The attenuation mechanism of seismic waves in northwestern Himalayas   总被引：3，自引：0，他引：3  

S. Mukhopadhyay  C. Tyagi  S. S. Rai 《Geophysical Journal International》2006,167(1):354-360

We analysed local earthquake waveforms recorded on a broad-band seismic network in northwestern Himalayas to compute the intrinsic and scattered attenuation parameters from coda waves. Similar to other tectonically active and heterogeneous regions, attenuation-frequency relation for western Himalaya is   Q ⁻¹ _c = (113 ± 7)  f ^(1.01±0.05)  where   Q_c   is the coda Q parameter. Intrinsic  ( Q ⁻¹ _i )  and scattering  ( Q ⁻¹ _s )  attenuations was separated using   Q_c   and direct S -wave Q data  ( Q_d )  . It is observed that estimated   Q ⁻¹ _c   is close to   Q ⁻¹ _i   and both of them are much larger than   Q ⁻¹ _s   suggesting that coda decay is predominantly caused by intrinsic attenuation. At higher frequencies, both the attenuation parameters   Q_c   and,   Q_d   are similar indicating that coda is predominantly composed of back-scattered S waves at these frequencies.  相似文献   

Backscattering from spherical elastic inclusions and accuracy of the Kirchhoff approximation for curved interfaces     

David W. Eaton 《Geophysical Journal International》2006,166(3):1249-1258

The Kirchhoff (or tangent plane) approximation, derived from the theoretically complete Kirchhoff–Helmholtz integral representation for the seismic wavefield, has been used extensively for the analysis of seismic-wave scattering from irregular interfaces; however, the accuracy of this method for curved interfaces has not been rigorously established. This paper describes an efficient Kirchhoff algorithm to simulate scattered waves from an arbitrarily curved interface in an elastic medium. Synthetic seismograms computed using this algorithm are compared with exact synthetics computed using analytical formulae for scattering of plane P waves by a spherical elastic inclusion. A windowing technique is used to remove strong internal reverberations from the analytical solution. Although the Kirchhoff method tends to underestimate the total scattering intensity, the accuracy of the approximation improves with increasing value of the wavenumber-radius product, kR . The arrival times and pulse shapes of primary reflections from the sphere are well approximated using the Kirchhoff approach regardless of curvature of the scattering surface, but the amplitudes are significantly underestimated for kR ≤ 5. The results of this work provide some new guidelines to assess the accuracy of Kirchhoff-synthetic seismograms for curved interfaces.  相似文献   

环境与减灾小卫星高光谱成像仪陆地气溶胶光学厚度反演   总被引：6，自引：1，他引：6  

孙林  柳钦火  陈良富  刘强 《遥感学报》2006,10(5):770-776

环境与减灾小卫星高光谱成像仪是搭载在中国将于2007年发射的环境与减灾小卫星A星（HJ-1A）上的传感器之一，在0．45—0．95μm波段范围内设置了约135个通道，空间分辨率为100m，回访周期约为96h。本文根据传感器的参数设置，使用HYPERION数据仿真模拟了HJ-1A高光谱数据，探讨了它在陆地气溶胶光学厚度探测中的应用。文中使用浓密植被算法反演了模拟数据的气溶胶光学厚度。结果表明，HJ-1A高光谱成像仪能够很好地应用于陆地气溶胶光学厚度的反演。  相似文献   

The Theory of Coda Wave Interferometry   总被引：7，自引：0，他引：7  

Roel Snieder 《Pure and Applied Geophysics》2006,163(2-3):455-473

Coda waves are sensitive to changes in the subsurface because the strong scattering that generates these waves causes them to repeatedly sample a limited region of space. Coda wave interferometry is a technique that exploits this sensitivity to estimate slight changes in the medium from a comparison of the coda waves before and after the perturbation. For spatially localized changes in the velocity, or for changes in the source location, the travel-time perturbation may be different for different scattering paths. The coda waves that arrive within a certain time window are therefore subject to a distribution of travel-time perturbations. Here I present the general theory of coda wave interferometry, and show how the time-shifted correlation coefficient can be used to estimate the mean and variance of the distribution of travel-time perturbations. I show how this general theory can be used to estimate changes in the wave velocity, in the location of scatterer positions, and in the source location.  相似文献   

