فصلنامه فیزیک زمین و فضا
سال سی و ششم شماره 3 (پاییز 1389)

Reflection and refraction of EM signals at both horizontal and vertical interfaces separate media of different electrical parameters, geoelectromagnetic methods have been developed and employed to recognize the geological features and particularly fault zones in many regions. To achieve higher lateral resolution and also greater depth penetration, the MT method is one of the most effective electromagnetic techniques to imagine the subsurface structures electrically. In 2006 wide frequency range of magnetotelluric measurements were carried out at the eastern part of the city of Arak in Iran to understand the crustal electrical conductivity of the region by putting emphasis on locating the fault zones. The electric and magnetic field components were acquired along a profile across the geological trend at 15 stations. A robust single site processing followed by the inversion and one dimensional as well as two dimensional modeling was performed. The inversion results revealed electrical conductivity structures in correlation with geological features. As a significant result, true locations of two major faults, Talkhab and Tabarteh Faults and a conductive block in between were recognized in the Arak area.

Based on 11 years of TOPEX/Poseidon satellite altimetry and coastal tide gauge sea level observations, four tidal constituents, namely O1, K1, M2 and S2, are modeled for the Persian Gulf and Oman Sea using a time-wise approach according to the following details. By selecting the cycle 100 as the reference, 772 points on 12 paths along the track of the altimetry satellite footprints over the Persian Gulf and Oman Sea are selected as the center points of 772 circular cells that were used to catch the repeated MGDR data from cycle 8 through 345 that fall within the aforementioned circular cells with a radius of 0.053?. In this way 772 time series of the sea level observations located at the center of the circular cells are developed. These data were further corrected for all effects whose correction parameters are given by the MGDR data files; except the tidal correction that was kept to become the source of information for our tidal modeling. The gaps in the 772 satellite derived time series are filled via inverse solution of the autocorrelation function that was applied to the existing sea level variation data. Besides, the 1 hourly time series of sea level observations at 17 tide gauges along the Iranian coast line at the Persian Gulf and Oman Sea were used both to check the validity of the tidal models developed by the altimetry data at the 772 point over the above mentioned sea areas and to increase the accuracy of satellite derived tidal models at the shallow waters. The equally spaced 772 satellite altimetry derived time series and 17 time series at the coastal tide gauge stations are subjected to Fourier analysis to obtain the major tidal constituents, which are next used as the initial value within a least square solution to obtain the adjusted tidal frequencies, their phase angles and amplitudes. The result of this step for the satellite derived time series were modeling of all the existing tidal constituents with periods greater than 20 days, as the repetition of the TOPEX/Poseidon satellite altimetry observation is 9.915 days, except for the crossover points where the repetition time period is half. Next, via forward modeling, the effects of the modeled tidal constituents were removed from the original 772 time series to remain with the residual time series that were re-orders according to their observation hour, without considering their observation date in order to develop 2 hourly residual time series, which were used to derive the other short period tidal constituents. The result of the numerical computation and the comparison of the satellite derive models with that obtained by tide gauge observations granted the success of the method and such a new tidal model for four tidal constituents namely, O1, K1, M2 and S2 is developed for the test area, i.e. Persian Gulf and Oman Sea.

exploration seismology, migration refers to a multi-channel processing step that attempts to spatially re-position events and improve focusing. Before migration, seismic data is usually displayed with traces plotted at the surface location of the receivers and with a vertical time axis. This means that dipping reflections are systematically mispositioned in the lateral coordinate and the vertical time axis needs a transformation to depth. Also problematic is the unfocused nature of seismic data before migration. Migration moves dipping reflections to their true subsurface positions and collapses diffractions, thus increasing spatial resolution and yielding a seismic image of the subsurface. Time migration which produces a migrated time section is appropriate as long as lateral velocity variations are mild to moderate. Dipping events on a stacked section call for time migration. Conflicting dips with different stacking velocities is one case in which a conventional stacked section differs from a zero-offset section. Thus, poststack migration which assumes that the stacked section is equivalent to a zero-offset is not valid to handle the case of conflicting dips. Instead, one needs to do prestack time migration.

The application of NMO (normal moveout) has been recognized as an effective method of generating quasi-zero-offset traces in traditional common-midpoint processing. Artifacts of the NMO method relate to the NMO-stretch effects. Conventional application of normal-moveout correction to a common-midpoint (CMP) reflection generates a stretch that increases with offset and decreases with zero-offset time. Shatilo and Aminzadeh (2000) introduced the technique which implies constant normal moveout (CNMO) for a finite time interval of a seismic trace. Perroud and Tygel (2004) introduced the implementation, called nonstretch NMO, automatically, which avoids the undesirable stretch effects that are present in the conventional NMO. They applied their new method (Nonstretch NMO) to shallow seismic data including high resolution (HR) seismic data and ground-penetrating radar (GPR) measurements.

