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84.
Ali Kiliçoğlu Ahmet Direnç Hasan Yildiz Murat Bölme Bahadir Aktuğ Mehmet Simav Onur Lenk 《Studia Geophysica et Geodaetica》2011,55(4):557-578
Turkish regional geoid models have been developed by employing a reference earth gravitational model, surface gravity observations
and digital terrain models. The gravimetric geoid models provide a ready transformation from ellipsoidal heights to the orthometric
heights through the use of GPS/leveling geoid heights determined through the national geodetic networks. The recent gravimetric
models for Turkish territory were computed depending on OSU91 (TG-91) and EGM96 (TG-03) earth gravitational models. The release
of the Earth Gravitational Model 2008 (EGM08), the collection of new surface gravity observations, the advanced satellite
altimetry-derived gravity over the sea, and the availability of the high resolution digital terrain model have encouraged
us to compute a new geoid model for Turkey. We used the Remove-Restore procedure based on EGM08 and applied Residual Terrain
Model (RTM) reduction of the surface gravity data. Fast Fourier Transformation (FFT) was then used to obtain the residual
quasigeoid from the reduced gravity. We restored the individual contributions of EGM08 and RTM to the whole quasi-geoid height
(TQG-09). Since the Helmert orthometric height system is adopted in Turkey, the quasi-geoid model (TQG-09) was then converted
to the geoid model (TG-09) by making use of Bouguer gravity anomalies and digital terrain model. After all we combined a gravimetric
geoid model with GPS/leveling geoid heights in order to obtain a hybrid geoid model (THG-09) (or a transformation surface)
to be used in GPS applications. The RMS of the post-fit residuals after the combination was found to be ± 0.95 cm, which represents
the internal precision of the final combination. And finally, we tested the hybrid geoid model with GPS/leveling data, which
were not used in the combination, to assess the external accuracy. Results show that the external accuracy of the THG-09 model
is ± 8.4 cm, a precision previously not achieved in Turkey until this study. 相似文献
85.
A GIS model for preliminary hazard assessment of swelling clays,a case study in Harran plain (SE Turkey) 总被引:2,自引:1,他引:1
Swelling behavior of clayey soils regarded as a hidden disaster, causes a great deal of damage in light hydraulic structures
such as drinking water network, irrigation pipes or open canal linings through which water can easily leak and penetrate into
soil during loading and unloading stages. Early identification during site investigation and laboratory testing is extremely
important to ensure that the appropriate design strategy is adopted. The clay soils having swelling potential are generally
found in arid and semi-arid regions, such as in the Şanlıurfa-Harran plain located in the southeast of Turkey. The problems
associated with swelling clays occurred for the lightweight hydraulic structures constructed in Turkey have been met during
the construction of irrigation structures in the scope of Southeast Anatolian Project as the Turkey’s greatest water resources
project. Therefore, the identification of such soils and the assessment of their swelling potential parameters are necessary
for hazard mitigation planning and land-use management. In this scope, extensive geotechnical study is executed for an investigation
area. The samples were obtained from the Harran plain where many irrigation canal structures are constructed, and geotechnical
study was performed on these samples. Atterberg limit tests often provide the basic field information to substantiate the
soil’s swelling nature. The plasticity index (PI) and liquid limit (LL) values are also used extensively for classifying swelling
soil and should always be determined during preliminary investigations. Because of this, LL and PI are assessed by using geographical
information system (GIS)-based computer software, and LL and PI contour maps are created. Swelling percentages of the soil
samples were carried out on both undisturbed and compacted soil samples by using direct methods. As a result of this extensive
study, the values of swelling percentages determined for each location are used to obtain the swelling potential hazard map
of the area by means of a GIS program. The results of GIS analyses for this area indicate that the analyses based on a lot
of data introduce meaningful results for this study. It is expected that these maps will be a useful tool for planners and
engineers in their efforts to achieve better land-use planning and to decide necessary remedial measures. 相似文献
86.