Chemical characteristics of fine particles during spring dust storm dominant period in two Chinese cities, Baotou and Wuwei     

Min HU Fan YANG Dingli YUE Song GUO Mengting WEN Wenting ZHANG Feng TIAN 《中国地球化学学报》2006,25(B08):221-221

Mineral dust, including dust storm, fugitive and floating dust are the most important components of ambient aerosol in northern China during spring. Meanwhile, it goes through heating period when fossil fuel combustion acts as a dominant source. In order to understand the chemical characteristics of fine particles in North China, two sampling sites was established in Baotou City in the west of Inner Mongolia Autonomous Region and Wuwei City near the edge of Gobi to collect fine particles （PM 2.5） samples in the spring of 2004 and 2005, and analyze mass concentrations, ionic species and some element components of PM 2.5. The results indicated that dust events elevated the mass concentrations of particulate matter significantly, not only for coarse particles, but also for fine particles. Crustal components, organic matter and sulfates are three major compositions in fine particles, their mass concentrations and relative abundances are closely related with the frequency of dust events and coal combustion. The characteristics of fine particles are obviously diverse under different atmospheric conditions. （1） When the dust storms take place, the wind speeds are often higher than 10 m/s. A plenty of mineral aerosol is input from long distance, and also local soil dust is blown up to suspend in the air as particles. Therefore, the concentrations of PM 2.5 are enhanced dramatically. Mass concentrations and relative percentages of crustal components in PM 2.5 increased significantly during dust events. The mineral dust may help gas to particle conversation by heterogeneous reactions. Increases in mass concentrations of SO4^2-, NO3^- and NH4^＋ and organic matter in PM 2.5 were observed evidently. But their fractions in PM 2.5 do not increase simultaneously due to dilution of storing wind. （2） When wind velocity is lower （〈3 m/s）, air pollutants are accumulated, both mass concentrations of SO4^2-, NO3^- and NH4^＋ and organic matter, and their fractions in PM 2.5 increase, indicating severe anthropogenic pollution. The chemical characteristics of PM 2.5 in Baotou and Wuwei are also influenced by their industrial structure, topography and meteorological conditions.  相似文献   

Monsoon control on trace metal fluxes in the deep Arabian Sea     

T. M. Balakrishnan Nair 《Journal of Earth System Science》2006,115(4):461-472

Particulate fluxes of aluminium, iron, magnesium and titanium were measured using six time-series sediment traps deployed in the eastern, central and western Arabian Sea. Annual Al fluxes at shallow and deep trap depths were 0.47 and 0.46 g m^-2 in the western Arabian Sea, and 0.33 and 0.47 g m^-2 in the eastern Arabian Sea. There is a difference of about 0.9–1.8 g m^-2y^-1 in the lithogenic fluxes determined analytically (residue remaining after leaching out all biogenic particles) and estimated from the Al fluxes in the western Arabian Sea. This arises due to higher fluxes of Mg (as dolomite) in the western Arabian Sea (6–11 times higher than the eastern Arabian Sea). The estimated dolomite fluxes at the western Arabian Sea site range from 0.9 to 1.35gm^-2y^-1. Fe fluxes in the Arabian Sea were less than that of the reported atmospheric fluxes without any evidence for the presence of labile fraction/excess of Fe in the settling particles. More than 75% of Al, Fe, Ti and Mg fluxes occurred during the southwest (SW) monsoon in the western Arabian Sea. In the eastern Arabian Sea, peak Al, Fe, Mg and Ti fluxes were recorded during both the northeast (NE) and SW monsoons. During the SW monsoon, there exists a time lag of around one month between the increases in lithogenic and dolomite fluxes. Total lithogenic fluxes increase when the southern branch of dust bearing northwesterlies is dragged by the SW monsoon winds to the trap locations. However, the dolomite fluxes increase only when the northern branch of the northwesterlies (which carries a huge amount of dolomite accounting 60% of the total dust load) is dragged, from further north, by SW monsoon winds. The potential for the use of Mg/Fe ratio as a paleo-monsoonal proxy is examined.  相似文献   

Nonlinear Response of Single Piles in Sand Subjected to Lateral Loads using k hmax Approach     

Sanjeev Kumar  Latika Lalvani  Maher Omar 《Geotechnical and Geological Engineering》2006,24(1):163-181

