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431.
Detecting and quantifying sources of non-stationarity via experimental semivariogram modeling 总被引:1,自引:0,他引:1
Miguel A. Cuba Oy Leuangthong Julian M. Ortiz 《Stochastic Environmental Research and Risk Assessment (SERRA)》2012,26(2):247-260
Conventional geostatistics often relies on the assumption of second order stationarity of the random function (RF). Generally,
local means and local variances of the random variables (RVs) are assumed to be constant throughout the domain. Large scale
differences in the local means and local variances of the RVs are referred to as trends. Two problems of building geostatistical
models in presence of mean trends are: (1) inflation of the conditional variances and (2) the spatial continuity is exaggerated.
Variance trends on the other hand cause conditional variances to be over-estimated in certain regions of the domain and under-estimated
in other areas. In both cases the uncertainty characterized by the geostatistical model is improperly assessed. This paper
proposes a new approach to identify the presence and contribution of mean and variance trends in the domain via calculation
of the experimental semivariogram. The traditional experimental semivariogram expression is decomposed into three components:
(1) the mean trend, (2) the variance trend and (3) the stationary component. Under stationary conditions, both the mean and
the variance trend components should be close to zero. This proposed approach is intended to be used in the early stages of
data analysis when domains are being defined or to verify the impact of detrending techniques in the conditioning dataset
for validating domains. This approach determines the source of a trend, thereby facilitating the choice of a suitable detrending
method for effective resource modeling. 相似文献
432.
Ground-motion prediction equations (GMPEs) for spectral accelerations have traditionally focused on the range of response
periods most closely associated with the dynamic characteristics of buildings. Providing predictions only in this period range
(from 0.1 to 2 or 3 s) has also accommodated the assumed limitations on the usable period range resulting from the processing
of accelerograms. There are, however, engineering applications for which estimates of spectral ordinates are required at shorter
response periods. Recent work has demonstrated that high-frequency spectral ordinates are relatively insensitive to record
processing, contrary to previous assumptions. In the light of this finding, additional regressions are performed to extend
a recent pan-European GMPE to higher response frequencies. This model and others that also include coefficients for spectral
ordinates at several high response frequencies are used to explore options for interpolating coefficients for equations that
do not provide good coverage in this range. The challenges and uncertainties associated with such interpolations are discussed.
The paper concludes that a set of standard response frequencies could be usefully established for future GMPEs. 相似文献
433.
434.
A self‐limiting bank erosion mechanism? inferring temporal variations in bank form and skin drag from high resolution topographic data
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Julian Leyland Stephen E. Darby Liliana Teruggi Massimo Rinaldi Daniele Ostuni 《地球表面变化过程与地形》2015,40(12):1600-1615
Fluvial bank erosion rates are often quantified by assuming that the erosion rate is a function of the excess (above a critical threshold) boundary shear stress applied by the flow. Research has shown that the form roughness induced by natural topographic bank features, such as slumps, spurs and embayments, is the dominant component of the spatially‐averaged total shear stress, meaning that form roughness provides an important control on bank erosion rates. However, measuring the relative components of the total shear stress for a natural system is not straightforward. In this work we use the method of Kean and Smith to partition the form and skin drag components of river bank roughness using a time series (2005–2011) of high‐resolution topographic surveys of an eroding bank of the Cecina River in central Italy. This method approximates the form drag component of the roughness along a longitudinal bank profile as a series of user‐defined Gaussian curves. The extracted metrics are used in conjunction with an estimate of the outer region flow velocity to partition the form and skin drag components of the total boundary shear stress according to the Kean and Smith analytical solution. The relative magnitude of the form and skin shear stress at each survey date is analysed alongside DEMs of difference to reveal that intense episodes of erosion are followed by periods of quiescence. We show that this is due to the protection offered by increased form drag roughness following erosion. We conceptualise the dynamic feedbacks that exist between river discharge, bank erosion processes and bank form roughness, into a simple model of the self‐limiting nature of river bank erosion. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
435.
