Journal of the Earth and Space Physics
Year:2007 | Volume:33 | Issue:1|Papers Count:18

The main point of this paper is to evaluate the perturbations in orbital elements of a low Earth orbiting satellite. The outcome of a numerical orbit integration process is the position and velocity vectors of satellite in an inertial coordinate system. The velocity and position vectors are converted into the corresponding orbital elements. Perturbations in a satellite motion affect the orbital elements in the sense of Keplerian motion. In this paper after introducing the perturbing forces acting on a satellite, the method of converting the position and velocity into the orbital elements is presented, and finally the perturbations in orbital elements of the low Earth orbiting satellite of CHAMP are evaluated. The numerical results show that, disregarding the geopotential perturbing forces, the air drag is the most predominant among other perturbing forces: rotational deformation, solar radiation, third body effect, solid Earth tide, ocean tide, and general relativity arranged by their magnitude respectively.

Utilization of MED method for seismic deconvolution which was suggested by Wiggins in 1978 is taken into consideration because it includes no limiting assumption and has a simple and iterative algorithm. This method's operator is only obtained according to the simplicity of output which is verified by varimax norm. Researchers have used norms resembling the varimax norm. Meanwhile, varimax norm because of utilizing toeplitiz matrix in the final equation has less estimation and therefore, more applications. One of the most important characteristics of this method is that no assumption for phase characteristic of seismic wavelet need be considered. In this research, the authors have attempted to study this characteristic by designing MED filter for wavelet with different phase characteristics.

Upcoming and downgoing wave separation is very important in VSP data interpretation. In this paper, we utilized the eigenimage idea of SVD method to separate upcoming and downgoing waves. This method is applied to synthetic and real data and the results are compared with that of the f-k method. The two dimensional Fourier method in order to be applied to separation of two types of waves requires a uniform sampling both in time and space. In addition, using a median filter for these purposes requires the seismic event to be aligned perfectly. According to the results of this study from the application of the two methods to synthetic and real data, the SVD method is preferred because there is no need for uniform sampling or perfect alignment of the seismic events. Furthermore, the nature of the eigenimage idea causes the noises to be cancelled from data while separating the waves. All the computer codes utilized in this study were developed by the authors.

This paper deals with the one-dimensional discrete wavelet transform (1D DWT) of four scaling coefficients are computed numerically by designing a convolutive operator. The near-zone contribution of the integral is calculated through wavelet transform and for the far-zone contribution the classic expansion of the spherical harmonics applied. Finally the geoidal heights are determined over a territory in Canada.

The goal of this research is study of sea level variations of Caspian Sea. For this purpose the satellite altimetry data of Topex/Posidon (T/P) and Jason-1, and coastal tide gauge observations at Anzali, Sdra, Noshahr harbors are used. The algorithmic steps of the research are as follows: (1) As the first step the data from T/P and Jason-1 satellites are collected and standard corrections are applied to the data. (2) The Caspian Sea is divided into cells and Sea level changes are studied within those cells. (3) The coastal tide gauge observations are analysis for periodic and non-periodic changes. (4) The same analysis as of step (3) is applied to the Satellite Altimetry data for recognition of periodic and non-periodic changes of the Caspian Sea level. For the steps (3) and (4) Fourier transform and Least Squares Spectrum analyses were performed. An almost steady slope from Southwest towards Northeast is observed in terms of orthometric heights of the mean Caspian Sea level. (5) Dynamic height of mean Caspian Sea level is computed and it was found out that the observed slope most have non gravimetric origin. The potential information needed for the computation of dynamic height of mean Caspian Sea level was provided by EGM96 geopotential model after being transformed into ellipsoidal harmonic coefficients and applied in ellipsoidal harmonic expansion of Earth’s gravitational field. Finally the methodology applied in this research is offered to the users via a software, which allows the sea level variation study of Caspian Sea via satellite altimetry observations.

Numerical experiments using Version 3 of the Regional Climate Model (RegCM3), coupled with the Biosphere-Atmosphere Transfer Scheme (BATS), are performed to study the role of topography and land cover of the Zagros ranges in the simulated mesoscale meteorological features over Iran. The simulation domain covers the area between 34 and 63 degrees East and 24 and 44 degrees North. The simulation period is the winter 1999: The horizontal resolution is set at 45 km × 45 km. The boundary conditions are determined from the (United States) National Center for Environmental Prediction (NCEP) - National Center for Atmospheric Research (NCAR) reanalysis. Three sets of numerical simulations are performed in different conditions, with changing the Zagros topography and land-use. For the first (control) run, the present topography and land-cover, as derived from the GTOPO30_3MIN and GLCC3MIN_BATS data, are used. For the second run, the Zagros ranges are flattened and the surface-level elevation of the grid points in the area is set using a linear interpolation between the grid points on the west and on the east of the ranges. The simulations show that excluding the Zagros does not change the average regional precipitation considerably, but changes the precipitation distribution by decreasing it in the west and increasing it in the central and eastern parts of the country. In line with theoretical expectations, the analysis also demonstrates the important role the Zagros topography plays in the formation of vorticity and divergence/convergence fields in the lower levels of the atmosphere. For the third run, the vegetation cover of the Zagros area was replaced with that of the desert. The results show that such a change results in only a minor decrease in the precipitation amount and the temperature of the eastern areas of the domain.

