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排序方式: 共有196条查询结果,搜索用时 203 毫秒
61.
Magnetic resonance sounding applied to aquifer characterization 总被引:3,自引:0,他引:3
Legchenko A Baltassat JM Bobachev A Martin C Robain H Vouillamoz JM 《Ground water》2004,42(3):363-373
Magnetic resonance sounding (MRS) is distinguished from other geophysical tools used for ground water investigation by the fact that it measures a magnetic resonance signal generated directly from subsurface water molecules. An alternating current pulse energizes a wire loop on the ground surface and the MRS signal is generated; subsurface water is indicated, with a high degree of reliability, by nonzero amplitude readings. Measurements with varied pulse magnitudes then reveal the depth and thickness of water saturated layers. The hydraulic conductivity of aquifers can also be estimated using boreholes for calibration. MRS can be used for both predicting the yield of water supply wells and for interpolation between boreholes, thereby reducing the number of holes required for hydrogeological modeling. An example of the practical application of MRS combined with two-dimensional electrical imaging, in the Kerbernez and Kerien catchments area of France, demonstrates the efficiency of the technique. 相似文献
62.
Mathias Göckede Tiina Markkanen Charlotte B. Hasager Thomas Foken 《Boundary-Layer Meteorology》2006,118(3):635-655
Horizontal heterogeneity can significantly affect the flux data quality at monitoring sites in complex terrain. In heterogeneous conditions, the adoption of the eddy-covariance technique is contraindicated by the lack of horizontal homogeneity and presence of advective conditions. In addition, uncertainty concerning the sources or sinks influencing a measurement compromises the data interpretation. The consideration of the spatial context of a measurement, defined by a footprint analysis, can therefore provide an important tool for data quality assessment. This study presents an update of an existing footprint-based quality evaluation concept for flux measurement sites in complex terrain. The most significant modifications in the present version are the use of a forward Lagrangian stochastic trajectory model for the determination of the spatial context of the measurements, and the determination of effective roughness lengths with a flux aggregation model in a pre-processing step. Detailed terrain data gathered by remote sensing methods are included. This approach determines spatial structures in the quality of flux data for varying meteorological conditions. The results help to identify terrain influences affecting the quality of flux data, such as dominating obstacles upwind of the site, or slopes biasing the wind field, so that the most suitable footprint regions for the collection of high-quality datasets can be identified. Additionally, the approach can be used to evaluate the performance of a coordinate rotation procedure, and to check to what extent the measured fluxes are representative for a target land-use type. 相似文献
63.
William F. Ruddiman Rose Marie L. Cline James D. Hays Warren L. Prell William F. Ruddiman Ted C. Moore Nilva G. Kipp Barbara E. Molfino George H. Denton Terence J. Hughes William L. Balsam Charlotte A. Brunner Jean-Claude Duplessy James L. Fastook John Imbrie Lloyd D. Keigwin Thomas B. Kellogg Peter R. Thompson 《Quaternary Research》1984,21(2):123-224
The final effort of the CLIMAP project was a study of the last interglaciation, a time of minimum ice volume some 122,000 yr ago coincident with the Substage 5e oxygen isotopic minimum. Based on detailed oxygen isotope analyses and biotic census counts in 52 cores across the world ocean, last interglacial sea-surface temperatures (SST) were compared with those today. There are small SST departures in the mid-latitude North Atlantic (warmer) and the Gulf of Mexico (cooler). The eastern boundary currents of the South Atlantic and Pacific oceans are marked by large SST anomalies in individual cores, but their interpretations are precluded by no-analog problems and by discordancies among estimates from different biotic groups. In general, the last interglacial ocean was not significantly different from the modern ocean. The relative sequencing of ice decay versus oceanic warming on the Stage 6/5 oxygen isotopic transition and of ice growth versus oceanic cooling on the Stage 5e/5d transition was also studied. In most of the Southern Hemisphere, the oceanic response marked by the biotic census counts preceded (led) the global ice-volume response marked by the oxygen-isotope signal by several thousand years. The reverse pattern is evident in the North Atlantic Ocean and the Gulf of Mexico, where the oceanic response lagged that of global ice volume by several thousand years. As a result, the very warm temperatures associated with the last interglaciation were regionally diachronous by several thousand years. These regional lead-lag relationships agree with those observed on other transitions and in long-term phase relationships; they cannot be explained simply as artifacts of bioturbational translations of the original signals. 相似文献
64.