Heming Xu Arthur J. Rodgers Ilya N. Lomov Oleg Y. Vorobiev 《Pure and Applied Geophysics》2014,171(3-5):507-521
Seismic source characteristics of low-yield (0.5–5 kt) underground explosions are inferred from hydrodynamic simulations using a granite material model on high-performance (parallel) computers. We use a non-linear rheological model for granite calibrated to historical near-field nuclear test data. Equivalent elastic P-wave source spectra are derived from the simulated hydrodynamic response using reduced velocity potentials. Source spectra and parameters are compared with the models of Mueller and Murphy (Bull Seism Soc Am 61:1675–1692, 1971, hereafter MM71) and Denny and Johnson (Explosion source phenomenology, pp 1–24, 1991, hereafter DJ91). The source spectra inferred from the simulations of different yields at normal scaled depth-of-burial (SDOB) match the MM71 spectra reasonably well. For normally buried nuclear explosions, seismic moments are larger for the hydrodynamic simulations than MM71 (by 25 %) and for DJ91 (by over a factor of 2), however, the scaling of moment with yield across this low-yield range is consistent for our calculations and the two models. Spectra from our simulations show higher corner frequencies at the lower end of the 0.5–5.0 kt yield range and stronger variation with yield than the MM71 and DJ91 models predict. The spectra from our simulations have additional energy above the corner frequency, probably related to non-linear near-source effects, but at high frequencies the spectral slopes agree with the f ?2 predictions of MM71. Simulations of nuclear explosions for a range of SDOB from 0.5 to 3.9 show stronger variations in the seismic moment than predicted by the MM71 and DJ91 models. Chemical explosions are found to generate higher moments by a factor of about two compared to nuclear explosions of the same yield in granite and at normal depth-of-burial, broadly consistent with comparisons of nuclear and chemical shots at the US Nevada Test Site (Denny, Proceeding of symposium on the non-proliferation experiment, Rockville, Maryland, 1994). For all buried explosions, the region of permanent deformation and material damage is not spherical but extends along the free surface above and away from the source. The effect of damage induced by a normally buried nuclear explosion on seismic radiation is explored by comparing the motions from hydrodynamic simulations with those for point-source elastic Green’s functions. Results show that radiation emerging at downward takeoff angles appears to be dominated by the expected isotropic source contribution, while at shallower angles the motions are complicated by near-surface damage and cannot be represented with the addition of a simple secondary compensated linear vector dipole point source above the shot point. The agreement and differences of simulated source spectra with the MM71 and DJ91 models motivates the use of numerical simulations to understand observed motions and investigate seismic source features for underground explosions in various emplacement media and conditions, including non-linear rheological effects such as material strength and porosity. 相似文献
436.
Lei Xing Julian P. Sachs HaiLong Zhang Li Li ZhongQiang Ji MeiXun Zhao 《中国科学:地球科学(英文版)》2016,59(5):981-988
Hydrogen isotopes in lipid biomarkers can trace past changes in the hydrologic cycle. Recent studies have revealed the potential of hydrogen isotopes in microalgal lipids for quantitatively reconstructing water δ~2H(δD) values and salinity. In this study we collected suspended particles along a salinity gradient from the Changjiang River Estuary(CRE), and measured δD values in fatty acids in these particles. The results indicated that δD values of water were correlated highly with salinity from the CRE, in agreement with the results from other estuaries. δD values in palmitic acid and stearic acid had a positive correlation with δD values of water from the CRE. Nevertheless, in the CRE, hydrogen isotope fractionation in fatty acids relative to water increased as salinity increased, opposite the trend in hydrogen isotope fractionation with salinity found in microalgal culture and field studies. We attribute the increase in hydrogen isotope fractionation as salinity increased to light availability, which was likely lower in the particle rich mixing zone at the end of the estuary, and potentially as well to multiple sources of fatty acids in the CRE. 相似文献
437.
Visualizing geomorphology: improving communication of data and concepts through engagement with the arts
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438.
Political geography in general and military geography in particular have paid little attention to the creation of security landscapes. Distinctive elements of the security landscape are identified at the macro-, the meso- and the microscales and applied to the development of Israel's security landscape since 1948, particularly in the Golan Heights and the Negev. The landscape intensifies from the core toward the periphery, expands with technological advances, and changes most at the micro-level. 相似文献
439.
隗合明 《地球科学与环境学报》1997,(4)
马鞍桥金矿床是产于南秦岭泥盆系中的大型微细浸染型金矿,沉积—叠加改造成矿特征典型。矿体沿上泥盆统上部层位稳定分布,多与围岩整合产出。矿石类型有破碎蚀变岩型、蚀变千枚岩型、炭质片岩型和含金石英脉型。矿石构造有浸染状、细脉浸染状、细脉状和角砾状。矿物成分主要为黄铁矿、磁黄铁矿和石英、绢云母等,金主要呈显微金、超显微金分布于矿物粒间或被吸附于矿物表面。矿床由多种地质作用经过多期多阶段形成,主要受构造、沉积地层和岩浆活动控制。区域性大型韧性剪切带控制金矿带、金矿区的展布,地区性二级脆性断裂控制金矿床的范围,局部的更次级层间破碎带、断裂裂隙带控制矿体的就位。含了地层为矿床形成提供了主要的成矿物质。岩浆活动为形成合矿流体提供了热源并是热液运移的主要热动力。 相似文献
440.
Recordings from strong-motion accelerographs are of fundamental importance in earthquake engineering, forming the basis for all characterizations of ground shaking employed for seismic design. The recordings, particularly those from analog instruments, invariably contain noise that can mask and distort the ground-motion signal at both high and low frequencies. For any application of recorded accelerograms in engineering seismology or earthquake engineering, it is important to identify the presence of this noise in the digitized time-history and its influence on the parameters that are to be derived from the records. If the parameters of interest are affected by noise then appropriate processing needs to be applied to the records, although it must be accepted from the outset that it is generally not possible to recover the actual ground motion over a wide range of frequencies. There are many schemes available for processing strong-motion data and it is important to be aware of the merits and pitfalls associated with each option. Equally important is to appreciate the effects of the procedures on the records in order to avoid errors in the interpretation and use of the results. Options for processing strong-motion accelerograms are presented, discussed and evaluated from the perspective of engineering application. 相似文献