Journal of the Earth and Space Physics
Year:2004 | Volume:30 | Issue:2|Papers Count:18

The normalized full gradient (NFG) method defined by Berezkin (1967, 1973 and 1998) is already used for detecting reservoirs. We apply the method for delineating near-surface gravity anomalies. 2-D rectangular prisms with different coordinates are used as the synthetic models to be detected by the method. The NFG is also applied to real microgravity data to determine the place and depth of a subterranean water tunnel (ghanat).      

The formation and characteristics of lee cyclones over the Alps and their impact on the Middle East weather are investigated for a one year period (2002). The synoptic charts of I.R. of Iran meteorological organization at different levels for this period as well as the ECMWF data have been used in this study. Relative vorticity at 500 hPa and thickness charts between 1000-500 hPa have been calculated in order to consider the effect of upper-level vorticity advection and low-level baroclinicity on the surface cyclogenesis. It is found that the lee cyclones have different seasonal characteristics, controlling cyclone formation and movement. The number of weak and intense cyclones in the Gulf of Genoa in the cold period of the year is significantly larger, especially as compared with the warm period. Conversely, the intensity of warm period cyclones is much higher than the cold period cyclones, as found in previous studies. The cold period cyclones can largely affect the Middle East and northwestern Iran due to their eastward movements, whereas the impact of warm period cyclones over Iran is very weak. The results suggest that the occurrences of intense snowfall in northwestern Iran in winter might be due to lee cyclones. During moderate periods of the year, the cyclones which are affected by the Black Sea might influence the weather of the Middle East and Iran.      

In this article the definitions and the basic mathematics of the 3D DMO are explained. Also the effect of Kirchhoff integral 3D DMO method is considered on the Gorgan real 3D seismic reflection data. To achieve this goal, some seismic sections of 3D stacked data plus some velocity spectra on some points are prepared after required preprocessing of the data. After 3D DMO corrections, the seismic sections are prepared on the same lines, and also velocity spectra are prepared on the same points. Comparing the sections before and after DMO correction, the effect of the 3D-DMO process will be revealed. The results have shown that the quality of the processed data has significantly improved by the 3D-DMO Kirchhoff integral method and the process cannot be eliminated from the processing sequence. The dip angles of the layers are between 0 to 50 degrees in the Gorgan area where the data are collected. The work is implemented by using PROMAX 7.1 software on IBM-590 system.      

We have used magnetotelluric equipment to investigate the possibility of an existing geothermal reservoir in an area close to the city of Khoy located in the north-west of Iran. Data of 60 stations were collected in this area of about 30 square kilometers. To perform the data processing procedure, we used a computer program called PROCMT, and for in interpretation of available data a computer program called GEOTOOLS was used. We recognized an east- west trend for the observed anomalies in this area. In two dimensional modeling of north-south profiles using rapid relaxation inversion (RRI) on TE and TM, low resistivity anomalies were found at a depth of 500 to 2000 meters.      

Lg phase is a regional phase and short period guided wave composed mainly of a sequence of multiple reflected post critical S-waves trapped in the crustal wave guide. Lg phase also can be described as superposition of a higher mode of Love and Rayleigh waves. The mechanisms proposed to explain the excitation of Lg phase by explosions are: shear waves generated by inhomogeneity near the source, scattering effects that transfer P-wave or surface wave (Rg) energy into Lg, S-wave energy generated by spall which is an efficient source of Lg, nongeometrical S* phase and P-wave converted S-wave trapped in the crustal wave guide. Lg phase is one of the main phases produced by an underground nuclear explosion even with low yield. This phase has a sharp beginning with high amplitude in the short period seismograms in regional distances. The group velocity of Lg phase is about 3.5 km/s. Lg phase is a proper phase for yield estimation of underground nuclear explosions because of having a symmetric and radial pattern, lack of high attenuation and less scattering of mb (Lg) compared with the magnitude derived from other phases. In this paper, 16 underground nuclear explosions at Semipalaninsk Test Site (STS, in east Kazakhstan) recorded by SRO, Mashhad, have been investigated. The yields of these explosions were announced by Stevens and Murphy (2001). Maximum peak to peak amplitude for periods of 1 to 1.5 seconds was read in the group velocity window of 3.2 to 3.6 km/s as the amplitude of Lg phase. Then mb(Lg) was determined by using the formula presented by Nuttli (1986a). The relationship between computed mb(Lg) and announced yield (Y in kt TNT) of 16 explosions of STS was derived as mb(Lg) = 0.564 log (Y) + 4.863. This relationship is recommended for yield estimation of STS nuclear explosions from SRO, Mashhad.      

