Journal of the Earth and Space Physics
Year:2010 | Volume:36 | Issue:2|Papers Count:13

The central Alborz Mountains of northern Iran (Figure 1) is a zone of intense active continental deformation and is the most heavily populated region of Iran, including the mega-city of Tehran with a population of over 10 million. The region is affected by numerous active faults, some of them having great seismic potential, and documented historical seismicity. Hence, the central Alborz is a prime candidate for seismic hazard evaluation studies. The choice of ground-motion model has a significant impact on risk estimates for a distance range of 50 to 200 km within an active seismic zone such as the central Alborz.To visualize the attenuation behavior of calculated amplitude and to find a best fit Smoother (LOWESS) robust curve-fitting method of Cleveland et al. (1992). The method requires no assumptions regarding the underlying form of the relationship between amplitude of acceleration spectrum and distance. The use of a theoretical function can distort the fit by forcing a number of hinge points rather than by the natural hinge points in the data trend.We only used 220 seismograms from 22 selected events with azimuthal gap less than 180 degrees. The data was bounded by azimuthal gap constrain to decrease successfully the radiation pattern effects on obtained spectral amplitudes. Two significant hinges are observed in 106 and 191 km. The first hinge is due to SmS phases received from Moho discontinuity beyond 106 km. The second hinge is related to the distance at which surface wave propagation becomes dominant. A small hinge was recognized in 55 km. We believe that it is because of a shallower discontinuity, possibly the Conrad discontinuity. To find the reasons behind the existence of the hinges, we model reflection coefficient versus distance for an Earth model having the Conrad or Moho discontinuity. We can estimate the depth of discontinuities using the calculated distances for the hinges (55 and 106 km).Using a simple model and mentioned hinges (55 and 106 km), the thickness of the Conrad and Moho are estimated as 24.5 and 46.5 km, respectively. The Moho thickness agrees well with that obtained from 1D tomography of the study region.

Distinguishable amplitude phenomena on surface seismic data often resulted from contrasts in the elastic parameters of subsurface layers. Various techniques have been involved to analyze and highlight such phenomena for their potential use as "Direct Hydrocarbon Indicator (DHI)". More recently, other techniques have been developed based on the variation of reflection coefficient with angle of incidence, conventionally called Amplitude-Versus-Offset (AVO).During the last twenty years the significance of AVO analysis for studying seismic reflection in oil exploration has been considered more importantly. In this work, first, a seismic line from a gas field and also a well are selected to indicate the results of the application of AVO analysis for detection of hydrocarbon reservoir in this field.In this project, using well logs and information obtained by core analysis, a synthetic seismogram has been built applying Zoeppritz equation. And Using Hampson-Russell software, AVO attributes have been extracted from synthetic seismogram. Then anomalies of these attributes have been investigated and compared with the anomalies from AVO attributes which were extracted from real seismic data to characterize the reservoir.It has been seen that the extracted attributes of the synthetic seismogram confirm the anomalies from real seismic data. Finally according to the obtained result, observed anomalies can be interpreted as a Gas Cap for this reservoir. This study is useful to identify reservoir and nonreservoirs and the results of this study are considered as input for detailed reservoir studies. In particular, knowing the reservoir physical and saturating fluid properties is of great importance in making plans for developing the reservoir.

Measurements of the tilt angle of potential fields can be a useful aid to their interpretation. There are several varieties of the methods in use, such as tilt angle, and the derivative of tilt angle. Tilt angle filters are used for exact edge detection of the rectangular bodies artificially by writing a MATLAB code. The gravity effects of these models are computed by standard algorithms. The edges of these complex rectangular bodies are determined appropriately by tilt angle. It is also applied on real gravity data. The residual gravity anomalies are used as the real data. Tilt angles are used to determine the edges of these residual anomalies. The results are compared with the results from the analytic signal method which is a classical edge detection method. This shows the better performance of the tilt angle filter method.

