全文获取类型
收费全文 | 1627篇 |
免费 | 67篇 |
国内免费 | 26篇 |
专业分类
测绘学 | 59篇 |
大气科学 | 126篇 |
地球物理 | 361篇 |
地质学 | 561篇 |
海洋学 | 119篇 |
天文学 | 340篇 |
综合类 | 8篇 |
自然地理 | 146篇 |
出版年
2022年 | 23篇 |
2021年 | 23篇 |
2020年 | 27篇 |
2019年 | 24篇 |
2018年 | 53篇 |
2017年 | 56篇 |
2016年 | 38篇 |
2015年 | 42篇 |
2014年 | 49篇 |
2013年 | 86篇 |
2012年 | 47篇 |
2011年 | 68篇 |
2010年 | 74篇 |
2009年 | 80篇 |
2008年 | 70篇 |
2007年 | 64篇 |
2006年 | 64篇 |
2005年 | 43篇 |
2004年 | 52篇 |
2003年 | 42篇 |
2002年 | 59篇 |
2001年 | 27篇 |
2000年 | 38篇 |
1999年 | 22篇 |
1998年 | 28篇 |
1997年 | 27篇 |
1996年 | 17篇 |
1995年 | 21篇 |
1994年 | 14篇 |
1993年 | 18篇 |
1992年 | 23篇 |
1991年 | 14篇 |
1990年 | 17篇 |
1989年 | 10篇 |
1987年 | 21篇 |
1986年 | 17篇 |
1985年 | 24篇 |
1984年 | 30篇 |
1983年 | 27篇 |
1982年 | 33篇 |
1981年 | 25篇 |
1980年 | 25篇 |
1979年 | 23篇 |
1978年 | 9篇 |
1977年 | 15篇 |
1976年 | 16篇 |
1975年 | 8篇 |
1974年 | 15篇 |
1973年 | 11篇 |
1971年 | 9篇 |
排序方式: 共有1720条查询结果,搜索用时 15 毫秒
881.
Joseph Masiero 《Icarus》2010,207(2):795-799
We have investigated the effect of rotation on the polarization of scattered light for the near-Earth asteroid (1943) Anteros using the Dual Beam Imaging Polarimeter on the University of Hawaii’s 2.2 m telescope. Anteros is an L-type asteroid that has not been previously observed polarimetrically. We find weak but significant variations in the polarization of Anteros as a function of rotation, indicating albedo changes across the surface. Specifically, we find that Anteros has a background albedo of pv = 0.18 ± 0.02 with a dark spot of pv < 0.09 covering <2% of the surface. 相似文献
882.
A second species of Neophrynichthys is described from 20 specimens collected from the Campbell Plateau, south of South Island, New Zealand. It differs from its sole congener, N. latus, in having 25–26 pectoral rays (versus 22–24), cirri on head more than twice as large, half as few (c. 50 versus >100), and generally absent from the mid‐region behind the eyes (versus being relatively dense), and in spotted colour pattern covering much less than 50% of the dorsal surface of the head (versus >50%). This new species is the sixth psychrolutid known from New Zealand. The genus Neophrynichthys is formally diagnosed by two synapomorphic characters unique among psychrolutids: having light spots on a dark ground colour and having an open fourth and fifth infraorbital, fifth and sixth cranial arch, medial extrascapular, and posterior tabular of the pterotic. 相似文献
883.
