launiainen  Ivan Mammarella  Timo Vesala 《Boundary-Layer Meteorology》2018,169(2):163-184

The three turbulent velocity components, water vapour (\(\text {H}_2\text {O}\)), carbon dioxide (\(\text {CO}_{2}\)), and methane (\(\text {CH}_{4}\)) concentration fluctuations are measured above a boreal peatland and analyzed using conditional sampling and quadrant analysis. The overarching question to be addressed is to what degree lower-order cumulant expansion methods describe transport efficiency and the relative importance of ejections and sweeps to momentum, \(\text {CH}_{4}\), \(\text {CO}_{2}\) and \(\text {H}_2\text {O}\) fluxes across a range of atmospheric flow regimes. The patchy peatland surface creates distinctly different source and sink distributions for the three scalars in space and time thereby adding to the uniqueness of the set-up. The measured and modelled fractional contributions to the momentum flux show that sweep events dominate over ejections in agreement with prior studies conducted in the roughness sublayer. For scalar fluxes, ejections dominate the turbulent fluxes over sweeps. While ejective motions persist longer for momentum transport, sweeping events persist longer for all three scalars. Third-order cumulant expansions describe many of the results detailed above, and the results are surprising given the highly non-Gaussian distribution of \(\text {CH}_{4}\) turbulent fluctuations. Connections between the asymmetric contributions of sweeps and ejections and the flux-transport term arising in scalar turbulent-flux-budget closure are derived and shown to agree reasonably well with measurements. The proposed model derived here is much simpler than prior structural models used to describe laboratory experiments. Implications of such asymmetric contributions on, (i) the usage of the now proliferating relaxed-eddy-accumulation method in turbulent flux measurements, (ii) the constant-flux assumption, and (iii) gradient-diffusion closure models are presented.  相似文献   

Multiscale finite-volume method for density-driven flow in porous media   总被引：1，自引：0，他引：1  

Ivan Lunati  Patrick Jenny 《Computational Geosciences》2008,12(3):337-350

The multiscale finite-volume (MSFV) method has been developed to solve multiphase flow problems on large and highly heterogeneous domains efficiently. It employs an auxiliary coarse grid, together with its dual, to define and solve a coarse-scale pressure problem. A set of basis functions, which are local solutions on dual cells, is used to interpolate the coarse-grid pressure and obtain an approximate fine-scale pressure distribution. However, if flow takes place in presence of gravity (or capillarity), the basis functions are not good interpolators. To treat this case correctly, a correction function is added to the basis function interpolated pressure. This function, which is similar to a supplementary basis function independent of the coarse-scale pressure, allows for a very accurate fine-scale approximation. In the coarse-scale pressure equation, it appears as an additional source term and can be regarded as a local correction to the coarse-scale operator: It modifies the fluxes across the coarse-cell interfaces defined by the basis functions. Given the closure assumption that localizes the pressure problem in a dual cell, the derivation of the local problem that defines the correction function is exact, and no additional hypothesis is needed. Therefore, as in the original MSFV method, the only closure approximation is the localization assumption. The numerical experiments performed for density-driven flow problems (counter-current flow and lock exchange) demonstrate excellent agreement between the MSFV solutions and the corresponding fine-scale reference solutions.  相似文献   

A comparison of winter mercury accumulation at forested and no-canopy sites measured with different snow sampling techniques     

Sarah J. Nelson  Kenneth B. Johnson  Kathleen C. Weathers  Cynthia S. Loftin  Ivan J. Fernandez  Jeffrey S. Kahl  David P. Krabbenhoft 《Applied Geochemistry》2008

Atmospheric mercury (Hg) is delivered to ecosystems via rain, snow, cloud/fog, and dry deposition. The importance of snow, especially snow that has passed through the forest canopy (throughfall), in delivering Hg to terrestrial ecosystems has received little attention in the literature. The snowpack is a dynamic system that links atmospheric deposition and ecosystem cycling through deposition and emission of deposited Hg. To examine the magnitude of Hg delivery via snowfall, and to illuminate processes affecting Hg flux to catchments during winter (cold season), Hg in snow in no-canopy areas and under forest canopies measured with four collection methods were compared: (1) Hg in wet precipitation as measured by the Mercury Deposition Network (MDN) for the site in Acadia National Park, Maine, USA, (2) event throughfall (collected after snowfall cessation for accumulations of >8 cm), (3) season-long throughfall collected using the same apparatus for event sampling but deployed for the entire cold season, and (4) snowpack sampling. Estimates (mean ± SE) of Hg deposition using these methods during the 91-day cold season in 2004–2005 at conifer sites showed that season-long throughfall Hg flux (1.80 μg/m²) < snowpack Hg (2.38 ± 0.68 μg/m²) < event throughfall flux (5.63 ± 0.38 μg/m²). Mercury deposition at the MDN site (0.91 μg/m²) was similar to that measured at other no-canopy sites in the area using the other methods, but was 3.4 times less than was measured under conifer canopies using the event sampling regime. This indicates that snow accumulated under the forest canopy received Hg from the overstory or exhibited less re-emission of Hg deposited in snow relative to open areas. The soil surface of field-scale plots were sprayed with a natural rain water sample that contained an Hg tracer (²⁰²Hg) just prior to the first snowfall to explore whether some snowpack Hg might be explained from soil emissions. The appearance of the ²⁰²Hg tracer in the snowpack (0–64% of the total Hg mass in the snowpack) suggests that movement of Hg from the soil into the snowpack is possible. However, as with any tracer study the ²⁰²Hg tracer may not precisely represent the reactivity and mobility of natural Hg in soils.  相似文献   

Coupling of watersheds and coastal waters: An introduction to the dedicated issue     

Ivan Valiela 《Estuaries and Coasts》1992,15(4):429-430

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Mayfly communities in a prealpine stream system of Switzerland     

Daniel Hefti  Ivan Tomka 《Aquatic Sciences - Research Across Boundaries》1991,53(1):20-38

The mayfly fauna of twelve localities belonging to a calcareous stream system of the Swiss Prealps was investigated during the last three years. The stream system was divided into three distinct topographical sections, each characterized by specific thermic and geomorphologic components. Thirty-five mayfly species, that belonged to different faunistic groups according to their respective longitudinal distribution, were found. A coefficient of interspecific association was determined for every pair of species. Non-parametric indices of diversity, as well as a parametric analysis, were calculated for the three stations investigated quantitatively. The faunistic classification of the twelve stations reflected the main topographical organization of the stream system.  相似文献   

Calendar of events     

Ivan Vaní?ek  Martin Vaní?ek 《Geotechnical and Geological Engineering》1991,9(2):154-154

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