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81.
Only with satellites it is possible to cover the entire Earth densely with gravity field related measurements of uniform quality
within a short period of time. However, due to the altitude of the satellite orbits, the signals of individual local masses
are strongly damped. Based on the approach of Petrovskaya and Vershkov we determine the gravity gradient tensor directly from
the spherical harmonic coefficients of the recent EIGEN-GL04C combined model of the GRACE satellite mission. Satellite gradiometry
can be used as a complementary tool to gravity and geoid information in interpreting the general geophysical and geodynamical
features of the Earth. Due to the high altitude of the satellite, the effects of the topography and the internal masses of
the Earth are strongly damped. However, the gradiometer data, which are nothing else than the second order spatial derivatives
of the gravity potential, efficiently counteract signal attenuation at the low and medium frequencies.
In this article we review the procedure for estimating the gravity gradient components directly from spherical harmonics coefficients.
Then we apply this method as a case study for the interpretation of possible geophysical or geodynamical patterns in Iran.
We found strong correlations between the cross-components of the gravity gradient tensor and the components of the deflection
of vertical, and we show that this result agrees with theory. Also, strong correlations of the gravity anomaly, geoid model
and a digital elevation model were found with the diagonal elements of the gradient tensor. 相似文献
82.
Saeid Jamshidi Ramin Bozorgmehry Boozarjomehry Mahmoud Reza Pishvaie 《Advances in water resources》2009
In pore network modeling, the void space of a rock sample is represented at the microscopic scale by a network of pores connected by throats. Construction of a reasonable representation of the geometry and topology of the pore space will lead to a reliable prediction of the properties of porous media. Recently, the theory of multi-cellular growth (or L-systems) has been used as a flexible tool for generation of pore network models which do not require any special information such as 2D SEM or 3D pore space images. In general, the networks generated by this method are irregular pore network models which are inherently closer to the complicated nature of the porous media rather than regular lattice networks. In this approach, the construction process is controlled only by the production rules that govern the development process of the network. In this study, genetic algorithm has been used to obtain the optimum values of the uncertain parameters of these production rules to build an appropriate irregular lattice network capable of the prediction of both static and hydraulic information of the target porous medium. 相似文献
83.
Consider a plane homogeneous harmonic SH wave incident upon an interface between two anelastic half-spaces. Computing the plane wave displacement and energy-flux-based reflection and transmission coefficients correctly requires determining the proper signs of the vertical slowness components of all the reflected and transmitted waves, i.e., determining which of the two values of the square root for a given vertical slowness should be chosen. For anelastic media, this can be problematic, as unphysical results can arise. Previous research has led to a specific recommendation on how to choose the signs. However, when this recommendation is employed, it is found via numerical experimentation that for certain values of the medium parameters, the energy-flux-based transmission coefficient T can be negative for certain supercritical values of the incidence angle, whereas physical reasoning suggests it should be zero. To investigate this seemingly unphysical result, an analytical determination of the mathematical conditions under which it occurs would be useful. Such a determination is performed in this article. Letting V 1 and V 2 be the wave speeds of homogeneous SH plane waves in the incidence and transmission media respectively, and Q 1 and Q 2 be the corresponding quality factors, with Q 1, Q 2 ? 1, it is found that if V 1 < V 2 and Q 1 < Q 2 (a common situation in the Earth), then T will be negative for part of the supercritical incidence angle range if 1 < Q 2/Q 1 ?? 2 ? (V 1/V 2)2 and for all of it if 2 (V 1/V 2)2 < Q 2/Q 1. 相似文献
84.
In time?–?distance helioseismology, wave travel times are measured from the cross-correlation between Doppler velocities recorded at any two locations on the solar surface. However, one of the main uncertainties associated with such measurements is how to interpret observations made in regions of strong magnetic field. Isolating the effects of the magnetic field from thermal or sound-speed perturbations has proved to be quite complex and has yet to yield reliable results when extracting travel times from the cross-correlation function. One possible way to decouple these effects is by using a 3D sunspot model based on observed surface magnetic-field profiles, with a surrounding stratified, quiet-Sun atmosphere to model the magneto-acoustic ray propagation, and analyse the resulting ray travel-time perturbations that will directly account for wave-speed variations produced by the magnetic field. These artificial travel-time perturbation profiles provide us with several related but distinct observations: i) that strong surface magnetic fields have a dual effect on helioseismic rays?–?increasing their skip distance while at the same time speeding them up considerably compared to their quiet-Sun counterparts, ii) there is a clear and significant frequency dependence of both skip-distance and travel-time perturbations across the simulated sunspot radius, iii) the negative sign and magnitude of these perturbations appears to be directly related to the sunspot magnetic-field strength and inclination, iv) by “switching off” the magnetic field inside the sunspot, we are able to completely isolate the thermal component of the travel-time perturbations observed, which is seen to be both opposite in sign and much smaller in magnitude than those measured when the magnetic field is present. These results tend to suggest that purely thermal perturbations are unlikely to be the main effect seen in travel times through sunspots, and that strong, near-surface magnetic fields may be directly and significantly altering the magnitude and lateral extent of sound-speed inversions of sunspots made by time?–?distance helioseismology. 相似文献
85.
