Deghosting by echo‐deblending

A marine source generates both a direct wavefield and a ghost wavefield. This is caused by the strong surface reflectivity, resulting in a blended source array, the blending process being natural. The two unblended response wavefields correspond to the real source at the actual location below the water level and to the ghost source at the mirrored location above the water level. As a consequence, deghosting becomes deblending (‘echo‐deblending’) and can be carried out with a deblending algorithm. In this paper we present source deghosting by an iterative deblending algorithm that properly includes the angle dependence of the ghost: It represents a closed‐loop, non‐causal solution. The proposed echo‐deblending algorithm is also applied to the detector deghosting problem. The detector cable may be slanted, and shot records may be generated by blended source arrays, the blending being created by simultaneous sources. Similar to surface‐related multiple elimination the method is independent of the complexity of the subsurface; only what happens at and near the surface is relevant. This means that the actual sea state may cause the reflection coefficient to become frequency dependent, and the water velocity may not be constant due to temporal and lateral variations in the pressure, temperature, and salinity. As a consequence, we propose that estimation of the actual ghost model should be part of the echo‐deblending algorithm. This is particularly true for source deghosting, where interaction of the source wavefield with the surface may be far from linear. The echo‐deblending theory also shows how multi‐level source acquisition and multi‐level streamer acquisition can be numerically simulated from standard acquisition data. The simulated multi‐level measurements increase the performance of the echo‐deblending process. The output of the echo‐deblending algorithm on the source side consists of two ghost‐free records: one generated by the real source at the actual location below the water level and one generated by the ghost source at the mirrored location above the water level. If we apply our algorithm at the detector side as well, we end up with four ghost‐free shot records. All these records are input to migration. Finally, we demonstrate that the proposed echo‐deblending algorithm is robust for background noise.     

加权人口密度连续分布模拟模型

人口分布是人文科学的重要研究对象。如何在数据不足的情况下获得较为符合实际的人口分布数据是业界的一大难题。为了解决这个问题,本文依照人口分布的规律,以人口统计数据、高程、海岸线、城市中心、河流和行政区划为影响因子,通过距离计算、重分类、加权计算等方法,建立了加权人口密度连续分布模拟模型,并进行了改进。基于2011年的福建省县级人口统计数据,本文在ArcGIS平台上模拟了该省的人口密度空间分布,模拟结果表明：各县人口总数误差与平均人口误差在40%以下的百分比分别为89%与84%,突出了福建省在城市中心、沿海、沿江,以及平原地区的人口聚集规律;各个县级行政区域内的数据集中,相对独立;县与县之间的区分边界数据又相互联系,变化连续,符合人口分布的实际情况。该模型相比于其他的人口模型易于理解,结构简单,可应用于各省的人口密度分布模拟,能有效地解决人口数据不足的问题。     

Mathematical modeling of colloid and virus cotransport in porous media: Application to experimental data

A conceptual mathematical model was developed to describe the simultaneous transport (cotransport) of viruses and colloids in three-dimensional, water saturated, homogeneous porous media with uniform flow. The model accounts for the migration of individual virus and colloid particles as well as viruses attached onto colloids. Viruses can be suspended in the aqueous phase, attached onto suspended colloids and the solid matrix, and attached onto colloids previously attached on the solid matrix. Colloids can be suspended in the aqueous phase or attached on the solid matrix. Viruses in all four phases (suspended in the aqueous phase, attached onto suspended colloid particles, attached on the solid matrix, and attached onto colloids previously attached on the solid matrix) may undergo inactivation with different inactivation coefficients. The governing coupled partial differential equations were solved numerically using finite difference methods, which were implemented explicitly or implicitly so that both stability and speed factors were satisfied. Furthermore, the experimental data collected by Syngouna and Chrysikopoulos [1] were satisfactorily fitted by the newly developed cotransport model.     