Evaluation of hydrocarbon saturation is very important in oil reservoirs calculations. Archie equations are the basic relations for evaluating rock saturation. The coefficients of these equations are determined by laboratory experiments. Archie presented these coefficients, which are constants for sandstone. However, since carbonate rocks show drastic variations in texture and pore type, Archie coefficients, which are more sensitive to pore type, should be determined for different types of carbonate rocks. Uncertainty in these coefficients causes many errors in saturation evaluation especially in the determination of the volume of oil in place. Cementation exponent is the main factor, which causes error in determining saturation. In this study Archie parameters (a and m) are determined in the laboratory for various petrofacies based on petrographic studies and CT-scan images. Due to high dependence of seismic wave velocity on pore shape in carbonate rocks, petrofacieses were also determined by using wave velocity deviation logs. Subsequently, Archie parameters were determined for each petrofacies. The results showed that cementation exponent increases with increasing velocity deviation values. It is concluded that correlation coefficient in different petrofacies obtained from velocity deviation logs is comparatively higher than the same for petrographic study. Considering the fact that petrophysical data are more accessible than the core-based petrography data, the method presented in this study seems to be a more useful approach in determining Archie coefficients.

Daily precipitation data from 38 stations in Iran are analyzed for the period of 1960-2001 to evaluate the possible long term trend in annual and seasonal precipitation. Statistical analysis using the run test shows that the annual and seasonal precipitation series are homogeneous at most of the stations, with the exception being the summer precipitation in the arid regions. The Mann-Kendall method was used to test the existence of trends, and the least square method to estimate the values of trend. The values of trend are expressed in terms of the percentage of mean annual and seasonal precipitation over the study period. There is a significant trend (at 90% level) in the annual precipitation 10 stations and in the number of rainy days at 21 stations. The results show that the annual precipitation has a decreasing trend at stations in the west, northwest and southeast and an increasing trend at most of the stations in the other parts of the country. The winter precipitation shows a trend very similar to that of the annual precipitation. Most of the stations have shown a reduced precipitation during the spring and increased precipitation in autumn. Large scale study of air mass trajectories and local evaluations through station meta data are required for exploring the possible causes of such changes. The results show that for the stations (seasons) where total precipitation has a positive, large and often significant trend, heavy precipitation shows an increasing trend and light precipitation shows a decreasing trend. For some other stations (seasons), where total precipitation has a large negative trend, there is negative trend in the upper intensity quantiles and a positive trend in the lower intensity quantiles.

One of the main steps in the solution of the gravimetric boundary value problems of geoid computations is the removal of the terrain effect. This computation requires knowledge of the topography of the Earth in terms of Digital Terrain Models (DTM). During recent years, thanks to satellite radar missions, several DTM’s with different resolutions are computed and presented to the geosciences community. In this paper two main DTM’s namely, GTOPO30 with 30 second resolution, the U.S. Geological Survey's EROS Data Center, and SRTM computed by computed by U.S. NASA within Shuttle Radar Topography Mission, are considered and are applied for local geoid computation without applying Stokes formula at a test region at the Coastal Pars of Iran. As the benchmark the geoidal heights at 50 GPS/Leveling stations within the test region are considered. Results of the computations indicate that SRTM could results in, on average, 30.05 cm better result at the GPS/Leveling point than GTOPO30 at the test region.

This paper is devoted to assessing the accuracy of the compact and the super-compact finite difference methods for spatial differencing of the linearized shallow water equations. The second-order centered, the fourth-order centered, the fourth-order compact, the sixth-order compact and the sixth-order super-compact schemes are used to carry out the spatial differencing of the linearized shallow water equations on Arakawa's A and C grids. For the frequency and group velocity of linear inertia gravity waves on different numerical methods, the sixth-order super-compact method shows a substantial improvement on traditional centered and compact finite difference schemes.

The crustal structure beneath 9 seismic stations, deployed in north-east of Iran is determined using receiver functions analysis. In this study, 4 stations in the Sabzavar-Taknar zone and 5 stations in the Binalud zone with more than 40 teleseismic earthquakes are used to calculate receiver functions. A crustal structure is suggested for the Sabzavar-Taknar zone with a Moho depth of 46-48km, results present the three main layers: The upper crust has a S wave velocity between 1.9-3.1 km/s and a 11 km thickness. The middle crust has S wave velocity between 3.0-3.5 km/s and an 18 km thickness. The lower crust has S wave velocity between 3.6-4.6 km/s and an 18 km thickness. The crustal structure that is suggested for the Binalud, is a Moho depth of 48-50 km and three main layers for crust: The upper crust has a S wave velocity between 2.4-3.3 km/s and a 10 km thickness. The middle crust has S wave velocity between 3.0-3.5 km/s and a 22 km thickness. The lower crust has S wave velocity between 3.6-4.6 km/s and a 17 km thickness.

In this paper, atmospheric circulations caused by airflow over mountains and hills with different experimental setups are investigated. It is known that in a stable atmosphere the airflow over wide mountains makes the air parcels move in a wave like pattern in the vertical plane. First, the steady state of the momentum equation in the zonal and vertical directions, the thermodynamic and continuity equations are linearized, neglecting friction and rotation. Then combining the linearized equations into a Helmholtz type equation for the vertical velocity, W (or stream function,) is solved using the Fourier transform method. The experiments are performed for one and two isolated mountain peaks with different heights and varying horizontal velocity and Scorer parameter using a one and two layer models. Results show that, if the Scorer parameter is larger in the lower layer then the airflow follows the shape of the mountain and associated troughs and ridges are deeper with increased mountain height and horizontal wind speed. Moreover, ascending and descending vertical motions are formed in the mountain side and leeward side with their cores located at around 3 km altitude. Their amplitudes are extended to higher levels up to 10 km.