This paper presents results of analysis of full-scale pile load test data of 14 piles embedded in either loose or medium dense sands. The analysis was performed using two methods, p–y curve approach and a more recently developed k_hmax approach. Comparison of the results obtained using both the methods is also presented. A step-by-step analysis procedure is presented for predicting lateral load deflection response of single piles in sand using the k_hmax approach. The results presented show that the k_hmax approach has promise over the p–y curve approach because of its simplicity and the fact that it provides upper- and lower-bound curves, which are valuable guides to making engineering decisions. For loose sands, a new range of k_hmax values is recommended to better predict the lateral load–deflection response of single piles.  相似文献   

Attenuation, transport and diffusion of scalar waves in textured random media     

L. Margerin   《Tectonophysics》2006,416(1-4):229

Most theoretical investigations of seismic wave scattering rely on the assumption that the underlying medium is statistically isotropic. However, deep seismic soundings of the crust as well as geological observations often reveal the existence of elongated or preferentially oriented scattering structures. In this paper, we develop mean field and radiative transfer theories to describe the attenuation and multiple scattering of a scalar wavefield in an anisotropic random medium. The scattering mean free path is found to depend strongly on the propagation direction. We derive a radiative transfer equation for statistically anisotropic random media from the Bethe–Salpeter formalism and propose a Monte Carlo method to solve this equation numerically. At longer times, the energy density is shown to obey a tensorial diffusion equation. The components of the diffusion tensor are obtained in closed form and excellent agreement is found between Monte Carlo simulations and analytical solutions of the diffusion equation. The theory has important potential implications for lithospheric models where scatterers are for example flat structures preferentially aligned along the surface. In this simple geometry, analytical expressions of the Coda Q parameter can be given explicitly in the diffusive regime. Our results suggest that pulse broadening and coda decay are controlled by different parameters, related to the eigenvalues of the diffusion tensor. These eigenvalues can differ by more than one order of magnitude. This theory could be applied to probe the anisotropy of length scales in the lithosphere.  相似文献   

云南地区尾波衰减特征研究          下载免费PDF全文

王伟君  刘杰  陈凌 《地震学报》2006,28(1):8-19

利用22个云南区域数字地震台网近震资料，研究了云南地区地震尾波在1~20 Hz内6个频段的衰减特征. 首先使用单次散射模型计算各台站的尾波衰减系数； 然后利用随机各向同性散射介质均匀分布的多次散射模型， 使用多流逝时间窗分析方法，计算各台站的平均自由程Le和地震反照率B0得到总衰减系数，并从中分离散射衰减系数 、吸收衰减系数 . 云南地区基本衰减特征是Le主要集中在10~30 km内， 在2~6 Hz之间有最大值；B0在1~2 Hz大致为0.5左右，在其它频段则小于0.5. 也即在1~2 Hz， 与 数值相当， 其它频段 逐渐大于，成为尾波总衰减的主要原因. 除1~2 Hz外，其它频段 值接近. 云南地区的Q-1尤其是， 在空间上有较大的差异，滇东略微高于滇西北， 滇西北高于滇西南地区. 与全球其它区域的研究结果比较发现，云南地区 略低于平均水平， 略高于平均水平， 而则处于其它区域结果的中间.   相似文献   

黑碳气溶胶研究进展Ⅰ：排放、清除和浓度   总被引：28，自引：0，他引：28  

许黎  王亚强  陈振林  罗勇  任万辉 《地球科学进展》2006,21(4):352-360

黑碳气溶胶是近几年非常活跃的一个研究课题，因为黑碳气溶胶吸收太阳和地球大气的辐射，对全球气候变暖有重要的影响。中国黑碳气溶胶的排放和浓度比同纬度的高，在全球气候变暖的大背景下，中国黑碳气溶胶一直受到国外的关注。综述了国内外黑碳气溶胶研究的最新进展，受篇幅的限制，文章分两篇，第一篇是黑碳气溶胶的排放、清除和浓度，第二篇是黑碳气溶胶的气候效应和拓展的研究领域。从黑碳排放的估算和大气浓度的测量方法进行描述，列出有关的测量结果，对黑碳气溶胶的排放和大气浓度进行国内外的初步比较分析。还对黑碳气溶胶排放和浓度测量误差进行了讨论，并对今后黑碳气溶胶研究提出了几点建议。  相似文献