全文获取类型
| 收费全文 | 1989篇 |
| 免费 | 274篇 |
| 国内免费 | 237篇 |
专业分类
| 测绘学 | 211篇 |
| 大气科学 | 433篇 |
| 地球物理 | 481篇 |
| 地质学 | 600篇 |
| 海洋学 | 142篇 |
| 天文学 | 27篇 |
| 综合类 | 94篇 |
| 自然地理 | 512篇 |
出版年
| 2024年 | 4篇 |
| 2023年 | 16篇 |
| 2022年 | 51篇 |
| 2021年 | 37篇 |
| 2020年 | 67篇 |
| 2019年 | 78篇 |
| 2018年 | 62篇 |
| 2017年 | 118篇 |
| 2016年 | 82篇 |
| 2015年 | 100篇 |
| 2014年 | 80篇 |
| 2013年 | 124篇 |
| 2012年 | 60篇 |
| 2011年 | 110篇 |
| 2010年 | 79篇 |
| 2009年 | 132篇 |
| 2008年 | 154篇 |
| 2007年 | 144篇 |
| 2006年 | 136篇 |
| 2005年 | 111篇 |
| 2004年 | 85篇 |
| 2003年 | 73篇 |
| 2002年 | 74篇 |
| 2001年 | 76篇 |
| 2000年 | 50篇 |
| 1999年 | 60篇 |
| 1998年 | 57篇 |
| 1997年 | 34篇 |
| 1996年 | 31篇 |
| 1995年 | 24篇 |
| 1994年 | 29篇 |
| 1993年 | 36篇 |
| 1992年 | 24篇 |
| 1991年 | 22篇 |
| 1990年 | 21篇 |
| 1989年 | 8篇 |
| 1988年 | 17篇 |
| 1987年 | 7篇 |
| 1986年 | 7篇 |
| 1985年 | 3篇 |
| 1984年 | 2篇 |
| 1983年 | 3篇 |
| 1982年 | 2篇 |
| 1981年 | 3篇 |
| 1980年 | 4篇 |
| 1978年 | 2篇 |
| 1954年 | 1篇 |
排序方式： 共有2500条查询结果，搜索用时 46 毫秒
961.
962.
Wave propagation simulation in a linear viscoacoustic medium 总被引：2，自引：0，他引：2
963.
964.
During experiments with digital stations in the period 1985–1987, twenty five earthquakes with magnitudes m
b
in the range 2.9 to 4.8 and epicentres located within the area 36°–42.3° (N) and 4.5°–13.6° (W) were recorded at Montemor (MOE) and Montachique (MTH). The three-component recordings were obtained by Geotech S13 instruments with 1 second period. A preliminary analysis of the recordings consisted in the determination of amplitudes and spectral contents of P and S waves, and led to the following observations: (1) The attenuation of waves is expressed by the equation V = exp(C
2).R
C
1. exp(C
3.M), where V stands for acceleration, velocity or displacement; M-magnitude; R-focal distance; C
1, C
2 and C
3 are constants to be obtained by least square fitting. The application of this equation led to C
1 of the order 1.7 for displacement, 1.8 for velocity and 2.0 for acceleration, with an average mean square error 0.8. (2) The ratios L/T (longitudinal/transversal amplitudes), for velocity and displacement, showed a tendency to reduce with increasing focal distance, being 2 for short distances (<50 km) and 1 for long distances (400 km). (3) The ratios S/P (S-wave/P-wave amplitudes), although with a large dispersion, showed a slight tendency for increasing with focal distance. (4) The predominant frequencies also showed a slight tendency to decrease with increasing focal distance and with magnitude. (5) The dependence of C
1 with frequency (3 to 12 Hz) is well behaved from 0.95 to 1.75 (for the velocity trace). 相似文献
965.
We consider the second-order differential equations ofP-SV motion in an isotropic elastic medium with spherical coordinates. We assume that in the medium Lamé's parameters , r
p
and compressional and shear-wave velocities , r, wherer is radial distance. With this regular heterogeneity both the radial functions appearing in displacement components satisfy a fourth-order differential equation which provides solutions in terms of exponential functions. We then consider a layered spherical earth in which each layer has heterogeneity as specified above. The dispersion equation of the Rayleigh wave is obtained using the Thomson-Haskel method. Due to exponential function solutions in each layer, the dispersion equation has similar simplicity, as in a flat-layered earth. The dispersion equation is further simplified, whenp=–2. We obtain numerical results which agree with results obtained by other methods. 相似文献
966.
967.
In tropical areas, when the zonal or meridional geostrophic equilibrium is destroyed, inertial gravity wave will be excited
by ageostrophic motion, with the dispersion of the wave, a new zonal or meridional geostrophic balance will be established
again, and it follows an invariant of semi-potential vorticity. Based on the invariant, it should be pointed out that the
direction of the zonal or meridional semi-geostrophic adaptation depends mainly upon the meridional characteristic scale of
initial disturbance. For the zonal (meridional) semi-geostrophic adaptation, if the zonal characteristic scale of initial
disturbance is big (small) enough, then the adaptation process always represents the mutual adjustment between the pressure
field and zonal (meridional) flow. 相似文献
968.
B. G. Hankin K. J. Beven 《Stochastic Environmental Research and Risk Assessment (SERRA)》1998,12(6):377-396
In a special opportunity, detailed measurements of the flow in an overbank flow in the Flood Channel Facility at HR Wallingford
were used in conjunction with tracer test data to assess the effectiveness of dispersion models based around random particle
tracking (RPT). Ten different RPT models based on different assumptions and levels of information about the nature of the
Lagrangian velocity field were investigated. Multiple simulations were used to calibrate variable parameters controlling the
average magnitude of the perturbations for each model by comparison with observed concentrations at one cross-section. The
calibrated models were then used to predict concentration distributions further downstream. Several of the calibrated models
showed close agreement between observed and predicted concentration distributions. The most complex models using the most
information about the velocity distributions were no better (and in some cases worse) in prediction than the simplest models
investigated. It would appear that our knowledge of the system, despite the quality of the experiments, is too uncertain to
infer a precise model structure. 相似文献
969.
Aircraft measurements of potential temperature and turbulent kinetic energy are used to examine the growth of the thermal internal boundary layer (TIBL) in sea-breeze flows on four selected days of a coastal fumigation study performed in 1995 at Kwinana in Western Australia. The aircraft data, together with radiosonde measurements taken on the same days, show a multi-layered low-level onshore flow in the vertical with a superadiabatic layer extending to about 50 m above the water surface on all four days. On the first three days the layer above the superadiabatic layer was neutral, typically 200 m deep, capped by a stably stratified region, whereas on the remaining day it was fully stable. The occurrence of the neutral layer on most experimental days contrasts with the more usual situation involving an entirely stable onshore flow. A composite approach based on both temperature and turbulence data is used to provide a pragmatic but self-consistent definition of the TIBL height. The data for the first three days indicate that the TIBL grows rapidly into the neutrally stratified region to the top of the region within about 2 km from the coast, with a very slow subsequent growth into the stable stratification aloft. On the other hand, the TIBL grows only to about 200 m within a distance of 7 km from the coast on the fourth day due to a strong stable stratification.An existing numerical TIBL model based on the slab approach, capable of describing the TIBL growth in both neutral and stable environments, and a recent analytical model, more efficient for operational use, are used to simulate the aircraft TIBL observations. The predictions by both models agree reasonably well with the data. 相似文献
970.