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221.
Adjustment of regularization in ill-posed linear inverse problems by the empirical Bayes approach 总被引:1,自引:0,他引:1
Yuji Mitsuhata 《Geophysical Prospecting》2004,52(3):213-239
Regularization is the most popular technique to overcome the null space of model parameters in geophysical inverse problems, and is implemented by including a constraint term as well as the data‐misfit term in the objective function being minimized. The weighting of the constraint term relative to the data‐fitting term is controlled by a regularization parameter, and its adjustment to obtain the best model has received much attention. The empirical Bayes approach discussed in this paper determines the optimum value of the regularization parameter from a given data set. The regularization term can be regarded as representing a priori information about the model parameters. The empirical Bayes approach and its more practical variant, Akaike's Bayesian Information Criterion, adjust the regularization parameter automatically in response to the level of data noise and to the suitability of the assumed a priori model information for the given data. When the noise level is high, the regularization parameter is made large, which means that the a priori information is emphasized. If the assumed a priori information is not suitable for the given data, the regularization parameter is made small. Both these behaviours are desirable characteristics for the regularized solutions of practical inverse problems. Four simple examples are presented to illustrate these characteristics for an underdetermined problem, a problem adopting an improper prior constraint and a problem having an unknown data variance, all frequently encountered geophysical inverse problems. Numerical experiments using Akaike's Bayesian Information Criterion for synthetic data provide results consistent with these characteristics. In addition, concerning the selection of an appropriate type of a priori model information, a comparison between four types of difference‐operator model – the zeroth‐, first‐, second‐ and third‐order difference‐operator models – suggests that the automatic determination of the optimum regularization parameter becomes more difficult with increasing order of the difference operators. Accordingly, taking the effect of data noise into account, it is better to employ the lower‐order difference‐operator models for inversions of noisy data. 相似文献
222.
Petrochemical evidence for off-ridge magmatism in a back-arc setting from the Yakuno ophiolite, Japan 总被引:2,自引:0,他引:2
Abstract The Permian ophiolite emplaced in the Yakuno area, Kyoto Prefecture, consists of metavolcanic sequences, metagabbro and a troctolitic intrusion. The metavolcanics are associated with thick mudstone through a contact that shows the flowage of lava over unconsolidated mud layers on the sea floor. The metavolcanics and metagabbro have rare earth element (REE) patterns that are similar to enriched (E)‐ and transitional (T)‐types ([La/Yb]N = 0.77–11.2) of mid‐oceanic ridge basalts (MORB), whereas their Nb/La ratios (0.40–1.20) are as low as those of back‐arc basin basalts (BABB). Cr‐spinels in the metavolcanic rocks have Cr? of 40–73 and an Fe3+? of 9–24, numbers which are comparable to the values of BABB. These lines of evidence suggest that the Yakuno ophiolite originated more likely from an early stage back‐arc basin rather than from an oceanic plateau, as has been suggested by some researchers. The troctolitic body that intrudes as a 0.5‐km long lens in the metagabbro is composed of troctolite, olivine gabbro and microgabbro. The troctolite is marked by an olivine–plagioclase crystallization sequence, different from the commonly observed olivine–clinopyroxene sequence in other mafic/ultramafic cumulates of the Yakuno ophiolite. The microgabbro, with a composition close to that of the parental magma of the troctolite, is depleted in light REE ([La/Yb]N = 0.18–0.55) so that it has an REE pattern that mimics normal (N)‐type MORB. The interstitial clinopyroxene of the troctolite has highly variable TiO2 contents (0.2–1.4 wt%), which is interpreted to result from postcumulus crystallization of heterogeneous intercumulus melts. The troctolitic intrusion may represent a late stage intrusion that formed in an off‐ridge environment during sea floor spreading of the back‐arc basin. The geochemical variation observed in the Yakuno ophiolite, ranging from N‐ to E‐MORB affinities, reflects the changes in both mantle source compositions and processes involved in magma generation during the evolution of the back‐arc basin. 相似文献
223.
Yuji Tanaka 《Journal of Oceanography》1992,48(4):461-472
Near surface vertical distributions of Japanese anchovy eggs (isolated and pelagic) were studied at western Wakasa Bay, Japan. Samples were collected in horizontal tows with four plankton nets simultaneously operated at different four depths of 0, 0.5, 1.0, and 2.0 m. The egg concentration was found to decrease exponentially with depth. The egg concentration profile can be explained by considering the balance of the eggs' ascent and vertical diffusion. Taking further into account vertical difference in rising/sinking rates of the eggs, a clear accumulation of the eggs on a pycnocline, observed elsewhere, also can be explained. Vertical eddy diffusivity in the surface layer in a calm sea was 1–10 cm2 s–1, as estimated from the egg concentration profile and the velocities of the eggs in the vertical direction. 相似文献
224.
225.
Hiroshi?IshidaEmail author Yuji?Watanabe Tatsuo?Fukuhara Sho?Kaneko Kazushi?Furusawa Yoshihisa?Shirayama 《Journal of Oceanography》2005,61(5):835-843
In order to evaluate the environmental impact associated with sequestration of carbon dioxide in the deep sea, a free fall
type field experimental device, the benthic chamber, was developed. In situ experiments to expose deep-sea communities to elevated concentrations of carbon dioxide (average of 20,000 ppm, 5,000 ppm
and control) were carried out using this device 3 times, viz., in the winter of 2002 and in the spring and the summer of 2003,
in the Kumano Trough at a depth of 2,000 m. In the long-term experiments (about two weeks in winter of 2002 and summer of
2003), the abundance of meiobenthos declined whereas that of bacteria increased under the condition of 20,000 ppm carbon dioxide
compared with the control. Among meiofauna, the abundance of foraminifers at the same concentration of carbon dioxide became
less than the control even in the short-term (3 days in spring of 2003) experiment, suggesting that organisms with a calcium
carbonate exoskeleton are more sensitive to the raised concentration of carbon dioxide. The respiration rate of the benthic
community exposed to 20,000 ppm was lower in the early stage of the experiment than in the latter half, whereas it was opposite
under the condition of 5,000 ppm. The increase of biological activity in the 20,000 ppm exposure group is probably due to
an increase of bacteria adapted to high carbon dioxide concentrations. The present results suggest that the influence of carbon
dioxide on the deep-sea benthic ecosystem does not follow a simple, linear relationship with concentration. 相似文献
226.