A stratigraphically consistent <20-cm-thick unit of microkrystite spherule and microtektite-bearing impact fallout ejecta overlying volcanic tuff of the 4th Shale Macroband (DGS4) of the Dales Gorge Member (2.47–2.50 Ga), Brockman Iron Formation, Hamersley Group, Western Australia, displays anomalous platinum group element (PGE) and other trace metal patterns. The unit has high Ir (13 ppb) and Pt (35 ppb), and low Pd (2.7 ppb) and Au (1.55–1.88 ppb). The low Pd/Ir ratios and low Cr/V suggest depletion in volatile PGE and metals relative to refractory PGE and V, contrasted to the ubiquitous high Pd/Ir of most terrestrial rocks. Marked depletion in the volatile Rare Earth Element (REE) abundances in stilpnomelane spherule cores is consistent with this model. The loss of volatile PGE, analogous to relations in 3.24 Ga impact fallout units of the Barberton greenstone belt (S3 and S4), suggests fractionation related to atmospheric spherule condensation. The microkrystite spherule unit locally incorporate fragments and up to meter-scale boulders of banded chert and stromatolite carbonate, suggesting tsunami transport postdating spherule deposition. DGS4 microkrystite spherules are dominated by stilpnomelane mantled by K-feldspar shells, which consist of inward-radiating fibrous feldspar aggregates suggestive of devitrification. The K and REE enrichment of spherule margins are contrasted to flat REE patterns of the stilpnomelane cores, suggesting adsorption of lithophile elements during settling of the spherules through the hydrosphere. K-feldspar shells contain submicron-scale Ni metal, oxide, sulfide and arsenide grains and euhedral needles of feldspar-exsolved ilmenite. Associated magnetite may have high nickel (<1.25% NiO). The generally mafic composition of the spherules and high Ni/Cr and Ni/Co are consistent with a target mafic-ultramafic crust, consistent with the lack of shock-metamorphosed quartz. Mixing calculations suggest a contribution of 2.5–3% projectile component to the impact-generated volatile cloud. Conservative mass balance estimates derived from the Ir and Pt flux, assuming global extent of a 10-cm-thick spherule unit and chondritic projectile composition, suggest an asteroid diameter on the scale of 30 km. Similar estimates are obtained from spherule sizes, which in DGS4 reach a mean diameter of 2.0 mm in aerodynamically elongate spherules. The evidence implies formation of an impact basin on the scale of 400 km in simatic/oceanic regions of the early Proterozoic crust. 相似文献
65.
Evolution of a post-batholith dike swarm in central coastal Queensland, Australia: arc-front to backarc? 总被引:8,自引:0,他引:8
Charlotte M. Allen 《Lithos》2000,51(4):331-349
A swarm of felsic and mafic dikes cuts a Late Carboniferous–Permian batholith called the Urannah Suite in central coastal Queensland. Late Permian–Triassic westward thrusting (Hunter–Bowen Orogeny) exposed this mid-crustal Suite and the crosscutting dikes, thus enabling examination of dikes that range in age from syn- to post-batholithic. Although both mafic and felsic dikes have the same dominant northerly strike, field, geochronologic and geochemical examination reveal that the swarm is composite; felsic dikes are older (285 Ma) and geochemically and isotopically distinct from mafic dikes (273–229 Ma). Dike compositions are compared to those of the host plutonic rocks, and to volcanic rocks the same age as the dikes. Whereas the felsic (older) dikes are compositionally similar to their host granites (initial 87Sr/86Sr>0.7045), the mafic (younger) dikes are isotopically (Sr, Nd, Pb) less radiogenic. Moreover, several different types of mafic dikes are evident based on geochemistry; most of these have mixed characteristics in terms of tectonic classification. All but two dikes of basalt and basaltic andesite composition classify as ‘with-in plate' on Ti–Zr–Y tectonic classification plots. Many of the basalts have high TiO2 contents (1.5–3.0 wt.%). Most of these have REE and spider diagram patterns like calc-alkaline tholeiites, the exceptions being a few alkali basalts recognized by their alkali content, and high Ti, Ce, Nb and Zr contents. When put into the context of all plutonic rocks in the area (late Paleozoic and Mesozoic), these dikes record a transition at 280 Ma, after which time, all magmatism in the region is less isotopically evolved (initial 87Sr/86Sr=0.7033–0.7044). A model of slab retreat and hinge movement to the east in the latest Permian explains the change of geochemical signature from arc-front to backarc at about 280 Ma. 相似文献
66.
