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排序方式: 共有249条查询结果,搜索用时 31 毫秒
101.
Clare M. Robertson Martin S. Brook Kat A. Holt Ian C. Fuller Douglas I. Benn 《New Zealand geographer》2013,69(1):14-25
Hooker Glacier in the central Southern Alps of New Zealand has undergone significant downwasting and recession (~2.14 km) during the last two centuries. High retreat rates (51 m a?1 1986–2001, 43 m a?1 2001–2011) have produced a large (1.22 km2) proglacial lake. We present a retreat scenario for Hooker Glacier. A retreat scenario predicts that the glacier terminus will stabilise >3 km up‐valley of the current lake outlet after 2028 when ice velocity equals calving rate. 相似文献
102.
Assimilating transient groundwater flow data via a localized ensemble Kalman filter to calibrate a heterogeneous conductivity field 总被引:2,自引:2,他引:0
Juxiu Tong Bill X. Hu Jinzhong Yang 《Stochastic Environmental Research and Risk Assessment (SERRA)》2012,26(3):467-478
A localized ensemble Kalman filter (EnKF) method is developed to assimilate transient flow data to calibrate a heterogeneous
conductivity field. To update conductivity value at a point in a study domain, instead of assimilating all the measurements
in the study domain, only limited measurement data in an area around the point are used for the conductivity updating in the
localized EnKF method. The localized EnKF is proposed to solve the problems of the filter divergence usually existing in a
data assimilation method without localization. The developed method is applied, in a synthetical two dimensional case, to
calibrate a heterogeneous conductivity field by assimilating transient hydraulic head data. The simulations by the data assimilation
with and without localized EnKF are compared. The study results indicate that the hydraulic conductivity field can be updated
efficiently by the localized EnKF, while it cannot be by the EnKF. The covariance inflation and localization are found to
solve the problem of the filter divergence efficiently. In comparison with the EnKF method without localization, the localized
EnKF method needs smaller ensemble size to achieve stabilized results. The simulation results by the localized EnKF method
are much more sensitive to conductivity correlation length than to the localization radius. The developed localized EnKF method
provides an approach to improve EnKF method in conductivity calibration. 相似文献
103.
Lunar meteoroid impact flashes provide a method to estimate the flux of the large meteoroid flux and thus their hazard to
spacecraft. Although meteoroid impacts on the Moon have been detected using video methods for over a decade, the difficulty
of manually searching hours of video for the rare, extremely brief impact flashes has discouraged the technique’s systematic
implementation. A prototype has been developed for the purpose of automatically searching lunar video records for impact flashes,
eliminating false detections, editing the returned possible flashes, and archiving and documenting the results. Several utilities
for measurement, analysis, and location of the flashes on the moon included in the program are demonstrated. Application of
the program to a year’s worth of lunar observations is discussed along with examples of impact flashes. 相似文献
104.
105.
The final stage in processing radar data so as to arrive at an estimated rain field typically involves a comparison of the preliminary radar-derived estimates of hourly rainfall with those observed by ground-based gauges. Often a mean field bias adjustment will then be applied using an age-weighted average of the individual gauge–radar comparisons. In this paper, a mean field bias adjustment is presented that uses the path-integrated rainfall estimates provided by microwave links together with information from gauges. It is shown to be at least as efficient as the current gauge-based procedure used by the UK Met Office to improve the accuracy of radar-based estimates of rainfall at the ground. 相似文献
106.