Elastic energy of seismic wave is lost through propagation into the earth. Various factors affect seismic energy. Some of them are frequency independent and recovered in processing steps. But other factors such as intrinsic absorption of medium are frequency dependent and cannot be recovered by processing methods. Lost energy caused by these factors is called attenuation. There are various methods to study the attenuation of seismic energy. Because of the frequency dependency behaviour of attenuation, it acts as a non-stationary quantity. Attenuation coefficient is usually studied in frequency domain based on power spectrum and statistical methods. Since Fourier transform does not consider the temporal variation of frequency content of seismic data, and due to the dependence of attenuation to frequency we used time-frequency tools in this study. Time-frequency transforms such as short-time Fourier transform, wavelet transform and S-transform are common tools in the processing and interpretation of seismic data. In this study, we used the Wigner-Ville distribution as a time-frequency tool to study the seismic wave attenuation.

The analytic signal for magnetic anomalies was initially defined as a “complex field deriving from a complex potential” (Nabighian, 1972). This function can be computed easily in the frequency domain, its real part is the horizontal derivative of the field and its imaginary part is the vertical derivative. Analytic signal processing and interpretation requires few initial assumptions regarding the source body geometry and magnetization and is particularly efficient at an early stage of the interpretation even if constraints are not available. For 2-D structures (Nabighian, 1972), the method assumes that the causative bodies have a polygonal cross-section with uniform magnetization. Such structures can also be considered as the superimposition of a finite number of magnetic steps. Narrow dikes and thin sheets can also be taken into account using a lower order of derivation; for example, the field itself instead of the horizontal derivatives. Nabighian (1972) demonstrates that the analytic signal has simple poles at each corner of the structures. The amplitude of the analytic signal is a bell-shaped symmetric function maximizing exactly over the top of each contact, with the width of the amplitude curve being related directly to the depth of the contact. This is also true for any of the derivatives of the signal (Nabighian, 1974); these properties can be used to locate the magnetic contacts and to estimate their depths. Extension of the 2-D analytic signal to three dimensions will allow more general interpretation procedures to be developed, the two-dimensionality assumption being no longer required. The relationship between the horizontal and vertical derivatives for the 3-D case was first derived by Nabighian (1984).

Temporal resolution of seismic data is proportional to the seismic band width. Seismic data still have not enough temporal resolution because of the band-limited nature of available data even if it is deconvolved. Lower and higher frequencies of seismic data spectrum are missing and cannot be recovered by the usual deconvolution methods. Because of absorption, high frequencies belonging to the spectrum are missing and recovery of lower frequencies is also a big deal (Lindseth, 1979). Many different deconvolution techniques have been developed to process the data obtained from various sources ranging of seismic data. especially, since for many years in seismic processing, they have been used to improve the temporal resolution of seismic data. In this paper we introduce a method that is the generalization of the autoregressive (AR) spectral extrapolation based method originally applied by Hakan Karsli (2006), which extrapolates the deconvolved seismic spectrum for recovery of missed frequencies. When reflectors are numerous, the seismic spectrum is complicated and extrapolation by AR-based methods is uncertain. The introduced method takes a certain part of both real and imaginary parts of the spectrum, where S/N is high compare to the rest of the spectrum, and extrapolates lower and higher portions of the spectrum using Singular Spectrum Analysis (SSA) and Autoregressive model. Experience shows that a 3–10 dB drop from the maximum amplitude of the spectrum of the source wavelet represents a high SNR portion of the spectrum. Because of the existence of unwanted noise, the usual regression algorithms do not lead to favorable results. In second step of extrapolation algorithm we decompose selected spectrum by SSA.

Electrical anisotropy in the earth, the effect of current density dependency on the electric field orientation in a medium, has been considered significantly in recent Magnetotelluric (MT) observations. Several suggestions for electrical anisotropy are based upon MT observations of "phase splits", analogous to shear wave splits in seismology. The MT phase data is accentuated more than its amplitude responses since shallow small scale conductivity heterogeneities cause a significant distortion in MT amplitude responses (known as Galvanic Distortion), while MT phase responses remain immune. To investigate the MT phase response we will use the tensor representation of the MT phase introduced by Caldwell et al. (2004). This representation has the advantage of considerably simplifying the analysis of the Galvanic distortion effect. Moreover no assumption about the dimensionality of the underlying regional conductivity structure is essential. The properties of a generally asymmetric "phase tensor" are best understood in terms of the tensor's graphical representation as an ellipse (figure1). The tensor principal axes and principal values correspond to the major and minor axes and the lengths of the corresponding ellipse radii, respectively

