青藏高原Pn波研究进展及问题     

李永华 《国际地震动态》2004,(2):6-8

简要总结了青藏高原地区Pn波速度结构、各向异性研究进展；介绍了Pn波速度结构、各向异性等在岩石圈结构、构造背景反映等方面的应用研究进展。分析了目前青藏高原Pn波研究中存在的一些问题。  相似文献   

应用Winsurf软件绘制磁场分布等值线图     

赖加成 《福建地震》2004,(3)

观测场地磁场梯度测量结果最直观的表述方式就是利用ΔF值绘制出的场地地磁场分布等值线图。Winsurf软件是Golden公司开发的一个三维绘图软件包 ,可以绘制各种等值线图和三维立体图。目前最高版本为 8.0。本文较为详细地介绍了利用这个软件的应用方法、步骤。  相似文献   

磁学方法在环境污染研究中的应用   总被引：12，自引：0，他引：12  

闫海涛  胡守云  朱育新 《地球科学进展》2004,19(2):230-236

环境磁学方法问世20多年来，因其快速、灵敏、经济、无破坏性和信息量大的特点，在环境研究中得到广泛的应用。磁学方法可用于传统的污染分析技术（化学）之前的预研究，而后者一般耗费大量人力、物力和时间；它也可以作为环境污染评估的代用指标。近年来，随着世界环境污染问题的日益严重和倍受关注，磁学方法被广泛地应用于环境污染研究。欧洲环境磁学专家在重建污染历史、监测现代环境污染、追踪与分离污染源等方面，开展了大量的工作，并积累了一定的经验。介绍了磁学方法用于污染研究的显著特点、主要用途、野外工作方法以及相关仪器；展示了在大气、土壤、潮滩、河流、湖泊以及海洋等不同环境系统中，利用磁手段研究污染的成果和最新进展；对 于一些难点、热点问题，如磁性机理的研究、多源污染的分离、强烈多变的人类活动对获取有意义磁讯号的影响等，做了相应的分析和总结。  相似文献   

Ambiguous Moment Tensors and Radiation Patterns in Anisotropic Media with Applications to the Modeling of Earthquake Mechanisms in W-Bohemia   总被引：3，自引：0，他引：3  

Rössler  D.  Rümpker  G.  Krüger  F. 《Studia Geophysica et Geodaetica》2004,48(1):233-250

Anisotropic material properties are usually neglected during inversions for source parameters of earthquakes. In general anisotropic media, however, moment tensors for pure-shear sources can exhibit significant non-double-couple components. Such effects may be erroneously interpreted as an indication for volumetric changes at the source. Here we investigate effects of anisotropy on seismic moment tensors and radiation patterns for pure-shear and tensile-type sources. Anisotropy can significantly influence the interpretation of the source mechanisms. For example, the orientation of the slip within the fault plane may affect the total seismic moment. Also, moment tensors due to pure-shear and tensile faulting can have similar characteristics depending on the orientation of the elastic tensor. Furthermore, the tensile nature of an earthquake can be obscured by near-source anisotropic properties. As an application, we consider effects of inhomogeneous anisotropic properties on the seismic moment tensor and the radiation patterns of a selected type of micro-earthquakes observed in W-Bohemia. The combined effects of near-source and along-path anisotropy cause characteristic amplitude distortions of the P, S1 and S2 waves. However, the modeling suggests that neither homogeneous nor inhomogeneous anisotropic properties alone can explain the observed large non-double-couple components.The results also indicate that a correct analysis of the source mechanism, in principle, is achievable by application of anisotropic moment tensor inversion.  相似文献   

Are open fractures necessarily aligned with maximum horizontal stress?     

Stephen E Laubach  Jon E Olson  Julia F.W Gale 《Earth and Planetary Science Letters》2004,222(1):191-195

Fluid flow in fractured rock is an increasingly central issue in recovering water and hydrocarbon supplies and geothermal energy, in predicting flow of pollutants underground, in engineering structures, and in understanding large-scale crustal behaviour. Conventional wisdom assumes that fluids prefer to flow along fractures oriented parallel or nearly parallel to modern-day maximum horizontal compressive stress, or S_Hmax. The reasoning is that these fractures have the lowest normal stresses across them and therefore provide the least resistance to flow. For example, this view governs how geophysicists design and interpret seismic experiments to probe fracture fluid pathways in the deep subsurface. Contrary to these widely held views, here we use core, stress measurement, and fluid flow data to show that S_Hmax does not necessarily coincide with the direction of open natural fractures in the subsurface (>3 km depth). Consequently, in situ stress direction cannot be considered to predict or control the direction of maximum permeability in rock. Where effective stress is compressive and fractures are expected to be closed, chemical alteration dictates location of open conduits, either preserving or destroying fracture flow pathways no matter their orientation.  相似文献   

Magnetic domain structures and stray fields of individual elongated magnetite grains revealed by magnetic force microscopy (MFM)     

C. Frandsen  S.L.S. Stipp 《Physics of the Earth and Planetary Interiors》2004,141(2):121-129

In order to clarify the relationship between microscopic and macroscopic magnetic features of a rock, we applied magnetic force microscopy (MFM) as a local probe on a sample with an intense natural remanent magnetisation, a norite from Heskestad, Norway. We studied in detail seven magnetite (Fe₃O₄) exsolution lamellae, five of them were about 30 μm long and a few micrometers wide, two were significantly shorter; all were located in their natural host, a grain of clinopyroxene. By combining MFM images of surface magnetisation with information about shape observed with optical microscopy (OM), the internal domain structure was determined for individual grains. In general, the lamellae were pseudo-single-domain grains with open-flux domain magnetisations parallel to their long axes. The domain sizes were, in cross-section, on the order of a micrometer for the longer lamellae and about 300 nm for the short lamellae. By increasing the separation between the MFM probe and the exposed end of the lamellae, information could be obtained about the decrease in stray fields for the individual grains. Close to the lamellae, stray fields were significant even for grains with multiple domains. The largest fields were found above the largest domains. A separation of approximately one domain width was found to be a characteristic distance, where stray fields from adjacent domains begin to cancel one another.  相似文献   