67.
Paleokarst system development in the San Andres Formation, Permian Basin, revealed by seismic characterization 总被引:1,自引:0,他引:1
Paleokarst systems are one of the major factors resulting in carbonate reservoir heterogeneity and compartmentalization. Nevertheless, few effective workflows have been proposed to map the 3D distribution of such systems. We describe a detailed seismic characterization approach integrating core, well log and rock physics analysis, to reveal a complex subsurface paleokarst system in the San Andres Formation, Permian basin, West Texas. In the area of high volume production, the collapsed paleokarst system is characterized by irregularly developed crackle and fracture breccias, mosaic breccias and cave fillings in the Upper San Andres Formation, which are delineated using seismic acoustic impedance. Along the transition from platform to basin, the paleokarst system is marked by a linear collapse including sags and small vertical faults that are recognizable in seismic imaging. Production data indicates that tight paleokarst zones cause reservoir compartmentalization and influence fluid communication between wells. The complex paleokarst system development is explained using a carbonate platform hydrological model, an outcrop analogue similar to modern marine hydrological environments within carbonate islands. Our method of model development for complex subsurface paleokarst systems may be applicable to other paleoenvironments. 相似文献
68.
Cassandra R. Fenton Reginald L. Hermanns Lars H. Blikra Peter W. Kubik Charlotte Bryant Samuel Niedermann Anette Meixner Mirjam M. Goethals 《Quaternary Geochronology》2011,6(5):437-452
Two rock avalanches in Troms County – the Grøtlandsura and Russenes – were selected as CRONUS-EU natural cosmogenic 10Be production-rate calibration sites because they (a) preserve large boulders that have been continuously exposed to cosmic irradiation since their emplacement; (b) contain boulders with abundant quartz phenocrysts and veins with low concentrations of naturally-occurring 9Be (typically < 1.5 ppb); and (c) have reliable minimum radiocarbon ages of 11,424 ± 108 cal yr BP and 10,942 ± 77 cal yr BP (1σ), respectively. Quartz samples (n = 6) from these two sites contained between 4.28 × 104 and 5.06 × 104 at 10Be/g using the 1.387 Myr 10Be half-life. Determination of these concentrations accounts for topographic and self-shielding, and effects on nuclide production due to isostatic rebound are shown to be negligible. Persistent, constant snow and moss cover cannot be proven, but if taken into consideration they may have reduced 10Be concentrations by 10%. Using the 10Be half-life of 1.387 Myr and the Stone scaling scheme, and accounting for snow- and moss-cover, we calculate an error-weighted mean total 10Be production rate of 4.12 ± 0.19 at/g/yr (1σ). A corresponding error-weighted mean spallogenic 10Be production rate is 3.96 ± 0.16 at/g/yr (1σ), respectively. These are in agreement within uncertainty with other 10Be production rates in the literature, but are significantly, statistically lower than the global average 10Be production rate. This research indicates, like other recent studies, that the production of cosmogenic 10Be in quartz is lower than previously established by other production-rate calibration projects. Similarly, our findings indicate that regional cosmogenic production rates should be used for determining exposure ages of landforms in order to increase the accuracy of those ages. As such, using the total 10Be production rate from our study, we determine an error-weighted mean surface-exposure age of a third rock avalanche in Troms County (the Hølen avalanche) to be 7.5 ± 0.3 kyr (1σ). This age suggests that the rock avalanche occurred shortly after the 8.2 kyr cooling event, just as the radiocarbon ages of the Grøtlandsura and Russenes avalanches confirm field evidence that those rock-slope failures occurred shortly after deglaciation. 相似文献
69.
