全文获取类型
收费全文 | 16865篇 |
免费 | 3040篇 |
国内免费 | 4022篇 |
专业分类
测绘学 | 2929篇 |
大气科学 | 1329篇 |
地球物理 | 2918篇 |
地质学 | 12000篇 |
海洋学 | 1560篇 |
天文学 | 798篇 |
综合类 | 1045篇 |
自然地理 | 1348篇 |
出版年
2024年 | 61篇 |
2023年 | 175篇 |
2022年 | 519篇 |
2021年 | 679篇 |
2020年 | 707篇 |
2019年 | 814篇 |
2018年 | 601篇 |
2017年 | 832篇 |
2016年 | 849篇 |
2015年 | 821篇 |
2014年 | 1039篇 |
2013年 | 1238篇 |
2012年 | 1163篇 |
2011年 | 1234篇 |
2010年 | 929篇 |
2009年 | 1137篇 |
2008年 | 1201篇 |
2007年 | 1151篇 |
2006年 | 1187篇 |
2005年 | 1037篇 |
2004年 | 966篇 |
2003年 | 786篇 |
2002年 | 763篇 |
2001年 | 600篇 |
2000年 | 627篇 |
1999年 | 501篇 |
1998年 | 399篇 |
1997年 | 398篇 |
1996年 | 309篇 |
1995年 | 265篇 |
1994年 | 214篇 |
1993年 | 193篇 |
1992年 | 144篇 |
1991年 | 89篇 |
1990年 | 87篇 |
1989年 | 57篇 |
1988年 | 51篇 |
1987年 | 29篇 |
1986年 | 28篇 |
1985年 | 9篇 |
1984年 | 4篇 |
1983年 | 6篇 |
1982年 | 2篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1978年 | 5篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1954年 | 7篇 |
1877年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
J. L. Mallet 《Mathematical Geology》1988,20(8):977-990
Lett= (x, y, z) be an ore grade function sampled at the nodes of a three dimensional regular grid. Ift
0 is a given cut-off, the ore deposit may be considered to be constituted by all the points (x, y, z) such that (x, y, z)>t
0. In fact, the ore deposit may be constituted by one or several disconnected bodies and two algorithms to display these bodies may be considered. The first consists in drawing intersections of the bodies with a series of equidistant parallel planes. The second consists in covering their surface with painted triangles. In both cases, how to remove hidden parts of the drawing is shown. An interactive approach of the problem also is considered.This paper presented at Emerging Concepts, MGUS-87 conference, Redwood City, California, 13–15 April 1987. 相似文献
992.
I. Lerche 《Mathematical Geology》1988,20(2):73-96
Using the logic for quantitative inversion of present-day downhole thermal indicators, the inversion procedure can be used to determine unknown, or poorly known, chemical and physical parameters as well as other geological quantities of interest which impact on burial history and thermal history of an evolving sedimentary basin. Some such quantities are: amount of erosion and timing of unconformities, paleo-overpressuring, stratigraphic age, timing of igneous intrusion and insertion temperature, overthrust timing and frictional heating, fault and slump timing, effects due to emplacement of a radiocative layer, and salt emplacement and dissolution timing. Combining a priori unknown values of these chemical/physical and geological parameters with unknown (a priori) paleoheat flux variations, a theoretical scheme, called thermal indicator tomography, is developed for the systematic determination of all parameters at the same time and on the same footing. Case histories will be discussed in subsequent papers in this series. 相似文献
993.
Mathematical models of hydrocarbon formation can be used to simulate the natural evolution of different types of organic matter and to make an overall calculation of the amounts of oil and/or gas produced during this evolution. However, such models do not provide any information on the composition of the hydrocarbons formed or on how they evolve during catagenesis.From the kinetic standpoint, the composition of the hydrocarbons formed can be considered to result from the effect of “primary cracking” reactions having a direct effect on kerogen during its evolution as well as from the effect of “secondary cracking” acting on the hydrocarbons formed.This report gives experimental results concerning the “primary cracking” of Types II and III kerogens and their modelling. For this, the hydrocarbons produced have been grouped into four classes (C1, C2–C5, C6–C15 and C15+). Experimental data corresponding to these different classes were obtained by the pyrolysis of kerogens with temperature programming of 4°C/min with continuous analysis, during heating, of the amount of hydrocarbons corresponding to each of these classes.The kinetic parameters of the model were optimized on the basis of the results obtained. This model represents the first step in the creation of a more sophisticated mathematical model to be capable of simulating the formation of different hydrocarbon classes during the thermal history of sediments. The second step being the adjustment of the kinetic parameters of “secondary cracking”. 相似文献
994.
