全文获取类型
收费全文 | 960篇 |
免费 | 38篇 |
国内免费 | 5篇 |
专业分类
测绘学 | 16篇 |
大气科学 | 68篇 |
地球物理 | 257篇 |
地质学 | 260篇 |
海洋学 | 85篇 |
天文学 | 189篇 |
综合类 | 12篇 |
自然地理 | 116篇 |
出版年
2021年 | 10篇 |
2020年 | 10篇 |
2019年 | 8篇 |
2018年 | 15篇 |
2017年 | 18篇 |
2016年 | 13篇 |
2015年 | 14篇 |
2014年 | 25篇 |
2013年 | 65篇 |
2012年 | 28篇 |
2011年 | 35篇 |
2010年 | 40篇 |
2009年 | 39篇 |
2008年 | 42篇 |
2007年 | 39篇 |
2006年 | 38篇 |
2005年 | 49篇 |
2004年 | 39篇 |
2003年 | 26篇 |
2002年 | 25篇 |
2001年 | 20篇 |
2000年 | 28篇 |
1999年 | 24篇 |
1998年 | 25篇 |
1997年 | 22篇 |
1996年 | 13篇 |
1995年 | 19篇 |
1994年 | 20篇 |
1993年 | 15篇 |
1992年 | 14篇 |
1991年 | 12篇 |
1990年 | 15篇 |
1989年 | 11篇 |
1988年 | 8篇 |
1987年 | 15篇 |
1986年 | 9篇 |
1985年 | 16篇 |
1984年 | 15篇 |
1983年 | 13篇 |
1982年 | 15篇 |
1981年 | 16篇 |
1980年 | 9篇 |
1979年 | 9篇 |
1978年 | 9篇 |
1977年 | 8篇 |
1976年 | 6篇 |
1975年 | 9篇 |
1974年 | 6篇 |
1973年 | 8篇 |
1972年 | 6篇 |
排序方式: 共有1003条查询结果,搜索用时 15 毫秒
71.
Assessment of Exploration Bias in Data-Driven Predictive Models and the Estimation of Undiscovered Resources 总被引:3,自引:0,他引:3
The spatial distribution of discovered resources may not fully mimic the distribution of all such resources, discovered and
undiscovered, because the process of discovery is biased by accessibility factors (e.g., outcrops, roads, and lakes) and by
exploration criteria. In data-driven predictive models, the use of training sites (resource occurrences) biased by exploration
criteria and accessibility does not necessarily translate to a biased predictive map. However, problems occur when evidence
layers correlate with these same exploration factors. These biases then can produce a data-driven model that predicts known
occurrences well, but poorly predicts undiscovered resources.
Statistical assessment of correlation between evidence layers and map-based exploration factors is difficult because it is
difficult to quantify the “degree of exploration.” However, if such a degree-of-exploration map can be produced, the benefits
can be enormous. Not only does it become possible to assess this correlation, but it becomes possible to predict undiscovered,
instead of discovered, resources.
Using geothermal systems in Nevada, USA, as an example, a degree-of-exploration model is created, which then is resolved into
purely explored and unexplored equivalents, each occurring within coextensive study areas. A weights-of-evidence (WofE) model
is built first without regard to the degree of exploration, and then a revised WofE model is calculated for the “explored
fraction” only. Differences in the weights between the two models provide a correlation measure between the evidence and the
degree of exploration.
The data used to build the geothermal evidence layers are perceived to be independent of degree of exploration. Nevertheless,
the evidence layers correlate with exploration because exploration has preferred the same favorable areas identified by the
evidence patterns. In this circumstance, however, the weights for the “explored” WofE model minimize this bias. Using these
revised weights, posterior probability is extrapolated into unexplored areas to estimate undiscovered deposits. 相似文献
72.
Although previous seismic monitoring studies have revealed several relationships between seismic responses and changes in reservoir rock properties, the quantitative evaluation of time‐lapse seismic data remains a challenge. In most cases of time‐lapse seismic analysis, fluid and/or pressure changes are detected qualitatively by changes in amplitude strength, traveltime and/or Poisson's ratio. We present the steps for time‐lapse seismic analysis, considering the pressure effect and the saturation scale of fluids. We then demonstrate a deterministic workflow for computing the fluid saturation in a reservoir in order to evaluate time‐lapse seismic data. In this approach, we derive the physical properties of the water‐saturated sandstone reservoir, based on the following inputs: VP, VS, ρ and the shale volume from seismic analysis, the average properties of sand grains, and formation‐water properties. Next, by comparing the in‐situ fluid‐saturated properties with the 100% formation‐water‐saturated reservoir properties, we determine the bulk modulus and density of the in‐situ fluid. Solving three simultaneous equations (relating the saturations of water, oil and gas in terms of the bulk modulus, density and the total saturation), we compute the saturation of each fluid. We use a real time‐lapse seismic data set from an oilfield in the North Sea for a case study. 相似文献
73.
74.
75.
