排序方式： 共有27条查询结果，搜索用时 953 毫秒
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K M Azam Chowdhury Wensheng Jiang Guimei Liu Md Kawser Ahmed Shaila Akhter 《海洋学报(英文版)》2022,41(4):23-39
In the northern Bay of Bengal, the existence of intense temperature inversion during winter is a widely accepted phenomenon. However, occurrences of temperature inversion during other seasons and the spatial distribution within and adjacent to the Bay of Bengal are not well understood. In this study, a higher resolution spatiotemporal variation of temperature inversion and its mechanisms are examined with mixed layer heat and salt budget analysis utilizing long-term Argo（2004 to 2020） and RAMA（2... 相似文献
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Yawar?HussainEmail author Frédéric?Satgé Muhammad?Babar?Hussain Hernan?Martinez-Carvajal Marie-Paule?Bonnet Martin?Cárdenas-Soto Henrique?Llacer?Roig Gulraiz?Akhter 《Theoretical and Applied Climatology》2018,131(3-4):1119-1132
The present study aims at the assessment of six satellite rainfall estimates (SREs) in Pakistan. For each assessed products, both real-time (RT) and post adjusted (Adj) versions are considered to highlight their potential benefits in the rainfall estimation at annual, monthly, and daily temporal scales. Three geomorphological climatic zones, i.e., plain, mountainous, and glacial are taken under considerations for the determination of relative potentials of these SREs over Pakistan at global and regional scales. All SREs, in general, have well captured the annual north-south rainfall decreasing patterns and rainfall amounts over the typical arid regions of the country. Regarding the zonal approach, the performance of all SREs has remained good over mountainous region comparative to arid regions. This poor performance in accurate rainfall estimation of all the six SREs over arid regions has made their use questionable in these regions. Over glacier region, all SREs have highly overestimated the rainfall. One possible cause of this overestimation may be due to the low surface temperature and radiation absorption over snow and ice cover, resulting in their misidentification with rainy clouds as daily false alarm ratio has increased from mountainous to glacial regions. Among RT products, CMORPH-RT is the most biased product. The Bias was almost removed on CMORPH-Adj thanks to the gauge adjustment. On a general way, all Adj versions outperformed their respective RT versions at all considered temporal scales and have confirmed the positive effects of gauge adjustment. CMORPH-Adj and TMPA-Adj have shown the best agreement with in situ data in terms of Bias, RMSE, and CC over the entire study area. 相似文献
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Michael S. Steckler S. Humayun Akhter Leonardo Seeber 《Earth and Planetary Science Letters》2008,273(3-4):367-378
We take a fresh look at the topography, structure and seismicity of the Ganges–Brahmaputra Delta (GBD)–Burma Arc collision zone in order to reevaluate the nature of the accretionary prism and its seismic potential. The GBD, the world's largest delta, has been built from sediments eroded from the Himalayan collision. These sediments prograded the continental margin of the Indian subcontinent by 400 km, forming a huge sediment pile that is now entering the Burma Arc subduction zone. Subduction of oceanic lithosphere with > 20 km sediment thickness is fueling the growth of an active accretionary prism exposed on land. The prism starts at an apex south of the GBD shelf edge at 18°N and widens northwards to form a broad triangle that may be up to 300 km wide at its northern limit. The front of the prism is blind, buried by the GBD sediments. Thus, the deformation front extends 100 km west of the surface fold belt beneath the Comilla Tract, which is uplifted by 3–4 m relative to the delta. This accretionary prism has the lowest surface slope of any active subduction zone. The gradient of the prism is only 0.1°, rising to 0.5° in the forearc region to the east. This low slope is consistent with the high level of overpressure found in the subsurface, and indicates a very weak detachment. Since its onset, the collision of the GBD and Burma Arc has expanded westward at 2 cm/yr, and propagated southwards at 5 cm/yr. Seismic hazard in the GBD is largely unknown. Intermediate-size earthquakes are associated with surface ruptures and fold growth in the external part of the prism. However, the possibility of large subduction ruptures has not been accounted for, and may be higher than generally believed. Although sediment-clogged systems are thought to not be able to sustain the stresses and strain-weakening behavior required for great earthquakes, some of the largest known earthquakes have occurred in heavily-sedimented subduction zones. A large earthquake in 1762 ruptured 250 km of the southern part of the GBD, suggesting large earthquakes are possible there. A large, but poorly documented earthquake in 1548 damaged population centers at the northern and southern ends of the onshore prism, and is the only known candidate for a rupture of the plate boundary along the subaerial part of the GBD–Burma Arc collision zone. 相似文献
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Thick forest cover and poor infrastructures are the major hindrances for detailed lithologic mapping in an inaccessible montane landscape. To overcome these limitations, we utilize a Landsat 5 TM image to map lithology using vegetation and drainage pattern as an indicator of underlying rock types in a heavily forested region of the Chittagong Hill Tracts area located in southeastern Bangladesh. We use supervised and unsupervised classifiers for a vegetation-based approach while on-screen digitization is used for drainage patterns-based mapping. Field observations were used for mapping lithology and evaluating accuracy. Overall, our results agree well with the current geologic map and improve it by providing a more spatially detailed distribution of the sandstone and shale. The performances of all approaches are good at the inner and outer flanks of anticlines located in the study area while the drainage pattern mapping performs best at the mid-flank area. 相似文献
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Pedrazas Micaela N. Cardenas M. Bayani Hosain Alamgir Demir Cansu Ahmed Kazi Matin Akhter Syed Humayun Wang Lichun Datta Saugata Knappett Peter S. K. 《Hydrogeology Journal》2021,29(4):1601-1610
Fluvio-deltaic aquifers are the primary source of drinking water for the people of Bangladesh. Such aquifers, which comprise the Ganges-Brahmaputra-Meghna Delta, are hydrogeologically heterogeneous. Because of widespread groundwater quality issues in Bangladesh, it is crucial to know the hydrostratigraphic architecture and hydrochemistry, as some aquifer units are contaminated, whereas others are safe. Geophysical methods provide a potentially effective and noninvasive method for extensive characterization of these aquifers. This study applies and investigates the limitations of using electrical resistivity imaging (ERI) for mapping the hydrostratigraphy and salinity of an aquifer-aquitard system adjacent to the Meghna River. Some electrical resistivity (ER) sections showed excellent correlation between resistivity and grain size. These suggest that ERI is a powerful tool for mapping internal aquifer architecture and their boundaries with finer-grained aquitards which clearly appear as low-ER zones. However, in parts of some ER sections, variations in electrical properties were determined by porewater resistivity. In these cases, low ER was indicative of brine and did not indicate the presence of finer-grained materials such as silt or clay. Accordingly, the following hydrostratigraphic zones with different resistivities were detected: (1) aquifers saturated with fresh groundwater, (2) a regional silt/clay aquitard, and (3) a deeper brine-saturated formation. In addition, shallow silt/clay pockets were detected close to the river and below the vadose zone. ERI is thus a promising technique for mapping aquifers versus aquitards; however, the observations are easily confounded by porewater salinity. In such cases, borehole information and groundwater salinity measurements are necessary for ground-truthing.