Investigation of pressure variations over stepped spillways using smooth particle hydrodynamics

In this paper the smoothed particle hydrodynamics, (SPH), technique is used to investigate the pressure distribution on steps located in the non-aerated flow region of a stepped spillway for different discharges typical of skimming flow conditions. The open source code 2D SPHysics has been employed after being validated against the laboratory model studies of flow over broad crested weirs and flow over stepped spillways. The numerical results, in terms of the water surface and velocity profiles at different sections, are in good agreement with the corresponding experimental results. The code is then applied to determine the pressure distribution on the vertical and horizontal step faces. Also, the aspects of the pressure pattern are described and the positions/magnitudes of the maximum and minimum pressure values are presented.     

音频大地电磁法强干扰压制试验研究

压制非相关噪声已经有多种比较成熟的技术方法,如多次叠加、远参考、Robust阻抗估计等,但这些方法对压制矿集区存在的强电磁噪声(通常属于相关噪声)基本没有效果.本文在前期提出的针对矿集区大地电磁强噪声压制的数学形态滤波基础上,开展了音频大地电磁法强干扰压制的试验研究.在四川西昌某AMT测点附近,布置了大功率电磁发送站,通过接地导线向地下供20A的方波电流,同时在供电期和间歇期用凤凰公司MTU-5A仪器观测4个水平分量的电磁场.在室内采用数学形态滤波和阈值法相结合的手段在时间域对含有人工源强干扰的电磁场数据进行处理,以视电阻率对比、频谱和极化方向等参数对处理效果进行评价.结果表明:结构元素恰当的数学形态滤波可以对人工源强干扰进行识别、去除,阈值法则可以消除形态滤波后的脉冲干扰,二者结合可以有效地压制AMT中的强干扰噪声,提高数据可靠性.本文结果对AMT实际资料处理具有重要的实用和参考价值.     

Power law statistics of cliff failures,scaling and percolation

Large cliff failures have often been characterized by their size in terms of the horizontal eroded area at the cliff top, and the maximum local retreat of the coastline. Field studies suggest that the frequencies of these two quantities decrease as power laws of their respective magnitudes, defining two different decay exponents. Moreover, the horizontal area increases as a power law of the maximum local retreat, identifying a third exponent. This observation indicates that the geometry of cliff failures are similar for different magnitudes. It is shown that, under a ‘scaling hypothesis’, the exponents satisfy a precise mathematical relation that relates the exponent of the distributions of magnitudes of events and their geometry – a relation that could be useful to validate the analysis of field catalogs. In order to understand the observed scenario, a numerical model of rocky coast erosion is developed. Despite its minimal character, it reproduces the observed cliff failure statistics. Strikingly, the model precisely reproduces the geometric similarity of cliff failures observed in the field. These results suggest that the theory of percolation, which underlies the erosion model, can possibly be used as a guide to decipher the physics of rocky coast erosion and could provide precise predictions of the statistics of cliff collapses. Copyright © 2015 John Wiley & Sons, Ltd.     

Alluvial floodplain classification by multivariate clustering and discriminant analysis for low‐relief glacially conditioned river catchments

River classifications provide useful frameworks to understand complex fluvial landscapes and to manage freshwater ecosystems. Alluvial floodplains for rivers in low‐relief glacially conditioned catchments of southern Ontario (Canada) are classified and tested using a sequence of multivariate statistical analyses. An original dataset of 109 floodplain sites is investigated using k‐means clustering, principal component analysis, and discriminant analysis statistical approaches. Four primary floodplain types are proposed representing basic morphological, stratigraphical, and sedimentological characteristics. Classifications are successfully discriminated by two principal dimensions: (1) stream power‐resistance; and (2) floodplain sedimentology. The latter is most efficiently represented by the availability of alluvial sand, and specifically a new variable defined as floodplain sand equivalent (FSE). Floodplain types are generally consistent with previous river classifications, however the glacial legacy requires refined classifications which account for inherited cobble bed materials and patterns of sand supply. Representing the residual variability of stream power‐resistance correlations, a third explanatory dimension of sediment transport is suggested, and may explain some within‐class variability in channel morphology. Balancing the opposing concepts of fluvial process domains and landform continuums, the potential for transitional floodplain types is also explored. The proposed first‐order alluvial floodplain classifications provide a basis from which to further investigate geomorphological diversity within the context of complex glacial legacy effects in low‐relief settings. Future research to reveal the spatial arrangement and linkages of distinct morphological groups within a regional landscape mosaic is expected to provide insights into patterns of post‐glacial fluvial adjustment. Copyright © 2014 John Wiley & Sons, Ltd.     