Investigation of upper crust anisotropy in Ghaen-Birjand region, east-central Iran     

Mohammad-Reza Gheitanchi  Zoya Zarifii 《地震学报(英文版)》2004,17(5):518-525

IntroductionWhenpropagatingthroughananisotropicmedium,ashearwavesplitsintotwo(quasi)shearwaveswithdifferentpropagationspeedsandpolarizedorthogonally.Owingtotherecentdevel-opmentofseismicobservationsystem,detectionofshearwavessplittingwithverysmalldelaytimesbetweenfasterandslowershearwavesbecameavailableandprovidedpowerfulapproachfordetectionofcrustalanisotropy.Crampin(1978)emphasizedtheroleofalignedmicrocracksasacauseofcrustalanisotropyandpointedoutthatforverticallyalignedmicrocracksthedirecti…  相似文献   

Magnetic fabrics and rock magnetism of Proterozoic dike swarm from the southern São Francisco Craton, Minas Gerais State, Brazil     

Maria Irene Bartolomeu Raposo  Alexandre O. Chaves  Paulo Lojkasek-Lima  Manoel Souza D'Agrella-Filho  Wilson Teixeira   《Tectonophysics》2004,378(1-2):43-63

Magnetic fabric and rock magnetism studies were performed on 32 mafic dikes of a Proterozoic dike swarm from the southern São Francisco Craton (SFC; Minas Gerais State, SE Brazil). Magnetic anisotropies were determined by applying anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of remanent magnetization (ARM). The latter was performed imposing both anhysteretic (total (AAR) and partial pAAR)) and isothermal remanence magnetizations (AIRM). Partial anhysteretic remanence anisotropy was performed based on remanent coercivity spectra from a pilot specimen of each site. In most sites, AMS is dominantly carried by ferromagnetic minerals, however, in some sites, the paramagnetic contribution exceeds 70% of bulk susceptibility. Rock magnetism and thin section analysis allow classifying the dikes as non-hydrothermalized and hydrothermalized. Magnetic measurement shows that the mean magnetic susceptibility is usually lower than 5×10⁻³ (SI). Ti-poor titanomagnetites up to pure magnetite pseudo-single-domain (PSD) grain sizes carry the majority of magnetic fabrics for non-hydrothermalized dikes whereas coarse to fine grained Ti-poor titanomagnetites carry the majority of magnetic fabrics for hydrothermalized dikes.Three primary AMS fabrics are recognized which are coaxial with ARM fabric, except for two dikes, from both non-hydrothermalized and hydrothermalized dikes. Normal AMS fabric surprisingly is not dominant (31%). The parallelism between AMS, pAAR_0–30, pAAR_30–60 and pAAR_60–90 fabrics in the hydrothermalized dikes indicates that magnetic grains formed due to late-stage crystallization or to remobilization of iron oxides due to hydrothermal alteration after dike emplacement have acquired a mimetic fabric coaxial with the primary fabric given by coarse-grained early crystallized Ti-poor titanomagnetites. This fabric is interpreted as magma flow in which the analysis of K_max inclination permitted the inference that the dikes were fed by horizontal or subhorizontal fluxes (K_max<30°). Intermediate AMS fabric is the most important (41%) in the investigated swarm. It is interpreted as due to vertical compaction of a static magma column with the minimum stress along the dike strike. ARM determinations for these sites also remained intermediate except for two dikes. In one of them, AIRM fabric resulted in normal AMS fabric while for the other AAR fabric resulted in inverse AMS fabric. A combination of AMS and ARM fabrics suggest that magmatic fabric for both dikes were overprinted by some late local event, probably related to Brasiliano orogenic processes after dike emplacement. InverseInverse AMS fabric is a minority (four dikes). ARM determinations also remained inverse suggesting a primary origin for inverse AMS fabric.  相似文献   

Analysis of geomagnetic field along seismic profile P4 of the International Project POLONAISE'97     

T. Grabowska  G. Bojdys 《Tectonophysics》2004,383(1-2):15-28

This paper presents relative secular variations of the total intensity of the geomagnetic field against a background of results of magnetic anomaly interpretation along seismic profile P4. Profile P4 crosses a Variscan folding zone in the Paleozoic Platform (PLZ), the Trans-European Suture Zone (TESZ), and the Polish part of the East European Craton (EEC). Secular geomagnetic field variations measured in 1966–2000 along a line adjacent to seismic profile P4 were analysed. The study of secular variations, reduced to the base recordings at the Belsk Magnetic Observatory, showed that the growth of geomagnetic field at the East European Craton was slower than in the Trans-European Suture Zone and the Paleozoic Platform.A 2D crustal magnetic model was interpreted as a result of magnetic modelling, in which seismic, geological and geothermal data were also used. The modelling showed that there were significant differences in the magnetic model for geotectonic units, which had been earlier determined based on deep seismic survey data. It should be noted that a fundamental change of trend of the relative secular variations was observed at the slope of the Precambrian Platform. After analysing the geomagnetic field observed along profile P4, the hypothesis that the contact between Phanerozoic and Precambrian Europe lies in Poland's territory can be proven.  相似文献   

Three-dimensional wave propagation in a general anisotropic poroelastic medium: phase velocity, group velocity and polarization   总被引：2，自引：0，他引：2  

M. D. Sharma 《Geophysical Journal International》2004,156(2):329-344

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