A quadrupole-based mass spectrometric system for determining noble gas (He, Ne, Ar, Kr and Xe) abundances in a liquid sample has been developed and applied to seawater equilibrated with the atmosphere at several temperatures. After known amounts of isotopic spikes (22Ne, 36Ar, 86Kr and 124Xe) were introduced into the preparation vacuum line, dissolved gases were extracted from seawater and well mixed with the spikes by ultrasonic vibration. Noble gases were purified using three-stage hot Ti getters and separated by two activated charcoal traps held at low temperature (liquid nitrogen and dry ice—ethanol). Noble gas abundances were measured by a relatively inexpensive quadrupole mass spectrometer based on the isotope dilution technique. Precisions obtained by repeated measurements for He, Ne, Ar, Kr and Xe are 1.5%, 0.4%, 0.4%, 0.4% and 0.6%, respectively. Noble gas abundances in air-saturated seawater at eight temperature steps (1.0°C, 5.5°C, 10.0°C, 14.4°C, 17.0°C, 21.3°C, 24.6°C, and 29.6°C) were measured using the analytical system. The overall variation of the abundances with temperature agrees well with those of NaCl brines reported in the literature. We propose here new constants to fit observed data to empirical equation of noble gas solubility in seawater with its temperature variation. 相似文献
227.
Katsuhisa Tanaka Kazumaro Okamura Katsunori Kimoto Hiroshi Yagi Masashi Kodama 《Journal of Oceanography》2007,63(1):143-148
CDB-P (phosphorus extractable by the citrate-dithionite-bicarbonate extraction procedure) was used as the indicator of bio-available
inorganic P in the suspended and surface sediments in the inner part of Ariake Bay. The CDB-P pool of suspended and surface
sediments in the tidal flat area was estimated to be 10 ton and 90 ton, respectively (total of 100 ton), which corresponds
to 1.5 times the average dissolved inorganic phosphorus standing stock. During the summer periods of hypoxia, the CDB-P concentration
in the surface sediments decreased with the decline of Eh, losing 43–47% of the CDB-P observed in April. These results suggest
that the phosphorus dynamics have been affected by the decrease of resuspended sediment concentrations brought about by the
reduction of the tidal currents and phosphate release from the surface sediments during frequently occurring summer hypoxia
events. 相似文献
228.
Sources of systematic error in the sampling procedure by Winkler method were examined. In the laboratory experiment, the dissolution of atmospheric oxygen into oxygenfree sea water during sampling amounts to 0.022 ml/l, and the effect of dissolved oxygen in sea water remaining in oxygen bottle is 0.019 ml/l. The effect of reagents added amounts to 0.017 ml/l. In the sampling procedures aboard, the effect of dissolution became bigger, and the table is presented for the correction to the reported value of dissolved oxygen measured by the “Manual of Oceanographic Observation”. The corrected value of dissolved oxygen in the routine analysis may not be correct within 0.02 ml/l. 相似文献
229.
The Yurihara oil and gas field is located on the southern edge of Akita Prefecture, northeastern Japan. In this area, drilling, surface geological surveys and many seismic surveys have been used to investigate the geological structure. Wells drilled into the Nishikurosawa Basalt Group (NBG) of Miocene age found oil and gas reservoirs at depths of 1.5–2 km. Oil and gas are now being produced commercially and further exploration is required in the surrounding areas. However, since the neighbouring areas are covered with young volcanic products from the Chokai volcano, and have a rough topography, the subsurface distribution of the NBG must be investigated using other methods in addition to seismic reflection. According to the well data, the resistivity of the NBG is comparatively higher than that of the overlying sedimentary formations, and therefore the magnetotelluric (MT) method is expected to be useful for the estimation of the distribution of the NBG. An MT survey was conducted along three survey lines in this area. Each line trended east–west, perpendicular to the regional geological strike, and was composed of about 25 measurement sites. Induction vectors evaluated from the magnetic field show that this area has a two-dimensional structure. The evaluated resistivity sections are in agreement with the log data. In conclusion, we were able to detect resistive layers (the NBG) below conductive layers. The results indicate that the NBG becomes gradually less resistive from north to south. In the centre of the northern line, an uplifted resistive area is interpreted as corresponding to the reservoir. By comparison with a seismic section, we prove the effectiveness of the integration of seismic and MT surveys for the investigation of the morphology and internal structure of the NBG. On other survey lines, the resistive uplifted zones are interpreted as possible prospective areas. 相似文献
230.
All professions began the countdown to the new world, considering the year 2000 to be the first year of the new millennium. Most of the actual impacts are already in place, including multinational businesses and the Euro as the first regional multinational currency. Most of the impacts have been harsh, and relief to individual consumers and professional people will be minor and felt mainly in enhanced ease of communication and media offerings. On the side of the conduct of the profession, the advantages will be far more narrow, as individual practitioners will be facing far stronger forms of competition for their services. Truly, the only way to survive will be for all of us to become more aware of the forces that impact engineering geology and the opportunities that engineering geologists have to provide their services, so basic to all forms of engineering, environmental protection and resource utilization. 相似文献