Anisotropic effective conductivity in fractured rocks by explicit effective medium methods

In this work, we assess the use of explicit methods for estimating the effective conductivity of anisotropic fractured media. Explicit methods are faster and simpler to use than implicit methods but may have a more limited range of validity. Five explicit methods are considered: the Maxwell approximation, the T‐matrix method, the symmetric and asymmetric weakly self‐consistent methods, and the weakly differential method, where the two latter methods are novelly constructed in this paper. For each method, we develop simplified expressions applicable to flat spheroidal “penny‐shaped” inclusions. The simplified expressions are accurate to the first order in the ratio of fracture thickness to fracture diameter. Our analysis shows that the conductivity predictions of the methods fall within known upper and lower bounds, except for the T‐matrix method at high fracture densities and the symmetric weakly self‐consistent method when applied to very thin fractures. Comparisons with numerical results show that all the methods give reliable estimates for small fracture densities. For high fracture densities, the weakly differential method is the most accurate if the fracture geometry is non‐percolating or the fracture/matrix conductivity contrast is small. For percolating conductive fracture networks, we have developed a scaling relation that can be applied to the weakly self‐consistent methods to give conductivity estimates that are close to the results from numerical simulations.     

Enhanced isotropic gradient operator

We present a two‐dimensional (2D) gradient operator that produces more accurate results than known traditional operators such as Ando, Sobel and the so‐called Isotropic operator. We further extend the derivation to three‐dimensional (3D), a powerful feature missing in all conventional operators. We start by constructing a parameterized formula that generically represents all 2D numerical gradient operators. We then solve for the required parameter by equating this numerical gradient with that obtained analytically from a single Fourier harmonic (or, equivalently here, a stationary plane wave). As this parameter is frequency‐ and direction‐dependent (by virtue of the underlying Fourier harmonic), we construe a pragmatic version of it that is independent of these two variables yet capable of significantly reducing the error associated with traditional operators. Extension to 3D is achieved similarly; it requires dealing with two parameters as opposed to only one in the 2D case. Synthetic and real‐data results confirm higher accuracy from this operator than from traditional ones.     

Edge‐on disk galaxies in the SDSS DR6: Fractions of bulgeless and other disk galaxies

The aim of this study is to determine the fractions of different spiral galaxy types, especially bulgeless disks, from a complete and homogeneous sample of 15 127 edge‐on disk galaxies extracted from the sixth data release from the Sloan Digital Sky Survey. The sample is divided in broad morphological classes and sub types consisting of galaxies with bulges, intermediate types and galaxies which appear bulgeless. A small fraction of disky irregulars is also detected. The morphological separation is based on automated classification criteria which resemble the bulge sizes and the flatness of the disks. Each of these broad classes contains about 1/3 of the total sample. Using strict criteria for selecting pure bulgeless galaxies leads to a fraction of 15% of simple disk galaxies. We compare this fraction to other galaxy catalogs and find an excellent agreement of the observed frequency of bulgeless galaxies. Although the fraction of simple disk galaxies in this study does not represent a “cosmic” fraction of bulgeless galaxies, it shows that the relative abundance of pure disks is comparable to other studies and offers a profound value of the frequency of simple disks in the local Universe. This fraction of simple disks emphasizes the challenge for formation and evolution models of disk galaxies since these models are hard pressed to explain the observed frequency of these objects (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)