Gary A. Dymski 《Geoforum》1996,27(4):439-452
This essay evaluates the evolution of Paul Krugman's ideas about economic geography between 1989 and 1996, focusing on the scope and intention of his work. While Krugman's geographical writings have acknowledged diverse research traditions, he has increasingly focused his efforts on formal spatial models that embody the methods of mainstream economic modeling. This emphasis reflects Krugman's view that formal modeling holds the most promise for understanding spatial aspects of urbanization. Krugman's interventions into economic geography have in turn allowed him to develop and articulate his own view of the future of social science: that is, the explanation of self-organizing behaviour should be the focal point of research, and formal modelling is the key means of advancing this agenda. 相似文献
76.
In the conventional model of floodplain sediment accumulation, mechanisms of floodplain growth are differentiated into lateral and vertical accretion processes, in which within-channel deposits are capped by overbank deposits. In the high-energy, gravel-based Squamish River, sediments laid down on bar surfaces are composed of trough and planar crossbedded coarse sands. These sequences contrast incongruously with adjacent floodplain deposits which are composed in large part of vertically accreted fine sands atop coarse alluvial gravels. Using element analysis it is inferred that bar platform sediments are stripped away by chute channels, which are subsequently infilled with lower-energy deposits. From this, a model of floodplain growth based on selective preservation of bar platform sands and prefrential preservation of vertically accreted deposits is proposed. This mechanism of sediment replacement occurs independent of channel planform type. 相似文献
77.
Michael E. Brownfield Ronald H. Affolter Gary D. Stricker Ricky T. Hildebrand 《International Journal of Coal Geology》1995,27(2-4)
Chromium contents obtained from 20 coal and 5 associated rock samples collected from the basal part of the Eocene Chuckanut Formation, in Skagit and Whatcom counties, northwest Washington, range between 30 and 300 ppm (mean 120 ppm whole-coal basis). The lenticular coals, ranging in rank from subbituminous to anthracite, and with an ash content of 12–46%, crop out along the western flank of the Cascade Range. Results of X-ray diffraction analysis of low-temperature ash show that the mineral matter in the coal samples consists predominantly of quartz and clay (kaolinite, illite and chlorite group). However, accessory minerals, isolated from the coal samples and analyzed by X-ray diffraction, scanning electron microscope and optical methods, contain angular fragments and euhedral crystals of the spinel group (chromite, magnetite and trevorite ), kaolinite-serpentine group (antigorite and chrysotile), chlorite group, amphibole group and pyroxene group minerals (augite, diopside and enstatite), all of which are commonly enriched in chromium.Although associated primarily with the inorganic fraction of the coal, concentrations of chromium in the samples show no statistically significant correlation with ash content. Localized concentrations of chromium in the coal are the result of natural contamination from the alteration of detrital chromium-bearing mineral grains introduced into the peat-forming mires from nearby Jurassic ophiolite bodies. The coals formed in the early Eocene, in rapidly subsiding small basins that developed during the uplift and erosion of the pre-Tertiary ophiolite terrain. Scattered bodies of source rock, random distribution of chromium-bearing minerals within the coal and sample heterogeneity account for the variation in Cr contents of the samples. 相似文献
78.
79.
Plagioclase from a progressively metamorphosed granodiorite changes as the metamorphic grade increases. Lower grade plagioclase are chemically inhomogeneous, with zoned rims containing distinct compositional levels of An0?3, An17, and An25. As grade increases the plagioclase becomes more chemically homogeneous with An0?3 rims dominating. Microcline inclusions are controlled by internal defects at lower grades and grain boundaries at higher grades. Myrmekite rims are developed at the highest grade. Rims are dependent on surface energy factors and occur at triple points, high angle lattice misfits and other high energy surfaces. At low grades, rims form at plagioclase-plagioclase contacts and at higher grades, at plagioclase-microcline contacts. These changes are due to impurity segregation and grain boundary migration, and an increase of the letter process at higher grades. 相似文献
80.
A method is presented for constructing the coronal magnetic field from photospheric magnetograms and observed coronal loops. A set of magnetic field lines generated from magnetogram data is parameterized and then deformed by varying the parameterized values. The coronal flux tubes associated with this field are adjusted until the correlation between the field lines and the observed coronal loops is maximized. A mathematical formulation is described which ensures that (i) the normal component of the photospheric field remains unchanged, (ii) the field is given in the entire corona over an active region, (iii) the field remains divergence-free, and (iv) electric currents are introduced into the field. It is demonstrated that a parameterization of a potential field, comprising a radial stretching of the field, can provide a match for a simple bipolar active region, AR 7999, which crossed the central meridian on 1996 November 26. The result is a non-force-free magnetic field with the Lorentz force being of the order of 10–5.5 g cm s–2 resulting from an electric current density of 0.079 A m–2. Calculations show that the plasma beta becomes larger than unity at a relatively low height of 0.25 r supporting the non-force-free conclusion. The presence of such strong non-radial currents requires large transverse pressure gradients to maintain a magnetostatic atmosphere, required by the relatively persistent nature of the coronal structures observed in AR 7999. This scheme is an important tool in generating a magnetic field solution consistent with the coronal flux tube observations and the observed photospheric magnetic field. 相似文献