Migration is a process that reconstructs an image of the earth's reflecting structure from elastic wavefield energy recorded at the surface in seismic traces. In seismic imaging, conventional processing is concentrates on producing a stacked section from common mid point gathers and is followed by a poststack migration, while in a prestack migration, first the events are migrated one by one, then those are stacked and the energy the common reflection point gathers goes to the surface of the earth. In this paper the effects of both post-and pre-stack time migration methods are compared by use of the Kirchhoff summation method on a seismic line in the central Iran area. The method of data collection is offend and the foldage is 48. The source of energy in acquisition is dynamite. The work is implemented by using PROMAX 6.0 software on ULTRA SUN system. The results are as follows: Continuity of reflectors will be improved after prestack migration. Velocity analysis after prestack migration is improved and provides a better harmony of the velocities. Because of the increasing of the reflector's amplitudes continuity, some geology structures appeared after prestack migration.      

An attempt is made to implement the objective analysis of Cressmann for some fields including sea level pressure, temperature and geopotential height at standard levels in an operational framework. The work presented here is a part of a complete automatic system for decoding, quality control and analysis of meteorological maps. The results are compared with the subjectively analyzed maps and are in good agreement. It is shown that the output of a numerical weather prediction model with 2.5ºand 5º horizontal resolution as the background field and 1200 km and 2500 km as the radius of influence for the surface and upper air analysis respectively give the best results.      

PSPC or split step Fourier migration is a method based on downward continuation of seismic wave field in frequency space and frequency-wave number domains. Compared with the phase shift method, this method is a superior algorithm with the ability to handle lateral variations in the velocity field. Therefore, the method is a depth migration, which works in the post stack domain. In this study, the ability, precision and validity of PSPC migration in the processing of real data sets with strong lateral velocity variations is verified. In order to achieve this objective, the velocity section used by migration algorithms has been improved in different steps by the iterative method of residual move-out analysis through applying pre stack Kirchhoff time migration. The improved velocity section was then used to apply pre stack Kirchhoff depth migration to original CMP gathers and PSPC migration to the stack section and the results were compared.    

The structures of thunderstorm outflows are studied using surface and sodar data at the Geophysics Station. Distinctive features associated with gust fronts are: a sudden drop in temperature (~ 3-8 °c), a sudden increase in wind speed (~10- 20 m/s), wind shift, a pressure rise (~2-3 mb), a humidity increment and rain. In this study, characteristics of 10 outflow samples are determined using sodar and surface meteorology data. Horizontal winds, maximum wind speed, propagation speed of gust fronts are computed using surface data. The charts of horizontal velocity, vertical velocity, direction and turbulence flow of wind (uv, uw, vw) are considered when the sodar was recording data. Vertical structure of horizontal wind is layered before the arrival of gust front on surface. In horizontal wind profiles, there are two maxima. When the horizontal component of wind is maximum, its vertical component wind becomes minimum, which is predictable through considering of vertical structure of gust front by continuity. Time variations of vertical component of wind speed show that Kelvin-Helmhotz instability is developed along the top of the gust front head near the surface. Estimated dimension of Kelvin-Helmholtz billows is about 10-20 km which appears to be larger than the ones observed by others. This may be due to the fact that these gust fronts occur over a sloping boundary. Contours of vertical and horizontal components of flow velocity and its horizontal direction of gust fronts show that these gust fronts have similar shapes but vary in sizes. This analysis also shows that the outflow has a complex and multi frontal structure. Further observational work is required for more detailed analysis of the gust front in vertical and horizontal extends.