全文获取类型
收费全文 | 45919篇 |
免费 | 20017篇 |
国内免费 | 44960篇 |
专业分类
测绘学 | 3926篇 |
大气科学 | 22965篇 |
地球物理 | 15070篇 |
地质学 | 41193篇 |
海洋学 | 17016篇 |
天文学 | 1414篇 |
综合类 | 5530篇 |
自然地理 | 3782篇 |
出版年
2024年 | 230篇 |
2023年 | 593篇 |
2022年 | 1152篇 |
2021年 | 1459篇 |
2020年 | 2498篇 |
2019年 | 5967篇 |
2018年 | 6250篇 |
2017年 | 5879篇 |
2016年 | 6016篇 |
2015年 | 5279篇 |
2014年 | 4997篇 |
2013年 | 5520篇 |
2012年 | 5234篇 |
2011年 | 5118篇 |
2010年 | 4948篇 |
2009年 | 4291篇 |
2008年 | 3560篇 |
2007年 | 3602篇 |
2006年 | 3176篇 |
2005年 | 2928篇 |
2004年 | 3103篇 |
2003年 | 2836篇 |
2002年 | 2656篇 |
2001年 | 2421篇 |
2000年 | 2030篇 |
1999年 | 2083篇 |
1998年 | 2059篇 |
1997年 | 2088篇 |
1996年 | 1682篇 |
1995年 | 1602篇 |
1994年 | 1480篇 |
1993年 | 1391篇 |
1992年 | 1135篇 |
1991年 | 870篇 |
1990年 | 808篇 |
1989年 | 664篇 |
1988年 | 591篇 |
1987年 | 435篇 |
1986年 | 356篇 |
1985年 | 282篇 |
1984年 | 298篇 |
1983年 | 188篇 |
1982年 | 217篇 |
1981年 | 159篇 |
1980年 | 111篇 |
1979年 | 115篇 |
1978年 | 49篇 |
1977年 | 43篇 |
1971年 | 53篇 |
1970年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
452.
Two different goals in fitting straight lines to data are to estimate a true linear relation (physical law) and to predict values of the dependent variable with the smallest possible error. Regarding the first goal, a Monte Carlo study indicated that the structural-analysis (SA) method of fitting straight lines to data is superior to the ordinary least-squares (OLS) method for estimating true straight-line relations. Number of data points, slope and intercept of the true relation, and variances of the errors associated with the independent (X) and dependent (Y) variables influence the degree of agreement. For example, differences between the two line-fitting methods decrease as error in X becomes small relative to error in Y. Regarding the second goal—predicting the dependent variable—OLS is better than SA. Again, the difference diminishes as X takes on less error relative to Y. With respect to estimation of slope and intercept and prediction of Y, agreement between Monte Carlo results and large-sample theory was very good for sample sizes of 100, and fair to good for sample sizes of 20. The procedures and error measures are illustrated with two geologic examples. 相似文献
453.
Slitless spectrograms obtained during the eclipse of 10 June 1972 have been analyzed to determine the height distribution of the D3 He line intensity.For undisturbed regions the maximum of D3 line intensity is confirmed to exist at about 1700km above the limb. Besides the above mentioned maximum, in plages a considerable intensity may be observed at low heights (h < 1000 km).An analysis of these observations for h > 1000 km has been carried out within the low temperature mechanism of triplet helium emission taking into account the helium ionization by XUV radiation. The density dependence of the 23
S level population at different XUV flux values has been calculated. Our observations give N
e 2 × 1010 cm–3 in the chromosphere at h = 2000 km. The probable coincidence of the H and He emission small filaments in the middle chromosphere is discussed. 相似文献
454.
3-D seismic structure of the source area of the 1993 Latur, India, earthquake and its implications for rupture nucleations 总被引:1,自引:0,他引:1
The Latur earthquake (Mw 6.1) of 29 September 1993 is a rare stable continental region (SCR) earthquake that occurred on a previously unknown blind fault. In this study, we determined detailed three-dimensional (3-D) P- and S-wave velocity (Vp, Vs) and Poisson's ratio (σ) structures by inverting the first P- and S-wave high-quality arrival time data from 142 aftershocks that were recorded by a network of temporary seismic stations. The source zone of the Latur earthquake shows strong lateral heterogeneities in Vp, Vs and σ structures, extending in a volume of about 90 × 90 × 15 km3. The mainshock occurred within, but near the boundary, of a low-Vp, high-Vs and low-σ zone. This suggests that the structural asperities at the mainshock hypocenter are associated with a partially fluid-saturated fractured rock in a previously unknown source zone with intersecting fault surfaces. This might have triggered the 1993 Latur mainshock and its aftershock sequence. Our results are in good agreement with other geophysical studies that suggest high conductivity and high concentration of radiogenic helium gas beneath the source zone of the Latur earthquake. Our study provides an additional evidence for the presence of fluid related anomaly at the hidden source zone of the Latur earthquake in the SCR and helps us understand the genesis of damaging earthquakes in the SCR of the world. 相似文献
455.
456.
457.
458.
459.
A sequential waveform method is developed to simulate the seismic response of basin-edge structure excited by a plane incident P-wave. The full procedure involves: (a) a previous parameterization of the investigated model using the seismic wave velocities and depths of the sedimentary stratifications; (b) an input motion determined from the records at stations installed on hard rock; (c) forward computation of the P-SV elastic wave field by means of a two-dimensional finite difference (FD) method; (d) the optimization of the model vector using simulated annealing technique and comparing the simulated seismic response of the tested structure with the observed wave field; (e) the correction of the initial model by trial-and-error by testing the differences between synthetics and observed data, and (f) the final solution obtained by iteration using the conjugate gradient algorithm. The search of an optimal basin-edge model has been parallel processed by varying the shapes and velocities of strata on the basis of the fitting of relative timing, amplitude and phase between the output and the observed data. The input motion and sensitivity have been checked and the validity of the method has been demonstrated by numeric analysis. Using the teleseismic records generated by 7 earthquakes recorded at 26 broadband seismic stations, we have studied the seismic velocity structure of the southern edge of the Jiyang depression located in the Bohai Bay basin, northern China. Two cross sections show an agreement between the velocity results and the geological sections available in the region. In addition, we obtain evidence of three hidden faults under the sections and features that suggest major extensions at the Paleogene. 相似文献
460.