by Xinya Li Bill X. Hu William C. Burnett Isaac R. Santos Jeffery P. Chanton 《Ground water》2009,47(4):558-568
Submarine ground water discharge (SGD) is now recognized as an important water pathway between land and sea. It is difficult to quantitatively predict SGD owing to its significant spatial and temporal variability. This study focuses on quantitative estimation of SGD caused by tidally induced sea water recirculation and a terrestrial hydraulic gradient. A two-dimensional hydrogeological model was developed to simulate SGD from a coastal unconfined aquifer in the northeastern Gulf of Mexico, where previous SGD studies were performed. A density-variable numerical code, SEAWAT2000, was applied to simulate SGD. To accurately predict discharge, various influencing factors such as heterogeneity in conductivity, uncertain boundary conditions, and tidal pumping were systematically assessed. The tidally influenced sea water recirculation zone and the fresh water–salt water mixing zone under various tidal patterns, tidal ranges, and water table heights were also investigated. The model was calibrated and validated from long-term, intensive measurements at the study site. The percentage of fresh SGD relative to total SGD ranged from 4% to 50% under normal conditions. Based on simulations of two field measurements in summer and spring, respectively, the fresh water ratios were 9% and 15%, respectively. These results support the hypothesis that the SGD induced by tidally driven sea water recirculation is much larger than terrestrial fresh ground water discharge at this site. The estimates of total and fresh SGD are at the low and high ends, respectively, of the estimation ranges obtained from geochemical tracers (e.g., 222 Rn). 相似文献
107.
Using data assimilation method to calibrate a heterogeneous conductivity field conditioning on transient flow test data 总被引:1,自引:1,他引:0
Juxiu Tong Bill X. Hu Jinzhong Yang 《Stochastic Environmental Research and Risk Assessment (SERRA)》2010,24(8):1211-1223
A data assimilation method is developed to calibrate a heterogeneous hydraulic conductivity field conditioning on transient
pumping test data. The ensemble Kalman filter (EnKF) approach is used to update model parameters such as hydraulic conductivity
and model variables such as hydraulic head using available data. A synthetical two-dimensional flow case is used to assess
the capability of the EnKF method to calibrate a heterogeneous conductivity field by assimilating transient flow data from
observation wells under different hydraulic boundary conditions. The study results indicate that the EnKF method will significantly
improve the estimation of the hydraulic conductivity field by assimilating continuous hydraulic head measurements and the
hydraulic boundary condition will significantly affect the simulation results. For our cases, after a few data assimilation
steps, the assimilated conductivity field with four Neumann boundaries matches the real field well while the assimilated conductivity
field with mixed Dirichlet and Neumann boundaries does not. We found in our cases that the ensemble size should be 300 or
larger for the numerical simulation. The number and the locations of the observation wells will significantly affect the hydraulic
conductivity field calibration. 相似文献
108.
Improved edge detection mapping through stacking and integration: a case study in the Bathurst Mining Camp
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Airborne geophysical surveys provide spatially continuous regional data coverage, which directly reflects subsurface petrophysical differences and thus the underlying geology. A modern geologic mapping exercise requires the fusion of this information to complement what is typically limited regional outcrop. Often, interpretation of the geophysical data in a geological context is done qualitatively using total field and derivative maps. With a qualitative approach, the resulting map product may reflect the interpreter's bias. Source edge detection provides a quantitative means to map lateral physical property changes in potential and non‐potential field data. There are a number of Source edge detection algorithms, all of which apply a transformation to convert local signal inflections associated with source edges into local maxima. As a consequence of differences in their computation, the various algorithms generate slightly different results for any given source depth, geometry, contrast, and noise levels. To enhance the viability of any detected edge, it is recommended that one combines the output of several Source edge detection algorithms. Here we introduce a simple data compilation method, deemed edge stacking, which improves the interpretable product of Source edge detection through direct gridding, grid addition, and amplitude thresholding. In two examples, i.e., a synthetic example and a real‐world example from the Bathurst Mining Camp, New Brunswick, Canada, a number of transformation algorithms are applied to gridded geophysical data sets and the resulting Source edge detection solutions combined. Edge stacking combines the benefits and nuances of each Source edge detection algorithm; coincident or overlapping and laterally continuous solutions are considered more indicative of a true edge, whereas isolated points are taken as being indicative of random noise or false solutions. When additional data types are available, as in our example, they may also be integrated to create a more complete geologic model. The effectiveness of this method is limited only by the resolution of each survey data set and the necessity of lateral physical property contrasts. The end product aims at creating a petrophysical contact map, which, when integrated with known lithological outcrop information, can be led to an improved geological map. 相似文献
109.
