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931.
932.
Alexander?L.?ForrestEmail author Hrund?ó.?Andradóttir Bernard?E.?Laval 《Aquatic Sciences - Research Across Boundaries》2012,74(2):361-374
The fate of inflows into lakes has been extensively studied during summer stratification but has seen relatively little focus
during the weak winter stratification, with or without ice-cover. Field observations are presented of groundwater inflow into
a shallow bay of a subarctic lake. Atmospheric forcing of the bay during the study period was extremely variable and coincided
with spring ice-cover break-up. Two dominant wind regimes were identified; (1) weak wind-forcing (wind speed <5 m s−1 or land-fast ice-cover), and (2) strong wind-forcing (wind speed >5 m s−1 and open water). At a relatively constant temperature of ~3.3°C, the groundwater inflow was closer to the temperature of
maximum density than the water in the main body of the lake, which during the observed winter stratification is ~1.2°C. During
weak wind-forcing, the stratification within Silfra Bay approximated two-layers as this denser groundwater formed a negatively
buoyant underflow. A calculated underflow entrainment rate of 2.8 × 10−3 agrees well with other underflow studies. During strong wind-forcing, the water column out to the mouth of the bay became
weakly stratified as the underflow was entrained vertically by wind-stirring. Observed periods of mixing can be predicted
to occur when turbulent kinetic energy (TKE) production by wind stirring integrated over the underflow hydraulic residence
time in the bay exceeds the potential energy associated with the stratification. A decrease of ice cover, as observed in the
studied subarctic lake over the last decade, will result in the underflow being more frequently exposed to the strong wind-forcing
regime during winter, thereby altering the winter distribution of groundwater inflow within the lake. 相似文献
933.
Alexander Velikoseltsev Karl Ulrich Schreiber Alexander Yankovsky Jon-Paul R. Wells Alexander Boronachin Anna Tkachenko 《Journal of Seismology》2012,16(4):623-637
In recent years, the measurement of rotational components of earthquake-induced ground motion became a reality due to high-resolution ring laser gyroscopes. As a consequence of the fact that they exploit the Sagnac effect, these devices are entirely insensitive to translational motion and are able to measure the rotation rate with high linearity and accuracy over a wide frequency band. During the last decade, a substantial number of earthquakes were recorded by the large ring lasers located in Germany, New Zealand, and USA, and the subsequent data analysis demonstrated reliability and consistency of the results with respect to theoretical models. However, most of the observations recorded teleseismic events in the far-field. The substantial mass and the size of these active interferometers make their near-field application difficult. Therefore, the passive counterparts of ring lasers, the fiber optic gyros can be used for seismic applications where the mobility is more important than extreme precision. These sensors provide reasonable accuracy and are small in size, which makes them perfect candidates for strong motion applications. The other advantage of fiber optic gyroscopes is that if the earthquake is local and shallow (like one occurred early this year at Canterbury, New Zealand), the large ring lasers simply do not have the dynamic range??the effect is far too large for these instruments. In this paper, we analyze a typical commercially available tactical grade fiber optic gyroscope (VG-951) with respect to the seismic rotation measurement requirements. The initial test results including translation and upper bounds of seismic rotation sensitivity are presented. The advantages and limitations of tactical grade fiber optic gyroscope as seismic rotation sensor are discussed. 相似文献
934.
We examined the behavior of different fractal dimensions when applied to study features of earthquake spatial distribution
on different types of data. We first examined simulated spatial fields of points of different clustering level, following
the so called Soneira-Peebles model. The model was chosen because it displays some similarity to the real clustering structure
of earthquakes occurring on hierarchically ordered faults. The analysis of the capacity, clustering and correlation dimensions
revealed that their behavior did not completely correlate with the clustering level of the simulated data sets. We also studied
temporal variations of the fractal coefficients, characterizing the spatial distribution of the 1999 İzmit-Düzce aftershock
sequence. The calculated coefficient values demonstrated analogous behavior like for the simulated data. They exposed different
variability in time, but for all of them a systematic fluctuation was observed before the occurrence of the Düzce earthquake.