相似文献8.
A theoretical investigation has been made on the Dust ion-acoustic (DIA) Gardner solitons (GSs) and double layers (DLs) in electronegative plasma consisting of inertial positive and negative ions, super-thermal (kappa distributed) electrons, and negatively charged static dust. The standard reductive perturbation method is employed to derive the Korteweg-de Vries (K-dV), modified K-dV (mK-dV), and standard Gardner equations, which admits solitary waves (SWs) and DLs solutions. It have been found that GSs and DLs exist for α around its critical value α c , where α c is the value of α corresponding to the vanishing of the nonlinear coefficient of the K-dV equation. The parametric regimes for the existence of both the positive as well as negative SWs and negative DLs are obtained. The basic features of DIA SWs and DLs are analyzed and it has been found that the polarity, speed, height, thickness of such DIA SWs and DLs structures, are significantly modified due to the presence of two types of ions and spectral index (κ) of super-thermal electrons. It has also been found that the characteristics of DIA GSs and DLs, are different from that of the K-dV solitons and mK-dV solitons. The relevance of our results to different interstellar space plasma situations are discussed. 相似文献
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Adnan Ahmad Khan Asam Farid Gulraiz Akhter Khyzer Munir James Small Zulfiqar Ahmad 《Surveys in Geophysics》2016,37(3):579-599
The study describes a methodology used to integrate legacy resistivity data with limited geological data in order to build three-dimensional models of the near subsurface. Variogram analysis and inversion techniques more typically found in the petroleum industry are applied to a set of 1D resistivity data taken from electrical surveys conducted in the 1980s. Through careful integration with limited geological data collected from boreholes and outcrops, the resultant model can be visualized in three dimensions to depict alluvium layers as lithological and structural units within the bedrock. By tuning the variogram parameters to account for directionality, it is possible to visualize the individual lithofacies and geomorphological features in the subsurface. In this study, an electrical resistivity data set collected as part of a groundwater study in an area of the Peshawar basin in Pakistan has been re-examined. Additional lithological logs from boreholes throughout the area have been combined with local outcrop information to calibrate the data. Tectonic activity during the Himalayan orogeny has caused uplift in the area and generated significant faulting in the bedrock resulting in the formation of depressions which are identified by low resistivity values representing clays. Paleo-streams have reworked these clays which have been eroded and replaced by gravel–sand facies along paleo-channels. It is concluded that the sediments have been deposited as prograding fan-shaped bodies and lacustrine deposits with interlayered gravel–sand and clay–silt facies. The Naranji area aquifer system has thus been formed as a result of local tectonic activity with fluvial erosion and deposition and is characterized by coarse sediments with high electrical resistivities. 相似文献
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Yawar Hussain Sadia Fida Ullah Muhammad Babar Hussain Abdul Qayyum Aslam Gulraiz Akhter Hernan Martinez-Carvajal Martin Cárdenas-Soto 《Environmental Earth Sciences》2017,76(2):84
The area of Thal Doab is located in the Indus Basin and is underlain by a thick alluvial aquifer called the Thal Doab aquifer (TDA). The TDA is undergone intense hydrological stress owing to rapid population growth and excessive groundwater use for livestock and irrigated agricultural land uses. The potential impact of these land uses on groundwater quality was assessed using a DRASTIC model in a Geographic Information System environment. Seven DRASTIC thematic maps were developed at fixed scale and then combined into a groundwater vulnerability map. The resultant vulnerability index values were grouped into four zones as low, moderate, high and very high. The study has established that 76% of the land area that is underlain by the TDA has a high to very high vulnerability to groundwater contamination mainly because of a thin soil profile, a shallow water table and the presence of soils and sediments with high hydraulic conductivity values. In addition, only 2 and 22% of the total area lie in low and moderate vulnerability zones, respectively. The outcomes of this study can be used to improve the sustainability of the groundwater resource through proper land-use management. 相似文献