全文获取类型
收费全文 | 72274篇 |
免费 | 1228篇 |
国内免费 | 552篇 |
专业分类
测绘学 | 1824篇 |
大气科学 | 5569篇 |
地球物理 | 14152篇 |
地质学 | 24628篇 |
海洋学 | 6055篇 |
天文学 | 16762篇 |
综合类 | 160篇 |
自然地理 | 4904篇 |
出版年
2020年 | 538篇 |
2019年 | 555篇 |
2018年 | 1272篇 |
2017年 | 1227篇 |
2016年 | 1553篇 |
2015年 | 1107篇 |
2014年 | 1709篇 |
2013年 | 3555篇 |
2012年 | 1750篇 |
2011年 | 2464篇 |
2010年 | 2231篇 |
2009年 | 3006篇 |
2008年 | 2704篇 |
2007年 | 2699篇 |
2006年 | 2512篇 |
2005年 | 2250篇 |
2004年 | 2197篇 |
2003年 | 2078篇 |
2002年 | 1974篇 |
2001年 | 1799篇 |
2000年 | 1668篇 |
1999年 | 1575篇 |
1998年 | 1538篇 |
1997年 | 1536篇 |
1996年 | 1244篇 |
1995年 | 1200篇 |
1994年 | 1135篇 |
1993年 | 1027篇 |
1992年 | 999篇 |
1991年 | 946篇 |
1990年 | 1036篇 |
1989年 | 935篇 |
1988年 | 868篇 |
1987年 | 1027篇 |
1986年 | 864篇 |
1985年 | 1153篇 |
1984年 | 1314篇 |
1983年 | 1260篇 |
1982年 | 1168篇 |
1981年 | 1094篇 |
1980年 | 1006篇 |
1979年 | 969篇 |
1978年 | 972篇 |
1977年 | 876篇 |
1976年 | 832篇 |
1975年 | 767篇 |
1974年 | 820篇 |
1973年 | 810篇 |
1972年 | 511篇 |
1971年 | 477篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
11.
This study evaluated the spatial variability of streambed vertical hydraulic conductivity (Kv) in different stream morphologies in the Frenchman Creek Watershed, Western Nebraska, using different variogram models. Streambed Kv values were determined in situ using permeameter tests at 10 sites in Frenchman, Stinking Water and Spring Creeks during the dry season at baseflow conditions. Measurements were taken both in straight and meandering stream channels during a 5 day period at similar flow conditions. Each test site comprised of at least three transects and each transect comprised of at least three Kv measurements. Linear, Gaussian, exponential and spherical variogram models were used with Kriging gridding method for the 10 sites. As a goodness-of-fit statistic for the variogram models, cross-validation results showed differences in the median absolute deviation and the standard deviation of the cross-validation residuals. Results show that using the geometric means of the 10 sites for gridding performs better than using either all the Kv values from the 93 permeameter tests or 10 Kv values from the middle transects and centre permeameters. Incorporating both the spatial variability and the uncertainty involved in the measurement at a reach segment can yield more accurate grid results that can be useful in calibrating Kv at watershed or sub-watershed scales in distributed hydrological models. 相似文献
12.
13.
In the atmospheric Čerenkov technique γ-rays are detected against the abundant background produced by hadronic showers. In
order to improve the signal to noise ratio of theexperiment, it is necessary to reject a significant fraction of hadronic
showers. Traditional background rejection methods based on image shape parameters have been extensively used for the data
from imaging telescopes. However, non-imaging Čerenkov telescopes have to develop very different means of statistically identifying
and removing cosmic ray events. Some of the parameters, which could be potentially important for non-imaging arrays, are the
temporal and spectral differences, the lateral distributions and density fluctuations of Čerenkov photons generated by γ-ray
and hadron primaries. Here we study the differences in fluctuations of Čerenkov photon density in the light pool at the observation
level from showers initiated by photons and those initiated by protons or heavier nuclei. The database of simulated events
for the PACT array has been used to evaluate the efficiency of the new technique. Various types of density fluctuations like
the short range and medium range fluctuations as well as flatness parameter are studied. The estimated quality factors reflect
the efficiencies with which the hadrons can be rejected from the data. Since some of these parameters are independent, the
cuts may be applied in tandem and we demonstrate that the proton rejection efficiency of ∼90% can be achieved. Use of density
fluctuations is particularly suited for wavefront sampling observations and it seems to be a good technique to improve the
signal to noise ratio.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
14.