Timing and source constraints on the relationship between mafic and felsic intrusions in the Emeishan large igneous province 总被引:8,自引:0,他引:8
Hong Zhong Ian H. Campbell Charlotte M. Allen Lie-Wen Xie 《Geochimica et cosmochimica acta》2011,75(5):1374-1395
Several I- and A-type granite, syenite plutons and spatially associated, giant Fe-Ti-V deposit-bearing mafic-ultramafic layered intrusions occur in the Pan-Xi (Panzhihua-Xichang) area within the inner zone of the Emeishan large igneous province (ELIP). These complexes are interpreted to be related to the Emeishan mantle plume. We present LA-ICP-MS and SIMS zircon U-Pb ages and Hf-Nd isotopic compositions for the gabbros, syenites and granites from these complexes. The dating shows that the age of the felsic intrusive magmatism (256.2 ± 3.0-259.8 ± 1.6 Ma) is indistinguishable from that of the mafic intrusive magmatism (255.4 ± 3.1-259.5 ± 2.7 Ma) and represents the final phase of a continuous magmatic episode that lasted no more than 10 Myr. The upper gabbros in the mafic-ultramafic intrusions are generally more isotopically enriched (lower εNd and εHf) than the middle and lower gabbros, suggesting that the upper gabbros have experienced a higher level of crustal contamination than the lower gabbros. The significantly positive εHf(t) values of the A-type granites and syenites (+4.9 to +10.8) are higher than those of the upper gabbros of the associated mafic intrusion, which shows that they cannot be derived by fractional crystallization of these bodies. They are however identical to those of the mafic enclaves (+7.0 to +11.4) and middle and lower gabbros, implying that they are cogenetic. We suggest that they were generated by fractionation of large-volume, plume-related basaltic magmas that ponded deep in the crust. The deep-seated magma chamber erupted in two stages: the first near a density minimum in the basaltic fractionation trend and the second during the final stage of fractionation when the magma was a low density Fe-poor, Si-rich felsic magma. The basaltic magmas emplaced in the shallow-level magma chambers differentiated to form mafic-ultramafic layered intrusions accompanied by a small amount of crustal assimilation through roof melting. Evolved A-type granites (synenites and syenodiorites) were produced dominantly by crystallization in the deep crustal magma chamber. In contrast, the I-type granites have negative εNd(t) [−6.3 to −7.5] and εHf(t) [−1.3 to −6.7] values, with the Nd model ages () of 1.63−1.67 Ga and Hf model ages () of 1.56−1.58 Ga, suggesting that they were mainly derived from partial melting of Mesoproterozoic crust. In combination with previous studies, this study also shows that plume activity not only gave rise to reworking of ancient crust, but also significant growth of juvenile crust in the center of the ELIP. 相似文献
70.
Mechanisms controlling silicon isotope distribution in the Eastern Equatorial Pacific 总被引:1,自引:0,他引:1
The distribution of silicon isotopes along a meridional transect at 140°W longitude in the Eastern Equatorial Pacific was used to test the hypothesis that δ30Si of silicic acid in surface waters should correlate with net silica production rates (gross silica production minus silica dissolution) rather than rates of gross silica production due to the opposing Si isotope fractionations associated with silica production and silica dissolution. Variations in δ30Si appeared significantly correlated with net silica production rates in equatorial surface waters and not with gross production rates. Around the Equator, values of δ30Si as low as deep water values occurred in the upper mesopelagic in a zone of net silica dissolution and high detrital biogenic silica content, where the release of low δ30Si silicic acid from opal dissolution would be expected to decrease δ30Si. The δ30Si of the deep water at 140°W appears constant for depths >2000 m and is similar to the deep water at 110°W. This study brings to light the importance of considering Si fractionation during diatom silica dissolution, the biological fractionation during silica production and physical factors such as currents and mixing with adjacent water masses when interpreting silicon isotope distributions. 相似文献