Through 10 years of support from the Minerals Management Service Association of American State Geologists' Continental Margins Program we have mapped along the Maine coast, seaward to the 100 m isobath. In all, 1,773 bottom sample stations were occupied, 3,358 km of side-scan sonar and 5,011 km of seismic reflection profiles were gathered. On the basis of these data, a surficial sediment map was created for the Maine inner continental shelf during the Year 8 project, and cores and seismic data were collected to evaluate sand thickness during Years 9 and 10. Sand covers only 8 % of the Maine shelf, and is concentrated seaward of beaches off southern Maine in water depths less than 60 m. Sand occurs in three depositional settings: (1) in shoreface deposits connected dynamically to contemporary beaches; (2) in submerged deltas associated with lower sea-level positions; and (3) in submerged lowstand shoreline positions between 50 and 60 m. Seismic profiles over the shoreface off Saco Bay, Wells Embayment, and off the Kennebec River mouth each imaged a wedge-shaped acoustic unit which tapered off between 20 and 30 m. Cores determined that this was sand that was underlain by a variable but thin (commonly 1 m) deposit of estuarine muddy sand and a thick deposit of glacial-marine mud. Off Saco Bay, more than 55 million m3 of sand exists in the shoreface, compared with about 22 million m3 on the adjacent beach and dunes. Seaward of the Kennebec River, a large delta deposited between 13 ka and the present time holds more than 300 million m3 of sand and gravel. The best sorted sand is on the surface nearshore, with increasing amounts of gravel offshore and mud beneath the surficial sand sheet. Bedforms indicate that the surficial sand is moved by waves to at least 55 m depth. Seaward of the Penobscot River, no significant sand or gravel was encountered. Muddy estuarine sediments overlie muddy glacial-marine sediment throughout the area offshore area of this river. No satisfactory explanation is offered for lack of a sandy delta seaward of Maine's largest river. Lowstand-shoreline deposits were cored in many places in Saco Bay and off the Kennebec River mouth. Datable materials from cores indicated that the lowstand occurred around 10.5 ka off the Kennebec. Cores did not penetrate glacial-marine sediment in the lowstand deposits, and seismic profiles were ambiguous about the vertical extent of sand in these units. For these reasons, no total thickness of sand was determined from the lowstand deposits, but given the area of the surficial sand, the volume is probably in the hundreds of millions of cubic meters. 相似文献
884.
Joseph J. Williams Stephen J. Brooks William D. Gosling 《Journal of Paleolimnology》2012,48(3):485-501
We present the first palaeolimnological investigation of chironomid larval assemblages from the Bolivian Eastern Cordillera. Taxonomic diagnoses are provided for the 10 chironomid taxa (subfamilies: Chironominae, Orthocladiinae and Tanypodinae) identified in the lake sediments. We compared changes in the chironomid assemblage from two Andean sites with previously reported palynological, charcoal and geochemical data, and highlight the potential of chironomid analysis to provide additional insights into environmental change in this region of high biodiversity over the last 18,000?years. At Lake Challacaba (17°33.257??S, 65°34.024??W; 3,400?m asl), the chironomid and geochemical data indicate periodic desiccation and hypersalinty of the basin c. 4,000?C3,460?cal?year BP. Increased abundance of Chironomus sp. at c. 1,000?cal?year BP suggests a change in human activity, supporting inferences from the pollen and spore records, which indicate elevated pastoral agriculture at this time. The greatest assemblage change in the chironomid record from Laguna Khomer Kocha Upper (17°16.514??S, 65°43.945??W; 4,153?m asl) occurred at c. 6,380?cal?year BP, concomitant with an increase in marsh woodland taxa, wetter conditions and a rising lake level at the end of a Holocene dry event. There is no apparent response in the chironomid assemblage to burning, however, at the onset of this dry event (c. 10,000?cal?year BP), which is the major transformative agent of the terrestrial vegetation. This study shows that chironomid assemblages in the tropical Andes responded to regional and local environmental changes, and in particular, that they were sensitive to adjustments in net moisture balance (water level fluctuations and salinity) and anthropogenic impacts (nutrient input). This suggests that within-lake processes are more important as drivers of chironomid assemblage composition than terrestrial vegetation or fire regime. Nevertheless, the full potential of subfossil chironomid analysis will only be realised once more modern autecological data are available. 相似文献
885.