Graham D. M. Andrews Michael J. Branney Bill Bonnichsen Michael McCurry 《Bulletin of Volcanology》2008,70(3):269-291
The 80 km long NNE-trending Rogerson Graben on the southern margin of the central Snake River Plain, Idaho, USA, hosts a rhyolitic
pyroclastic succession, 200 m thick, that records a period of successive, late-Miocene, large-volume explosive eruptions from
the Yellowstone–Snake River Plain volcanic province, and contemporaneous extension. The succession, here termed the Rogerson
Formation, comprises seven members (defined herein) and records at least eight large explosive eruptions with numerous repose
periods. Five high-grade and extremely high-grade ignimbrites are intercalated with three non-welded ignimbrites and two volcaniclastic
deposits, with numerous repose periods (palaeosols) throughout. Two of the ignimbrites are dominantly rheomorphic and lava-like
but contain subordinate non-welded pyroclastic layers. The ignimbrites are typical Snake River Plain high-silica rhyolites,
with anhydrous crystal assemblages and high inferred magmatic temperatures (≤ 1,025°C). We tentatively infer that the Jackpot
and Rabbit Springs Members may have been emplaced from the Bruneau–Jarbidge eruptive centre on the basis of: (1) flow lineation
trends, (2) crystal assemblage, and (3) radiometric age. We infer that the overlying Brown’s View, Grey’s Landing, and Sand
Springs Members may have been emplaced from the Twin Falls eruptive centre on the basis of: (1) kinematic indicators (from
the east), and (2) crystal assemblage. Furthermore, we have established the contemporaneous evolution of the Rogerson Graben
from the emplacement of the Jackpot Member onwards, and infer that it is similar to younger half-graben along the southern
margin of the Snake River Plain, formed by local reactivation of Basin and Range structures by the northeastwardly migration
of the Yellowstone hot-spot.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
110.
Susan Ringrose Bill Downey Dane Genecke Francis Sefe Bernard Vink 《Journal of Arid Environments》1999,43(4):375
Quaternary sedimentation in the western Makgadikgadi basin of north central Botswana is evaluated on the basis of new evidence from satellite imagery and sedimentological analyses. Thematic Mapper imagery interpretation, combined with field evidence, has led to the identification of geomorphological features which are mainly composed of light grey calcareous sandstones (formerly calcretes) overlain by dark grey sands. The literature suggests that palaeolake Makgadikgadi I formed and developed intermittently after initial downwarping in the early–mid Pleistocene. The calcareous sandstones were formed when calcium carbonate precipitation took place in pre-existing Kalahari sands along the western shoreline of Makgadikgadi I. Field evidence, supported by X-ray diffraction and SEM analyses, indicates that CaCO3precipitated mainly in marshy conditions around plant roots and stems and in association with bacteria in embayments along the lakeshore. The sandstones thickened and became partially indurated as a result of increasing palaeolake levels. Deposition was terminated by renewed tectonism which uplifted the shoreline zone relative to the lake basin, leading to falling palaeolake levels. Post-uplift reworking led to case hardening and pedogenic calcrete formation in the upper sections of the calcareous sandstones. Sedimentary conditions altered during the late Pleistocene. Extensive distributaries from the proto-Okavango system incised the shoreline ridge contributing to the filling of Makgadikgadi II. Satellite data suggest that the proto-Okavango rivers formed a series of fan deltas at this time along the western Makgadikgadi basin. Widespread dispersal of fluvial grey sands took place as a result of basin tilting which led to anastomosing channels flowing southward possibly around 18,000 B.P. These results, although preliminary in nature, augment previous geomorphological analyses by adding some detail in terms of depositional environments and by providing a tentative age and origin for the ubiquitous grey sands. 相似文献