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41.
全波形反演是一种利用地震波传播的动力学特征来获取地下介质物性参数的反演方法,可为揭示地下精细结构提供重要依据。本文以弹性波方程作为数学模型来模拟地震波传播规律并进行相应的反演方法研究。为提高计算效率与反演结果的准确性,可将近似解析离散化(NAD)算子用于频率域弹性波方程的正演模拟。本文在频率域NAD离散的基础上推导阻抗矩阵的稀疏分块结构与反演目标函数对模型参数的梯度计算公式,由此建立基于NAD算子的频率域弹性波全波形反演方法。为验证该方法的有效性,文中通过数值实验对多种典型介质模型进行反演计算,均得到了理想的反演结果。
相似文献42.
The application of the saddlepoint approximation to reliability analysis of dynamic systems is investigated. The failure event in reliability problems is formulated as the exceedance of a single performance variable over a prescribed threshold level. The saddlepoint approximation technique provides a choice to estimate the cumulative distribution function (CDF) of the performance variable. The failure probability is obtained as the value of the complement CDF at a specif ied threshold. The method requires computing the saddlepoint from a simple algebraic equation that depends on the cumulant generating function (CGF) of the performance variable. A method for calculating the saddlepoint using random samples of the performance variable is presented. The applicable region of the saddlepoint approximation is discussed in detail. A 10-story shear building model with white noise excitation illustrates the accuracy and effi ciency of the proposed methodology. 相似文献
43.
The Ocean Drilling Program (ODP) initiated drilling at Site 1256D in the Guatemala Basin, about 1,000 km off the East Pacific
Rise to penetrate plutonic rocks, anticipated to be relatively shallow in this region, formed at an ultra-fast spreading rate.
IODP Expedition E312 successfully drilled into gabbros at ~1,150 m in basement. Multi-channel seismic traces show weak laterally
coherent sub-basement reflections at borehole depths. Synthetic reflectivity seismograms were computed using a Ricker wavelet
and impedance profiles from borehole sonic logs. These seismograms show significant sub-basement amplitude peaks. A zero-offset
vertical seismic profile, shot on E312, was processed to investigate the authenticity of these reflections and their relationship
to borehole geology. A dual scheme of the median filtering and F–K dip filtering was used. Tests with synthetic seismograms
indicate the approach is effective at reasonable SNR levels. Downgoing energy is clearly identified but negligible upgoing
energy is visible over random noise. These results indicate that lava flows and igneous contacts in upper ocean crust have
significant topography on lateral scales less than the Fresnel Zone (~300 m) due to igneous and tectonic processes. 相似文献
44.
45.
Microbial community dynamics and biodegradation of polycyclic aromatic hydrocarbons in polluted marine sediments in Hong Kong 总被引:1,自引:0,他引:1
Dynamics of microbial community and biodegradation of polycyclic aromatic hydrocarbons (PAHs) in polluted marine sediments, artificially spiked with a mixture of PAHs (fluorene, phenanthrene, fluoranthene and pyrene), were examined for a period of 60 days. Microbial communities were characterised by bacterial counts, ester-linked fatty acid methyl ester (EL-FAME) analysis and denaturing gradient gel electrophoresis (DGGE). A noted reduction in species diversity occurred only in the high PAH level treatment at onset. Both EL-FAME and DGGE demonstrated a marked shift in microbial community, in all the PAH level treatments, afterwards, with increases in the number of fatty acid degraders, the relative abundance of fatty acid biomarkers for gram-negative bacteria and a decrease in species diversity. The shift was also accompanied by the significant decrease in PAH concentrations. By the end of the experiment, diversity indices, based on both approaches, recovered when PAH concentrations declined to their background levels, except in the high PAH level treatment. 相似文献
46.
Eugene Yee 《Boundary-Layer Meteorology》2001,98(3):517-527
A simple analytical model is developed for the meanupcrossing rate of plume concentration fluctuations assuming that thisprocess can be well approximated by a lognormal process. The resultingexpression requires only the specification of the in-plume fluctuationintensity and in-plume Taylor micro-time scale and, hence, does notexplicitly involve the joint probability density function of theconcentration and its derivative. The analytical model provides agood fit to some field measurements of the mean upcrossing rate ina dispersing plume. 相似文献
47.
This study analyzed the uncertainty of inversion and the resolution limit in the presence of noise by means of statistical experiments. The exhaustive method is adopted to obtain the global optimal solution in each experiment. We found that even with small level of noise, solutions fluctuate in a large range for the thin bed. The distribution of solutions in the presence of noise is closely related to the spread of the cost function in the absence of noise. As a result, the area of a certain neighborhood around the true solution on the spread of the cost function in the absence of noise is used to evaluate the uncertainty of inversion and the resolution limit in the presence of noise. In the case that the SNR (signal-to-noise ratio) is 5 in this study, solutions focus around the true solution with a very small uncertainty only when the bed thickness is greater than the reciprocal of the double predominant frequency of the convoluting wavelet. 相似文献
48.
