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This letter proposes a new model for the second-order statistics of spatial texture in synthetic aperture radar images. The autocovariance function is locally approximated by a two-dimensional anisotropic Gaussian kernel (AGK) to characterize texture by its local orientation and anisotropy. The estimation of texture parameters at a given scale is based on the gradient structure tensor operator and does not require the explicit computation of the autocovariance. Finally, a new filter called AGK minimum mean square error (MMSE) that takes into account this spatial information is introduced and compared with the refined MMSE filter. The proposed filter has better performance in terms of texture preservation and structure enhancement 相似文献
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Major element compositional overlap exists between microspherules of different microtektite layers or strewnfields. For this reason, microspherules of similar composition cannot, a priori, be assumed to belong to the same microtektite event and those of different compositions cannot, a priori, be assumed to result from different events. Nevertheless, despite major element compositional overlap between microspherules of different strewnfields, multivariate factor analysis shows microtektites and related microspherules of three stratigraphically different late Eocene layers to follow recognizably different compositional trends. The microtektite population of the North American strewnfield (Globorotalia cerroazulensis Zone) follows compositionally well defined trends and is characterized by high concentrations of SiO2, Al2O3, and TiO2. The microspherule population of the slightly older crystal-bearing Globorotalia cerroazulensis Zone microspherule layer is more heterogeneous and characterized by microspherules which are relatively enriched in FeO and MgO and relatively impoverished in SiO2 and TiO2. The microspherule population of the oldest microspherule layer in the uppermost Globigerapsis semiinvoluta Zone is highly heterogeneous and characterized by microspherules which are relatively enriched in CaO and impoverished in Al2O3 and Na2O. Individual microspherules of this oldest late Eocene horizon often exhibit major element compositions similar to those of the lower Gl. cerroazulensis Zone layer and occasionally exhibit major element compositions similar to North American layer microtektites. Nevertheless, late Eocene microspherule occurrences can be assigned to appropriate late Eocene microtektite horizons on the basis of major element compositional trends. 相似文献
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G. Keller S.L. D'Hondt C.J. Orth J.S. Gilmore P.Q. Oliver E.M. Shoemaker E. Molina 《Meteoritics & planetary science》1987,22(1):25-60
Recent discoveries of microtektite and related crystal bearing microspherule layers in deep-sea sediments of the west equatorial Pacific DSDP Sites 292, 315A and 462, off-shore New Jersey in Site 612 and in southern Spain have confirmed the presence of at least three microspherule layers in Late Eocene sediments. Moreover, these discoveries have extended the North American strewn field from the Caribbean and Gulf of Mexico region to the northwest Atlantic, and have established a third strewn field in western equatorial Pacific and Indian Ocean which may extend to the Mediterranean. Stratigraphically the oldest microspherule layer occurs in the planktonic foraminifer Globigerapsis semiinvoluta Zone about 0.5 m.y. prior to the closely spaced crystal bearing microspherule layer and North American microtektite layer in the Globorotalia cerroazulensis Zone. Major element composition of the G. semiinvoluta Zone layer and the crystal bearing microspherule layer overlap, but there is a clear trend towards higher Al2O3 and FeO values in SiO2 equivalent microspherules of the latter layer. The G. semiinvoluta Zone microspherules also contain a higher percentage of non-crystalline spherules (microtektites) than the crystal bearing microspherule layer, but lower than the North American microtektite layer. Excess iridium due to an abrupt increase in supply is associated with the middle crystal bearing microspherule layer and to a lesser extent with the other two layers. But, Ir excess due to concentration as a result of carbonate loss was also observed at two sites (462, 612). The three late Eocene microspherule layers do not precisely coincide with planktonic foraminiferal species extinctions, but a major faunal assemblage change is associated with the G. semiinvoluta Zone layer. Abundant pyrite is present in the North American microtektite layer of DSDP Site 612 suggesting reducing conditions possibly due to a sudden influx of biologic matter (dead bodies) to the ocean floor, and the crystal bearing microspherule layer coincides with five radiolarian extinctions. All three microspherule layers are associated with decreased carbonate possibly due to sudden productivity changes, increased dissolution as a result of sea-level and climate fluctuations, or the impact events. 相似文献
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大洋钻探对洋底以下生命的探索 总被引：4，自引：0，他引：4
StevenD’Hondt 《地球科学进展》2003,18(5):759-763
对ODP采集的沉积物样品的地球化学研究揭示，在全世界所有的大洋底以下都发现了细菌活动，洋底沉积物中的生物圈可能构成了地球生物数量的 1／3，该生物圈通过调节海洋沉积物的成岩作用和大洋玄武岩的风化，影响到长期的全球生物地球化学循环。
DSDP最早提出了海洋沉积物中细菌活动性的证据，ODP的一系列航次提供了各种不同环境下深部生物圈细菌群落以及细菌参与的地球化学过程纪录。ODP201是第一个专门致力于研究洋底以下生命的大洋钻探航次，它记录了从富有机质的边缘沉积到贫有机质的开阔海沉积环境的洋底以下细菌活动性和细菌群落的连续性，201航次还揭示，古海洋环境对现在正活跃于深海沉积物中的原核生物群落造成了影响。 相似文献
DSDP最早提出了海洋沉积物中细菌活动性的证据，ODP的一系列航次提供了各种不同环境下深部生物圈细菌群落以及细菌参与的地球化学过程纪录。ODP201是第一个专门致力于研究洋底以下生命的大洋钻探航次，它记录了从富有机质的边缘沉积到贫有机质的开阔海沉积环境的洋底以下细菌活动性和细菌群落的连续性，201航次还揭示，古海洋环境对现在正活跃于深海沉积物中的原核生物群落造成了影响。 相似文献
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