Our analysis revealed that although fractal coefficients could be applied to measure earthquake clustering, they should be
used with caution, trying to figure out the best coefficient for a certain data set. 相似文献
935.
David Alexander 《New Zealand geographer》2008,64(2):157-161
Abstract: The occurrence of nuisance macro-algae in estuaries is a worldwide problem. Monitoring algal cover has proven difficult in the past, in part due to the lack of adequate, universally accepted mapping protocols. This paper discusses a methodology to map algal cover which was successfully trialled in the Avon-Heathcote Estuary in January 2008. 相似文献
936.
The use of paleolimnology to reconstruct the collapse of the Fraser River sockeye salmon (Oncorhynchus nerka) populations following the landslides at the Hells Gate section of the Fraser canyon (British Columbia, Canada) is explicitly
tested. Construction of the Canadian Pacific Railway caused a series of landslides in 1913–1914, partially blocking the Fraser
River, preventing spawning salmon migration, and causing a near-complete collapse of upstream salmon stocks. We selected three
sockeye nursery lakes upstream of Hells Gate, which varied in spawner density, migration length, and lake catchment characteristics.
In each of the lakes, geochemical (stable nitrogen isotopes and C:N) and biological (diatoms) proxies failed to register the
impact of a dramatic decrease in marine-derived nutrients (MDN). Additional variations in sockeye abundance, documented by
the onset of commercial fishing and modern escapement records, were also not imprinted on the sediment record. Changes in
diatom assemblages are coincident with 20th century climate warming and local catchment disturbances and are not attributable
to variability in MDN subsidies. These results suggest that MDN do not remain within lakes in the Fraser River drainage long
enough to become faithfully archived in the sediment record or that the lakes do not receive sufficient MDN to produce a recognizable
sedimentary signature. 相似文献
937.
Alexander A. Prokopenko Christopher St. G. C. Kendall 《Journal of Paleolimnology》2008,40(4):1185-1192
This paper seeks to arrive at a consistent interpretation of (1) the age model, (2) the grain size record, and (3) seismic reflection data from Lake Hovsgol (a.k.a Khubsugul or Hövsgöl), Mongolia, reported by Fedotov et al. (2007, earlier by Fedotov et al. 2002, 2004). In their most recent contribution, the grain size record of the KDP-01 drill core is interpreted as a climatic signal while little consideration is given to lake-level changes and hence to basin-wide changes in depositional setting evident from seismic profiles; also, a nearly linear age model is at odds with the seismic evidence for a major angular unconformity in the sediment strata. The lack of regional seismic stratigraphic analysis has thus led to an improbable interpretation of the Lake Hovsgol sediment grain size record and ultimately to an improbable scenario of Mongolian glaciation history. Using the available seismic profiles, here we show that the drill core penetrated several transgressive/regressive sedimentary sequences and a major angular unconformity. Therefore, the drilled sediment section cannot represent continuous sediment accumulation and the Brunhes age model across the unconformity cannot be nearly linear; the time interval representing a hiatus remains to be determined. The assumed nearly linear age/depth relationship in the upper 23 m above the angular unconformity is also an unlikely relationship, given the evidence of repeated changes in lake level, and hence in the depositional setting and sedimentation rates. We further propose a qualitative reference model for changes in the Lake Hovsgol depositional setting (presented as a step-by-step animation – see supplementary material) based on manually ‘backstripping and rebuilding’ the seismic pattern. We argue that this model provides a useful template of the likely sediment facies changes in the deep axial part of the Hovsgol basin: our crude model in fact captures the major depositional trends in the KDP-01 drill core section located some 10 km NW along the seismic line. We contend that changes in the depositional setting provide the first-order control on sediment grain size in the Hovsgol record. Our study provides important new constraints on the nature of sedimentary proxy records in Lake Hovsgol and on their interpretation as a record of Mongolian glaciation history. 相似文献
938.
939.
940.