首页 | 官方网站   微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   4177篇
  免费   184篇
  国内免费   73篇
地球科学   4434篇
  2022年   30篇
  2021年   46篇
  2020年   47篇
  2019年   62篇
  2018年   85篇
  2017年   104篇
  2016年   115篇
  2015年   104篇
  2014年   134篇
  2013年   258篇
  2012年   130篇
  2011年   229篇
  2010年   162篇
  2009年   226篇
  2008年   234篇
  2007年   207篇
  2006年   189篇
  2005年   168篇
  2004年   136篇
  2003年   158篇
  2002年   132篇
  2001年   81篇
  2000年   83篇
  1999年   57篇
  1998年   61篇
  1997年   53篇
  1996年   66篇
  1995年   48篇
  1994年   55篇
  1993年   40篇
  1992年   48篇
  1991年   37篇
  1990年   33篇
  1989年   35篇
  1988年   35篇
  1987年   39篇
  1986年   38篇
  1985年   55篇
  1984年   44篇
  1983年   49篇
  1982年   50篇
  1981年   57篇
  1980年   48篇
  1979年   39篇
  1978年   47篇
  1977年   28篇
  1976年   29篇
  1975年   24篇
  1974年   31篇
  1973年   29篇
排序方式: 共有4434条查询结果,搜索用时 15 毫秒
1.
A prototype electromagnetic vibrator, referred to here as E-Vib, was upgraded and developed for broadband hardrock and mineral exploration seismic surveys. We selected the iron oxide mine in Blötberget, central Sweden, for a test site in 2019 for the newly developed E-Vib because of the availability of earlier seismic datasets (from 2015 to 2016) for verification of its performance for hardrock imaging purposes. The two-dimensional data acquisition consisted of a fixed geometry with 550 receiver locations spaced at every 5 m, employing both cabled and wireless seismic recorders, along an approximately 2.7 km long profile. The E-Vib operated at every second receiver station (i.e. 10 m spacing) with a linear sweep of 2–180 Hz and with a peak force of 7 kN. The processing workflow took advantage of the broadband signal generated by the E-Vib in this challenging hardrock environment with varying ground conditions. The processed seismic section shows a set of reflections associated with the known iron oxide mineralization and a major crosscutting reflection interpreted to be from a fault system likely to be crosscutting the mineralization. The broadband source data acquisition and subsequent processing helped to improve signal quality and resolution in comparison with the earlier workflows and data where a drophammer seismic source was used as the seismic source. These results suggest new possibilities for the E-Vib source for improved targeting in hardrock geological settings.  相似文献   
2.
The term capture, related to the source of water derived from wells, has been used in two distinct yet related contexts by the hydrologic community. The first is a water‐budget context, in which capture refers to decreases in the rates of groundwater outflow and (or) increases in the rates of recharge along head‐dependent boundaries of an aquifer in response to pumping. The second is a transport context, in which capture zone refers to the specific flowpaths that define the three‐dimensional, volumetric portion of a groundwater flow field that discharges to a well. A closely related issue that has become associated with the source of water to wells is streamflow depletion, which refers to the reduction in streamflow caused by pumping, and is a type of capture. Rates of capture and streamflow depletion are calculated by use of water‐budget analyses, most often with groundwater‐flow models. Transport models, particularly particle‐tracking methods, are used to determine capture zones to wells. In general, however, transport methods are not useful for quantifying actual or potential streamflow depletion or other types of capture along aquifer boundaries. To clarify the sometimes subtle differences among these terms, we describe the processes and relations among capture, capture zones, and streamflow depletion, and provide proposed terminology to distinguish among them.  相似文献   
3.
High-biomass red tides occur frequently in some semi-enclosed bays of Hong Kong where ambient nutrients are not high enough to support such a high phytoplankton biomass. These high-biomass red tides release massive inorganic nutrients into local waters during their collapse. We hypothesized that the massive inorganic nutrients released from the collapse of red tides would fuel growth of other phytoplankton species. This could influence phytoplankton species composition. We tested the hypothesis using a red tide event caused by Mesodinium rubrum (M. rubrum) in a semi-enclosed bay, Port Shelter. The red tide patch had a cell density as high as 5.0×105 cells L?1, and high chlorophyll a (63.71 μg L?1). Ambient inorganic nutrients (nitrate: \(\rm{NO}_3^-\), ammonium: \(\rm{NH}_4^+\), phosphate: \(\rm{PO}_4^{3-}\), silicate: \(\rm{SiO}_4^{3-}\)) were low both in the red tide patch and the non-red-tide patch (clear waters outside the red tide patch). Nutrient addition experiments were conducted by adding all the inorganic nutrients to water samples from the two patches followed by incubation for 9 days. The results showed that the addition of inorganic nutrients did not sustain high M. rubrum cell density, which collapsed after day 1, and did not drive M. rubrum in the non-red-tide patch sample to the same high-cell density in the red tide patch sample. This confirmed that nutrients were not the driving factor for the formation of this red tide event, or for its collapse. The death of M. rubrum after day 1 released high concentrations of \(\rm{NO}_3^-\), \(\rm{PO}_4^{3-}\), \(\rm{SiO}_4^{3-}\), \(\rm{NH}_4^+\), and urea. Bacterial abundance and heterotrophic activity increased, reaching the highest on day 3 or 4, and decreased as cell density of M. rubrum declined. The released nutrients stimulated growth of diatoms, such as Chaetoceros affinis var. circinalis, Thalassiothrix frauenfeldii, and Nitzschia sp., particularly with additions of \(\rm{SiO}_4^{3-}\) treatments, and other species. These results demonstrated that initiation of M. rubrum red tides in the bay was not directly driven by nutrients. However, the massive inorganic nutrients released from the collapse of the red tide could induce a second bloom in low-ambient nutrient water, influencing phytoplankton species composition.  相似文献   
4.