Three finite element codes, namely TELEMAC, ADCIRC and QUODDY, are used to compute the spatial distributions of the M2, M4 and M6 components of the tide in the sea region off the west coast of Britain. This region is chosen because there is an accurate
topographic dataset in the area and detailed open boundary M2 tidal forcing for driving the model. In addition, accurate solutions (based upon comparisons with extensive observations)
using uniform grid finite difference models forced with these open boundary data exist for comparison purposes. By using boundary
forcing, bottom topography and bottom drag coefficients identical to those used in an earlier finite difference model, there
is no danger of comparing finite element solutions for “untuned unoptimised solutions” with those from a “tuned optimised
solution”. In addition, by placing the open boundary in all finite element calculations at the same location as that used
in a previous finite difference model and using the same M2 tidal boundary forcing and water depths, a like with like comparison of solutions derived with the various finite element
models was possible. In addition, this open boundary was well removed from the shallow water region, namely the eastern Irish
Sea where the higher harmonics were generated. Since these are not included in the open boundary, forcing their generation
was determined by physical processes within the models. Consequently, an inter-comparison of these higher harmonics generated
by the various finite element codes gives some indication of the degree of variability in the solution particularly in coastal
regions from one finite element model to another. Initial calculations using high-resolution near-shore topography in the
eastern Irish Sea and including “wetting and drying” showed that M2 tidal amplitudes and phases in the region computed with TELEMAC were in good agreement with observations. The ADCIRC code
gave amplitudes about 30 cm lower and phases about 8° higher. For the M4 tide, in the eastern Irish Sea amplitudes computed with TELEMAC were about 4 cm higher than ADCIRC on average, with phase
differences of order 5°. For the M6 component, amplitudes and phases showed significant small-scale variability in the eastern Irish Sea, and no clear bias between
the models could be found. Although setting a minimum water depth of 5 m in the near-shore region, hence removing wetting
and drying, reduced the small-scale variability in the models, the differences in M2 and M4 tide between models remained. For M6, a significant reduction in variability occurred in the eastern Irish Sea when a minimum 5-m water depth was specified. In
this case, TELEMAC gave amplitudes that were 1 cm higher and phases 30° lower than ADCIRC on average. For QUODDY in the eastern
Irish Sea, average M2 tidal amplitudes were about 10 cm higher and phase 8° higher than those computed with TELEMAC. For M4, amplitudes were approximately 2 cm higher with phases of order 15° higher in the northern part of the region and 15° lower
in the southern part. For M6 in the north of the region, amplitudes were 2 cm higher and about 2 cm lower in the south. Very rapid M6 tidal-phase changes occurred in the near-shore regions. The lessons learned from this model inter-comparison study are summarised
in the final section of the paper. In addition, the problems of performing a detailed model–model inter-comparison are discussed,
as are the enormous difficulties of conducting a true model skill assessment that would require detailed measurements of tidal
boundary forcing, near-shore topography and precise knowledge of bed types and bed forms. Such data are at present not available. 相似文献
49.
大别造山带的汇聚演化历史一直存在不同的认识,来自扬子陆块和华北陆块显生宙以来的地层记录以及北淮阳地区的地层记录可能为两个陆块的汇聚模式提供制约。通过对扬子陆块与华北陆块显生宙以来岩相古地理进行比较,其中3个阶段相似, 2个阶段差别很大。以浊积岩发育作为俯冲阶段的标志,磨拉石建造出现作为闭合的标志,显生宙以来,大别造山带曾经历了两次俯冲和两次汇聚,俯冲时间分别是晚奥陶世—早志留世和早三叠世,汇聚的时间分别为中 晚泥盆世和中三叠世,早期的汇聚具有软碰撞特征,以上认知得到了北淮阳地区地层、古生物和沉积记录的支持,同时也与大别山造山带榴辉岩的同位素年龄数据基本吻合。高压 超高压变质岩在晚三叠世处于快速折返阶段,早侏罗世以前到达地表,其平均折返隆升速率为39~66mm/a。 相似文献
50.
The West Kunlun orogenic belt is located at the conjunction of the paleo-Asian tectonic system and the Tethys tectonic system.
Petrological and mineralogical studies of the Early Cambrian metamorphic surface crust in this region have shown that in case
the metamorphism reached low-temperature granulate facies, the typical mineral assemblage is biotite-garnet-silimanite-K feldspar-plagioclase-quartz.
The peak metamorphic temperatures are within the range of 720–740°C and the pressure is 0.6 GPa ±. Three types of metamorphic
zircon have been detected in the metamorphic rocks: the complex inclusion-bearing type ; the early relic zircon inclusion-bearing
type; and the inclusion-free type. SHRIMP age determination of these three types of metamorphic zircon have revealed that
these zircons were formed principally during 400–460 Ma, indicating that pre-Cambrian metamorphic surface crust rocks underwent
low-temperature granulite facies metamorphism during the Caledonian. In combination with the geological characteristics of
this region, it is considered that when the oceanic basin was closed, there occurred intense intracontinental subduction (type
A), bringing part of the Early Cambrian metamorphic basement in this region downwards to the lower crust. Meanwhile, there
were accompanied with tectonic deformation at deep levels and medium- to high-grade metamorphism. This study provided important
chronological and mineralogical evidence for the exploration of the evolutionary mechanism and process of the West Kunlun
Early Paleozoic.
Part of the results from the research project “ Research on the West Kunlun pre-Cambrian tectonic events” under the program
“ Research on the important geological problems of China’ s pre-Cambrian” (No. 200113900070) sponsored by the China National
Geological Surveying Bureau. 相似文献