Shirin Mohammadkhan Hasan Ahmadi Mohammad Jafari 《Arabian Journal of Geosciences》2011,4(1-2):331-338
Soil erosion is the dominating factor of damaging roads in Iran. Roads are a critical component of civilization. Developing and maintaining the economic activity that is vital for the quality of modern life would be difficult without roads. Accelerated erosion and increased sediment yields resulting from changes in land use are critical environmental problems, and one of the important changes is road construction. Construction and condition of roadways are associated with direct and indirect impacts on soil erosion. This paper presents the relationship between soil erosion and some characteristics such as distance to roadway, slope, and parent material in Latian Watershed, Tehran Province, Iran. Soil erosion map was interpreted using aerial photos and GIS technology. Buffer zones, subdivided into 12 strips, each 0.2 km wide, which were located alongside roadway. The distribution patterns of various types of erosion were also identified by GIS by overlaying road buffer strips and soil erosion map. The results show that soil erosion landform such as land slide, badland, bank erosion, channel erosion, etc. found in buffer zones along both sides of roads. 相似文献
87.
The system of ST Aquarii has been observed inB andV filters at KOTTAMIA Observatory in Egypt. Two light curves have been obtained and new times of minima have been calculated.The light curves have been analysed using the theory of Fourier transform of light changes in frequencydomain, and new geometrical and physical elements have been obtained. 相似文献
88.
We use the Vlasov equations for ions and electrons to develop a theory of a double layer in which there are both free and trapped electrons and ions. We find the equations which replace the Langmuir condition and the Bohm conditions and by numerically solving the resultant differential equation we find for particular choices of distribution functions the potential distribution in the layer. We discuss the applicability of this theory to solar flares, and show that conditions in solar flares may be such that double layers can exist for which the free particles have a power-law energy distribution. These particles will be accelerated in a double layer and may in this way account for the production of high-energy particles during the impulsive phase of solar flares. 相似文献
89.
The time-dependent interaction of the granulation velocity field with a magnetic flux tube is investigated here. It is seen
that when a magnetic field line is displaced normal to itself so as to simulate thebuffeting action of granules, a flow of gas is initiated along the field. By choosing a lateral velocity field which is consistent
with observations of granules, it is found that the resulting gas motion is a downward flow with a velocity compatible with
the observed downflow in isolated photospheric flux tubes. It is therefore proposed that the observed photospheric downflow
is a manifestation of the interaction of granules with flux tubes. 相似文献
90.
Arsenic in shallow groundwater of Bangladesh: investigations from three different physiographic settings 总被引:1,自引:0,他引:1
M. Aziz Hasan K. Matin Ahmed Ondra Sracek Prosun Bhattacharya Mattias von Brömssen Sandra Broms Johan Fogelström M. Lutful Mazumder Gunnar Jacks 《Hydrogeology Journal》2007,15(8):1507-1522
Occurrences of arsenic (As) in the Bengal Basin of Bangladesh show close relationships with depositional environments and sediment textures. Hydrochemical data from three sites with varying physiography and sedimentation history show marked variations in redox status and dissolved As concentrations. Arsenic concentration in groundwater of the Ganges Flood Plain (GFP) is characteristically low, where high Mn concentrations indicate redox buffering by reduction of Mn(IV)-oxyhydroxides. Low DOC, \( {\text{HCO}}^{ - }_{3} \), \( {\text{NH}}^{ + }_{4} \) and high \( {\text{NO}}^{ - }_{3} \) and \( {\text{SO}}^{{2 - }}_{4} \) concentrations reflect an elevated redox status in GFP aquifers. In contrast, As concentration in the Ganges Delta Plain (GDP) is very high along with high Fe and low Mn. In the Meghna Flood Plain (MFP), moderate to high As and Fe concentrations and low Mn are detected. Degradation of organic matter probably drives redox reactions in the aquifers, particularly in MFP and GDP, thereby mobilising dissolved As. Speciation calculations indicate supersaturation with respect to siderite and vivianite in the groundwater samples at MFP and GDP, but groundwater in the GFP wells is generally supersaturated with respect to rhodochrosite. Values of log PCO2 at MFP and GDP sites are generally higher than at the GFP site. This is consistent with Mn(IV)-redox buffering suggested at the GFP site compared to Fe(III)-redox buffering at MFP and GDP sites. 相似文献