The soil factor is crucial in controlling and properly modeling the initiation and development of ephemeral gullies (EGs). Usually, EG initiation has been related to various soil properties (i.e. sealing, critical shear stress, moisture, texture, etc.); meanwhile, the total growth of each EG (erosion rate) has been linked with proper soil erodibility. But, despite the studies to determine the influence of soil erodibility on (ephemeral) gully erosion, a universal approach is still lacking. This is due to the complex relationship and interactions between soil properties and the erosive process. A feasible soil characterization of EG erosion prediction on a large scale should be based on simple, quick and inexpensive tests to perform. The objective of this study was to identify and assess the soil properties – easily and quickly to determine – which best reflect soil erodibility on EG erosion. Forty‐nine different physical–chemical soil properties that may participate in establishing soil erodibility were determined on agricultural soils affected by the formation of EGs in Spain and Italy. Experiments were conducted in the laboratory and in the field (in the vicinity of the erosion paths). Because of its importance in controlling EG erosion, five variables related to antecedent moisture prior to the event that generated the gullies and two properties related to landscape topography were obtained for each situation. The most relevant variables were detected using multivariate analysis. The results defined 13 key variables: water content before the initiation of EGs, organic matter content, cation exchange capacity, relative sealing index, two granulometric and organic matter indices, seal permeability, aggregates stability (three index), crust penetration resistance, shear strength and an erodibility index obtained from the Jet Test erosion apparatus. The latter is proposed as a useful technique to evaluate and predict soil loss caused by EG erosion. Copyright © 2018 John Wiley & Sons, Ltd.  相似文献   
5.
6.
Mineralogy and Petrology - Mineralized NYF and LCT pegmatites occur throughout the northeast-trending Neoproterozoic Damara Belt, Namibia. Mineralization in the pegmatites varies geographically,...  相似文献   
7.
The isotopic composition of evaporites can shed light on their environment of precipitation and their subsequent recycling processes. In this study, we performed Sr, O and S isotopic analyses on evaporitic sulphates in the halokinetic Sivas Basin. The main objectives were to decipher the age and origin of the evaporites responsible for the salt tectonics, and to test whether diapir dissolution acts as the source of younger evaporitic layers in continental mini‐basins. The Sr isotopes demonstrate that the first evaporites precipitated from seawater during the Middle–Late Eocene. The similar isotopic values measured in the halokinetic domain confirm that the Eocene evaporites triggered the salt tectonics and were continuously recycled in Oligo‐Miocene mini‐basins as lacustrine to sabkha evaporites. Modern halite precipitates suggest that the dissolution and recycling of diapiric halite is ongoing. This study demonstrates the efficiency of isotopic analyses in constraining evaporite recycling processes in continental halokinetic domains.  相似文献   
8.
The detrital zircon population in quartzitic conglomerates from the northern Tanzania Craton yield ages between 2640 Ma and 2790 Ma which includes most of the igneous history from this part of the craton. The igneous evolution is characterised by mafic volcanism with an oceanic plateau‐like geochemical signature at ~2800 Ma followed by diorite and tonalite–trondhjemite–granodiorite dominated magmatism between 2790 and 2700 Ma, which transitioned into more evolved high‐K magmatism between 2700 and 2620 Ma. The εHf values of the detrital zircons range from +2.4 to ?1.4 and change with time from radiogenic Hf pre‐2700 Ma (98% positive εHf) to unradiogenic Hf post‐2700 Ma (41% positive εHf). The petrological progression from mafic to felsic crust is reflected in the detrital age distribution and Hf isotopes and is consistent with juvenile mafic crust slowly maturing into more evolved felsic crust through a series of successive partial melting events in an oceanic‐plateau‐like environment.  相似文献   
9.
The Northland region of New Zealand includes numerous high-value, macrophyte-dominated dune lakes. Recent water policy reforms offer limited guidance on managing for aquatic macrophytes. In addition, dune lake histories are poorly known as regular monitoring dates to 2005 AD. Here, ca. 4000 years of lake functional behaviour is reconstructed from sedimentary archives in two Northland dune lakes (Humuhumu and Rotokawau). Results demonstrated that macrophyte dominance is sensitive to catchment erosion and hydrological drawdown. Degradation of macrophyte communities occurred in the nineteenth and twentieth centuries, earlier at Lake Humuhumu than Lake Rotokawau (post-1880 AD and post-1930 AD, respectively). In both lakes, increased erosional influx reduced macrophyte productivity, before later increases to wider trophic state (post-1970 AD). Lake-level decline is linked to increased nutrient loading at Lake Rotokawau but less so, Lake Humuhumu which is more strongly groundwater-fed. In Northland dune lakes, water-level reduction and erosional influx from land use have driven macrophyte degradation.  相似文献   
10.
River discharge and nutrient measurements are subject to aleatory and epistemic uncertainties. In this study, we present a novel method for estimating these uncertainties in colocated discharge and phosphorus (P) measurements. The “voting point”‐based method constrains the derived stage‐discharge rating curve both on the fit to available gaugings and to the catchment water balance. This helps reduce the uncertainty beyond the range of available gaugings and during out of bank situations. In the example presented here, for the top 5% of flows, uncertainties are shown to be 139% using a traditional power law fit, compared with 40% when using our updated “voting point” method. Furthermore, the method is extended to in situ and lab analysed nutrient concentration data pairings, with lower uncertainties (81%) shown for high concentrations (top 5%) than when a traditional regression is applied (102%). Overall, for both discharge and nutrient data, the method presented goes some way to accounting for epistemic uncertainties associated with nonstationary physical characteristics of the monitoring site.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司    京ICP备09084417号-23

京公网安备 11010802026262号