The authors suggested acceptance criteria for rural dispersion models’ performance measures in this journal in 2004. The current paper suggests modified values of acceptance criteria for urban applications and tests them with tracer data from four urban field experiments. For the arc-maximum concentrations, the fractional bias should have a magnitude <0.67 (i.e., the relative mean bias is less than a factor of 2); the normalized mean-square error should be <6 (i.e., the random scatter is less than about 2.4 times the mean); and the fraction of predictions that are within a factor of two of the observations (FAC2) should be >0.3. For all data paired in space, for which a threshold concentration must always be defined, the normalized absolute difference should be <0.50, when the threshold is three times the instrument’s limit of quantification (LOQ). An overall criterion is then applied that the total set of acceptance criteria should be satisfied in at least half of the field experiments. These acceptance criteria are applied to evaluations of the US Department of Defense’s Joint Effects Model (JEM) with tracer data from US urban field experiments in Salt Lake City (U2000), Oklahoma City (JU2003), and Manhattan (MSG05 and MID05). JEM includes the SCIPUFF dispersion model with the urban canopy option and the urban dispersion model (UDM) option. In each set of evaluations, three or four likely options are tested for meteorological inputs (e.g., a local building top wind speed, the closest National Weather Service airport observations, or outputs from numerical weather prediction models). It is found that, due to large natural variability in the urban data, there is not a large difference between the performance measures for the two model options and the three or four meteorological input options. The more detailed UDM and the state-of-the-art numerical weather models do provide a slight improvement over the other options. The proposed urban dispersion model acceptance criteria are satisfied at over half of the field experiments. 相似文献
886.
K. S. Krishnamohan K. Mohanakumar P. V. Joseph 《Theoretical and Applied Climatology》2012,109(1-2):271-282
Cyclonic storms having maximum winds of 34 knots and above that had genesis in north Indian Ocean have been studied with respect to the eastward passage of Madden–Julian Oscillation (MJO). In the three decades (1979–2008), there were a total of 118 cyclones reported in which 96 formed in the region chosen (0–15oN, 60oE–100oE) for the study. Although the percentage of MJO days inducing cyclogenesis is small, it is found that tropical cyclone genesis preferentially occurred during the convective phase of MJO. This accounted for 44 cyclones of the total 54 cyclones (i.e., 81.5%) formed under MJO amplitude 1 and above. The study has shown that, when the enhanced convection of MJO is over the maritime continent and the adjoining eastern Indian Ocean, it creates the highest favorable environment for cyclogenesis in the Bay of Bengal. During this phase, westerlies at 850 hPa are strong in the equatorial region south of Bay of Bengal creating strong cyclonic vorticity in the lower troposphere along with the low vertical wind shear. 相似文献
887.
We present small-scale laboratory models of oceanic subduction in which plates motion is imposed by lateral boundary conditions. The oceanic plate moves trenchward at constant speed and subducts below a fixed overriding plate. In this configuration, the long-term process of subduction is not steady-state. Slab interaction with the upper mantle-lower mantle boundary results in periods of slab flattening during which the dip of the slab diminishes, followed by periods of slab steepening. The overriding plate tectonic regime is influenced by the dynamics of subduction, slab anchoring favouring trench perpendicular shortening. When the slab is anchored, slab flattening further favours shortening, while slab steepening favours extension or smaller shortening rates. Non-steady-state long-term subduction may explain part of the variability of slab geometries evidenced by statistical analyses of present-day subduction zones. Experiments suggest that, despite boundary conditions applied on the converging plates do not change, tectonics pulses within the overriding plate may be caused by this non-steady-state behaviour. 相似文献
888.