本文在调研我国西北内陆盆地中的红层与砂岩型铀矿关系的基础上,提出了一个以沉积相和红化成因为基础的红层分类:把内陆盆地中的红层分为陆解阶段形成的冲积平原红层和浅湖红层;成岩后生阶段形成的河流冲积红层、三角洲红层和沙漠红层;以及表生风化阶段形成的溅泼红(次生红层)等6类红层。并依据这6类红层的特征、产出构造环境,以及含矿主岩到各类红层的Eh、pH、Th/U、Fc~(3+)/Fc~(2+)、Sr/Ba等地化特征值的变化,确定了它们与砂岩型铀矿的关系,即陆解阶段岩石红化时为开放环境,活性铀易于流失,对成矿不利;而成岩后生阶段,岩石红化时的环境为封闭系统,对成矿有利,能形成铀矿床。 相似文献
995.
996.
The Hikurangi Margin is a region of oblique subduction with northwest-dipping intermediate depth seismicity extending southwest from the Kermadec system to about 42°S. The current episode of subduction is at least 16–20 Ma old. The plate convergence rate varies along the margin from about 60 mm/a at the south end of the Kermadec Trench to about 45 mm/a at 42°S. The age of the Pacific lithosphere adjacent to the Hikurangi Trench is not known.The margin divides at about latitude 39°S into two quite dissimilar parts. The northern part has experienced andesitic volcanism for about 18 Ma, and back-arc extension in the last 4 Ma that has produced a back-arc basin onshore with high heaflow, thin crust and low upper-mantle seismic velocities. The extension appears to have arisen from a seawards migration of the Hikurangi Trench north of 39°S. Here the plate interface is thought to be currently uncoupled, as geodetic data indicate extension of the fore-arc basin, and historic earthquakes have not exceededM
s=7.South of 39°S there is no volcanism and a back-arc basin has been produced by downward flexure of the lithosphere due to strong coupling with the subducting plate. Heatflow in the basin is normal. Evidence for strong coupling comes from historic earthquakes of up to aboutM
s=8 and high rates of uplift on the southeast coast of the North Island.The reason for this division of the margin is not known but may be related to an inferred increase, from northeast to southwest, in the buoyancy of the Pacific lithosphere. 相似文献
997.
三维地震勘探在十年内取得了显著的地质效果和经济效益。三维地震数据处理技术也得到了迅速的发展。但是,与二维相比,三维地震数据处理技术有许多不完善之处。本文对其中几个主要问题进行分析并提出解决方法。这些问题是三维叠加速度分析、三维静校正和叠前三维偏移等处理方法。 相似文献
998.
999.
The determination of uranium series disequilibria in fluvial environments is proposed as a method of calculating catchment mass balances. The technique is based on two main principles. Firstly, 234U is more mobile than 238U, especially during the early stages of weathering. Secondly, uranium is far more mobile than either thorium or protactinium. Consequently, teaching during weathering results in the loss of the uranium found in the fresh rock, leaving the two immobile daughters behind. The ratio of uranium carried by sediment to that dissolved, US/UW can, therefore, be determined from river water and sediment isotopic activity ratios. Fluxes of uranium can then be calculated from average concentrations in the water and the associated sediment, from which a sediment yield can be inferred. The Witham catchment in Lincolnshire has been used to test the proposed method. A US/UW ratio of between 5 and 7 is determined and a sediment yield of 2.51 ± 2.12 tonnes yr?1 km?2 is proposed. Although some problems concerning environmental chemistry have arisen, the validity of the approach is confirmed by the close correspondence between the results obtained and those inferred by earlier workers using more conventional methods. 相似文献
1000.
A suite of marl samples from the evaporitic series of the Mulhouse basin (France; Lower Oligocene) was studied for its biomarker content, in particular its polar constituents. Novel series of 3-carboxyalkyl steranes and 15-oxo triaromatic ketones were identified by synthesis. The 3-carboxyalkyl steranes probably originate from highly polar precursors yet unreported in living organisms. Our data suggest that micro-algae could be the major source of these compounds which seem to be indicators of high algal input rather than characteristic of evaporitic environments. The 15-oxo triaromatic ketones could be oxidation products of triaromatic steroid hydrocarbons formed during diagenesis, although their formation during work-up procedure could not be excluded. 相似文献