86.
Non-parametric trend analysis of the aridity index for three large arid and semi-arid basins in Iran 总被引：2，自引：1，他引：1
Hossien Ahani Mehrzad Kherad Mohammad Reza Kousari Lieke van Roosmalen Ramin Aryanfar Seyyed Mashaallah Hosseini 《Theoretical and Applied Climatology》2013,112(3-4):553-564
Currently, an important scientific challenge that researchers are facing is to gain a better understanding of climate change at the regional scale, which can be especially challenging in an area with low and highly variable precipitation amounts such as Iran. Trend analysis of the medium-term change using ground station observations of meteorological variables can enhance our knowledge of the dominant processes in an area and contribute to the analysis of future climate projections. Generally, studies focus on the long-term variability of temperature and precipitation and to a lesser extent on other important parameters such as moisture indices. In this study the recent 50-year trends (1955–2005) of precipitation (P), potential evapotranspiration (PET), and aridity index (AI) in monthly time scale were studied over 14 synoptic stations in three large Iran basins using the Mann–Kendall non-parametric test. Additionally, an analysis of the monthly, seasonal and annual trend of each parameter was performed. Results showed no significant trends in the monthly time series. However, PET showed significant, mostly decreasing trends, for the seasonal values, which resulted in a significant negative trend in annual PET at five stations. Significant negative trends in seasonal P values were only found at a number of stations in spring and summer and no station showed significant negative trends in annual P. Due to the varied positive and negative trends in annual P and to a lesser extent PET, almost as many stations with negative as positive trends in annual AI were found, indicating that both drying and wetting trends occurred in Iran. Overall, the northern part of the study area showed an increasing trend in annual AI which meant that the region became wetter, while the south showed decreasing trends in AI. 相似文献
87.
H. R. Pourghasemi H. R. Moradi S. M. Fatemi Aghda E. A. Sezer A. Goli Jirandeh B. Pradhan 《Environmental Earth Sciences》2014,71(8):3617-3626
The aim of the presented study is to assess the fractal dimension (D) and the geometrical characteristics (length and width) of the landslides identified in North of Tehran, Iran. At first, the landslide locations (528 landslides) were identified by interpretation of aerial photographs, satellite images and field surveys, and then to calculate the fractal dimension (D), we used the computer programming named as FRACEK. In the next step, geometrical characteristics of each landslide such as length (L) and width (W) were calculated by ArcGIS software. The landslide polygons were digitized from the mentioned landslide inventory map and rotated based on movement direction. The fractal dimension for all landslides varied between 1.665 and 1.968. Subsequently, the relationship between the length/width ratios and theirs fractal D values for 528 landslides was calculated. The results showed that correlation coefficients (R), which are different regression models such as exponential, linear, logarithmic, polynomial, and power, between D and L/W ratio are relatively high, respectively (0.75, 0.75, 0.76, 0.78, and 0.75). It can be concluded that the fractal dimension values and geometry characteristics of landslides would be useful indices for the management of hazardous areas, susceptible slopes, land use planning, and landslide hazard mitigation. 相似文献
88.
Saleh Yousefi Hamid Reza Moradi Saskia Keesstra Hamid Reza Pourghasemi Oldrich Navratil Janet Hooke 《国际地球制图》2019,34(3):276-292
In the present study, we investigate the effects of urbanization growth on river morphology in the downstream part of Talar River, east of Mazandaran Province, Iran. Morphological and morphometric parameters in 10 equal sub-reaches were defined along a 11.5 km reach of the Talar River after land cover maps were produced for 1955, 1968, 1994, 2005 and 2013. Land cover types changed extremely during the study period. Residential lands were found to have increased in area by about 1631%, while forest land and riparian vegetation decreased in by approximately 99.9 and 96.2%, respectively. The results of morphometric and morphological factors showed that average channel width (W) for all 11.5 km of the study river decreased by 84% during the study period, while the flow length increased by about 2.14%. 相似文献
89.
The seasonal and spatial variability of airborne dust deposits and associated trace metals such as Pb, Co, Ni, Cd, Mn in 15 sites surrounding a heavily industrialized region in south Esfahan (central Iran) are investigated. Total deposit rates (TDR) of dusts and trace metals are analyzed, contamination factor (CF) and pollution load index (PLI) are also calculated. Furthermore, correlation and cluster analysis are performed to identify the source of the pollution. The highest dust-TDR (15.97 g m−2 per season), the highest concentration of trace metals, CF and PLI are recorded in summer because of the lack of precipitation, high temperature, and drought conditions in this season. Pb and Cd show the highest CF values. In the towns near the two major steel mills in the region (i.e., Esfahan Steel Company and Mobarekeh Steel Complex), the CF values for Pb and Cd are about 13 and 12, respectively (i.e., 13 and 12 times higher than the pre-industrial values, respectively). The spatial distribution maps of the dust deposit rate, dust-borne trace metals, and the obtained PLI of the trace metals in the study area reveal that the two major industries in the region are the main sources of dust and trace metal distribution. 相似文献