Isolation of resonance in acoustic backscatter from elastic targetsusing adaptive estimation schemes
It has been shown that at certain frequencies the acoustic backscatter from elastic targets exhibits certain resonance behavior which closely relates to the physical properties of the target such as dimension, thickness, and composition. The purpose of this paper is to develop an automated approach for identifying the presence of resonance in the acoustic backscatter from an unknown underwater target by isolating the resonance part from the specular contribution. An adaptive transversal filter structure is used to estimate the specular part of the backscatter and consequently the error signal would provide an estimate of the resonance part. An important aspect of this scheme lies in the fact that it does not require an underlying model for the elastic return. The adaptation rule is based upon fast recursive least squares (RLS) learning. The approach taken in this paper is general in the sense that it can be applied to targets of unknown geometry and thickness and, further, does not require any a priori information about the target and/or the environment. Test results on acoustic data are presented which indicate the effectiveness of the proposed approach 相似文献
15.
16.
17.
18.
Parametric transduction offers valuable advantages for underwater acoustic communications. Perhaps the most significant benefit is the fact that high directivity is achieved by means of a physically small transmit transducer. This feature may, ultimately, be employed to permit long-range, low-frequency communication using a compact source. The high directivity is desirable to combat multipath propagation and to achieve data communications in water which is shallow by comparison with range. A real-time, high data-rate “model” differential phase shift keying (DPSK) communication system has been constructed and demonstrated. This system uses parametric transduction, with a 300-kHz primary frequency and a 50-kHz secondary frequency. Experimental results show that the system can be employed to combat multipath propagation in shallow water and can achieve high data-rate text and color image transmission at 10 and 20 kb s-1 for 2-DPSK and 4-DPSK, respectively, through a transmission bandwidth of 10 kHz. The “model” system was developed to confirm performance predictions for a future, operational long-range link employing a 50-kHz primary frequency and a 5-kHz secondary frequency 相似文献
19.
In astronomical photometry, the sensitivity of observations is limited by the dark counts of the photomultiplier tube. In the present work, the effect of dark count noise in photon counting systems is investigated by theory and experimental measurements. Dark counts are considered to be originating from two sources, namely: dc fluctuations and random pulses.Experimental measurements were carried out to determine noise effects in different operating regions of noise dominance. The results provide strong evidence that: in normal operating mode, where the effect of random pulses is dominant, dark counts do not follow Poisson statistics. The observed noise shows strong (1/f) power spectrum, where the observed noise power is found to increase with time of observation.The results are important in photon counting systems operating under dark count limited mode. The conclusions drawn can be useful in obtaining more accurate error estimates and in assessing astronomical photometric observations and data reduction techniques. 相似文献
20.
The 362.156 GHz absorption spectrum of H2O2 in the Mars atmosphere was observed on September 4 of 2003, employing the James Clerk Maxwell Telescope (JCMT) sub-millimeter facility on Mauna Kea, Hawaii. Radiative transfer analysis of this line absorption yields an average volume mixing ratio of 18±0.4 ppbv within the lower (0-30 km) Mars atmosphere, in general accordance with standard photochemical models (e.g., Nair et al., 1994, Icarus 111, 124-150). Our derived H2O2 abundance is roughly three times greater than the upper limit retrieved by Encrenaz et al. (2002, Astron. Astrophys. 396, 1037-1044) from infrared spectroscopy, although part of this discrepancy may result from the different solar longitudes (Ls) of observation. Aphelion-to-perihelion thermal forcing of the global Mars hygropause generates substantial (>200%) increases in HOx abundances above ∼10 km altitudes between the Ls=112° period of the Encrenaz et al. upper limit measurement and the current Ls=250° period of detection (Clancy and Nair, 1996, J. Geophys. Res. 101, 12785-12590). The observed H2O2 line absorption weakens arguments for non-standard homogeneous (Encrenaz et al., 2002, Astron. Astrophys. 396, 1037-1044) or heterogeneous (Krasnopolsky, 2003a, J. Geophys. Res. 108; 2003b, Icarus 165, 315-325) chemistry, which have been advocated partly on the basis of infrared (8 μm) non-detections for Mars H2O2. Observation of Mars H2O2 also represents the first measurement of a key catalytic specie in a planetary atmosphere other than our own. 相似文献