S. S. C. Shenoi N. Nasnodkar G. Rajesh K. Jossia Joseph I. Suresh A. M. Almeida 《Journal of Earth System Science》2009,118(5):483-496
This paper describes the variability in the diurnal range of SST in the north Indian Ocean using in situ measurements and tests the suitability of simple regression models in estimating the diurnal range. SST measurements obtained
from 1556 drifting and 25 moored buoys were used to determine the diurnal range of SSTs. The magnitude of diurnal range of
SST was highest in spring and lowest in summer monsoon. Except in spring, nearly 75–80% of the observations reported diurnal
range below 0.5°C. The distributions of the magnitudes of diurnal warming across the three basins of north Indian Ocean (Arabian
Sea, Bay of Bengal and Equatorial Indian Ocean) were similar except for the differences between the Arabian Sea and the other
two basins during November–February (winter monsoon) and May. The magnitude of diurnal warming that depended on the location
of temperature sensor below the water level varied with seasons. In spring, the magnitude of diurnal warming diminished drastically
with the increase in the depth of temperature sensor. The diurnal range estimated using the drifting buoy data was higher
than the diurnal range estimated using moored buoys fitted with temperature sensors at greater depths.
A simple regression model based on the peak solar radiation and average wind speed was good enough to estimate the diurnal
range of SST at ∼1.0 m in the north Indian Ocean during most of the seasons except under low wind-high solar radiation conditions
that occur mostly during spring. The additional information on the rate of precipitation is found to be redundant for the
estimation of the magnitude of diurnal warming at those depths. 相似文献
889.
Hrishikesh Samant Ashwin Pundalik Joseph D’souza Hetu Sheth Keegan Carmo Lobo Kyle D’souza Vanit Patel 《Journal of Earth System Science》2017,126(1):9
The Panvel flexure is a 150-km long tectonic structure, comprising prominently seaward-dipping Deccan flood basalts, on the western Indian rifted margin. Given the active tectonic faulting beneath the Panvel flexure zone inferred from microseismicity, better structural understanding of the region is needed. The geology of Elephanta Island in the Mumbai harbour, famous for the ca. mid-6th century A.D. Hindu rock-cut caves in Deccan basalt (a UNESCO World Heritage site) is poorly known. We describe a previously unreported but well-exposed fault zone on Elephanta Island, consisting of two large faults dipping steeply east–southeast and producing easterly downthrows. Well-developed slickensides and structural measurements indicate oblique slip on both faults. The Elephanta Island fault zone may be the northern extension of the Alibag–Uran fault zone previously described. This and two other known regional faults (Nhava–Sheva and Belpada faults) indicate a progressively eastward step-faulted structure of the Panvel flexure, with the important result that the individual movements were not simply downdip but also oblique-slip and locally even rotational (as at Uran). An interesting problem is the normal faulting, block tectonics and rifting of this region of the crust for which seismological data indicate a normal thickness (up to 41.3 km). A model of asymmetric rifting by simple shear may explain this observation and the consistently landward dips of the rifted margin faults. 相似文献
890.
Monsoon onset over Kerala (India) which occurs every year is a major climatic phenomenon that involves large scale changes in wind, rainfall and sea surface temperature (SST). Over the last 150 years, the date of monsoon onset over Kerala (DMOK) has varied widely, the earliest being 11 May, 1918 and the most delayed being 18 June, 1972. DMOK has a long term (1870–2014) mean of 01 June and standard deviation of 7–8 days. We have studied the inter-annual and decadal time scale variability of DMOK and their relation with SST. We found that SST anomalies of large spatial scale similar to those in El Nino/La Nina are associated with the inter-annual variability in DMOK. Indian Ocean between latitudes \(5^{\circ }\hbox {S}\) and \(20^{\circ }\hbox {N}\) has two episodes of active convection associated with monsoon onset over Kerala (MOK), one around DMOK and the other about six weeks earlier (called pre-monsoon rain peak or bogus monsoon onset) and in between a two week period of suppressed convection occurs over north Indian Ocean. A prominent decadal time scale variability was found in DMOK having large and statistically significant linear correlation with the SST gradient across the equator over Indian and Pacific oceans, the large correlation persisting for several months prior to the MOK. However, no linear trend was seen in DMOK during the long period from 1870 to 2014. 相似文献