During the last several years, application of numerical weather prediction models in the country have become common in both research and operations, even though systematic verification of the models’ results using statistical methods has rarely been conducted (Sodoudi et al. 2009). This paper aims at comparing the MM5 24-hour precipitation forecasts with the corresponding observations using standard scores for categorical forecasts associated with 2×2 contingency tables for different precipitation thresholds over different nine sub-regions of Iran. Comparison is conducted for +24h/+48h/+72h forecasts for a four winter month period from December 2004 to March 2005. Performance of the model results were assessed for all available synoptic and climatological stations scattered across the country at three different precipitation thresholds. The 0.1 mm/24h threshold was considered as the rain/no rain event. The other two intervals are: 0.1-10 and greater than 10 mm/24h for light and heavy precipitation respectively. Based on the long term means of precipitation of different parts of Iran, nine different sub-regions were defined and verification was conducted for the whole country and nine different sub-regions separately. In this study for verification scores the quantity of precipitation is considered as a dichotomous variable by considering different precipitation thresholds. The standard approach is to record the frequencies with which the precipitation was observed and forecasted in a two-by-two table, and then to quantify forecast quality with summary measures of the table. The structure of a typical contingency table is presented in table.

Previous studies show that the North Atlantic Oscillation (NAO) is the dominant mode of variability in the northern hemisphere winter with marked climatic effects on its downstream regions. In this article the effects of some important forcing terms in the time tendency equation of the Eddy Kinetic Energy (EKE) in critical positive months (CPM) and critical negative months (CNM) of the NAO are studied using NCEP/NCAR reanalysis data. The data covered span the years 1950-2005 for the winter months (December to February). The critical months are defined on the basis of the monthly index of NAO and include 29 CPM and 33 CNM. The selected forcing terms include baroclinic conversion (BCC), barotropic conversion (BTC), convergence of total energy flux (CTF) and ageostrophic flux (CAF). The ensemble mean of vertical average of these forcing terms as well as the baroclinic generation (BCG) term, representing the generation of available potential energy, are computed over an area from 0 to 90E and 20N to 70N for CPM and CNM. In addition to the usual ensemble mean, to avoid cancellation, separate averages are also taken of the positive and negative values of the foregoing quantities

The compact finite difference schemes have been found to give simple ways of reaching the objectives of high accuracy and low computational cost. During the past two decades, the compact schemes have been used extensively for numerical simulation of various fluid dynamics problems. These methods have also been applied for numerical solution of some prototype geophysical fluid dynamics problems (e.g., shallow water equations). Most of the compact finite difference schemes are symmetric (usually with 3 or 5 point stencil) and finding each derivative requires a matrix inversion. However, by splitting the derivative operator of a central compact scheme into one-sided forward and backward operators, a family of compact MacCormack-type schemes can be derived. While these classes of compact schemes are as accurate as the original central compact methods used to derive the one-sided forward and backward operators, they need less computational work per point. In addition, the one-sided nature of the method is an essential advantage of the method especially when severe gradients are present. These two features (i.e. high accuracy and low computational cost) makes the compact MacCormack-type scheme an attractive candidate for numerical models of the atmosphere and oceans. This work focuses on the application of the fourth-order compact MacCormack-type scheme for numerical solution of the unsteady and non-linear Rossby adjustment problem (one and two dimensional cases). The second-order MacCormack method is also used for numerical solution of the equations. In the one-dimensional case, a single layer shallow water model is used to study the unsteady and nonlinear Rossby adjustment problem. The conservative form of the two-dimensional shallow water equations is used to study the unsteady and nonlinear Rossby adjustment problem in the two-dimensional case. For both cases, the time evolution of a fluid layer initially at rest with a discontinuity in the height filed is considered for numerical simulations.

The conductivity distribution across the Trans-European Suture Zone (TESZ) is presented based on the measurements along a 400 km northeastern directed profile, starting from the German-Polish Basin, crossing the TESZ and ending at the East European Craton. Two-dimensional inversion was applied to magnetotelluric transfer functions and magnetovariational responses corresponding to 38 long-period simultaneously observed sites. Input data for the inversion procedure were created by rotating all transfer functions to strike direction obtained from strike and dimensionality analysis. The results show a thick sedimentary cover, several crustal inhomogeneities and a deep conductive structure below the center of the TESZ. In order to achieve a stable model, several sensitivity analysis were carried out.