On 26 December 2003 at 05:26LT (01:26GMT), a catastrophic earthquake Mw 6.6, seismic moment (6-9 x1018 Nm) struck the city of Bam in the Kerman province of southeast Iran. The intense shaking in the city caused the complete collapse of nearly every building in the central parts of the city including many of the newer buildings, killing about 40000 people officially. The city lies to the east of the Nyband- Gowk-Sarvestan fault system and on which several powerful earthquakes have occurred over the past 23 years [Berberian et al., 1984; Berberian and Qorashi, 1994; Berberian et al., 2001]. There are no recorded historical earthquakes at Bam. Most of the citadel of Arg-e- Bam, one of world heritage sites inscribed by UNESCO, which was constructed by mud brick about 2000 years ago was destroyed in this earthquake. Four powerful earthquakes (M>6) since 1981 have occurred near the Gowk fault zone [Berberian et al., 1984, 2001]. The Gowk fault zone, a predominantly right-lateral strike-slip zone that extends from 50 km west of Bam northward [Walker and Jackson, 2002], has also been associated with several large historical earthquakes [Ambraseys and Melville, 1982; Berberian and Yeats, 1999].After 26 December 2003 many aftershocks and earthquakes have occurred in Bam, southeast Iran. The last event with M=4.9 (NEIC) occurred on 26 April 2007. In this study, we calculated the ground motion parameters of the latest event using digital accelerograms of the Building and Housing Research Center (BHRC). We calculated seismic Moment (Mo) and Moment magnitude (Mw) of the 03/26/2007 Bam earthquake. For acceleration, Velocity and Displacement diagrams; we determined flat portion (K) and corner frequency (fc) by correction of base line and band pass filtering of Fourier spectrum. We calculated average seismic Moment (Mo=3.6×1016 Nm) and Mw 5; the answers conform to the results by other methods computation. We determined the location of the epicenter, our motivation for these computations was the determination of the difference of epicentral location with Maximum 36 km. In this study, we determined the focal mechanism of this earthquake; strike angle (N51oW), Dip89º westwards and Rake 138º, by using first P motion of the accelerograms and seismograms Data.The occurrence of a strong earthquake such as that of the 26 April 2007 indicates that the Bam region is still seismically active.

The deep internal structure of the crust can be determined using appropriate seismic and electromagnetic methods. The natural source magneto telluric (MT) method is the most suitable electromagnetic technique for probing into the deep crust. A long period magneto telluric data set obtained in the Southern Chilean Andes is investigated in this paper. Dimensionality analysis shows that the data may be regarded as with a strike 2D direction aligned to the N-S direction. Results of a joint inversion of different MT data types indicate relatively high conductive structures in the middle to deep crust beneath the volcanic arc.

Linear filters are used for a wide range of magnetic science including noise attenuation, spatial derivatives, upward and downward continuation and reduction to the pole. The majority of these filters are spatially-invariant, meaning that the filter has a constant wave number response over the whole signal. In contrast, many signals which encountered real problems such as magnetic signals, typically exhibit a spatially-varying wave number content which motivates us to design filters with spatially-varying wave number responses. This leads to better preservation of anomaly gradients in the calculated derivatives than is possible using conventional Fourier or space domain smoothing techniques.Fourier transform because of its stability, the simple physical interpretation of the transform coefficients and the diagonalisation of spatially invariant linear operators in the Fourier domain play an important role in magnetic processing. However, Fourier filters cannot be designed to adapt to local properties of the signal or to generate spatially varying filters.One of the methods for generating spatially-varying filters is based on the continuous wavelet transform (CWT) which provides new powerful tools in magnetic data processing. The wavelet transform is ideal for analysing signals such as magnetic signals that contain short duration, transient features. Wavelet techniques can be used to provide solutions to problems that are difficult or impossible to solve using conventional global techniques such as Fourier-based methods. The wavelet transform preserves both spatial and wave number information about a signal allowing us to design a range of spatially varying filters that act on the wavelet coefficients. This method provides robust and efficient new frameworks for designing filters that are impractical to implement using conventional space or wave number domain techniques. This method is compared with other techniques in upward-downward continuation. We demonstrate the application of spatially-varying scale filters to the problem of upward and downward continuation from a level observation surface to a new irregular-height surface. Downward continuation is the most difficult of these operations as it is highly numerically unstable and is very sensitive to high wave number noise. For comparison, conventional methods of downward continuation, such as the Taylor-series and chessboard methods are used which for stabilizing a global low-pass filter are applied to the data to attenuate any high wave number noise that may create difficulties in the continuation procedure. The wavelet implementation produces a superior result compared with conventional techniques such as Taylor-series and chessboard algorithms.In this study, the wavelet approach combined with the exponential smoothing filter produces sharper images than either the chessboard or Taylor-series methods that are clearly evident in the case study and synthetic examples. In contrast, the Taylor-series cosine roll-off filter is designed to ensure that the downward continuation is stable over the largest continuation distance. The chessboard method should theoretically be able to behave like the wavelet method by adjusting the amount of smoothing applied to each downward continuation slice. However, the sliding-rule filter does not appear to be as effective as the wavelet exponential filter. The large differences in the performance of each of these downward continuation methods highlight the significance in the choice of smoothing method. In addition, the wavelet exponential filter has the advantage of being locally adapted to the signal, which means that we do not need to over smooth the signal when the local downward continuation distances are small but the chessboard method suffers from over smoothing which is needed to prevent artifacts from continuing downward below the shallowest magnetic sources. One advantage of the wavelet and chessboard methods over the Taylor-series method is simplicity to automatically generate the parameters needed to design the smoothing filters for the wavelet and chessboard methods. In contrast, the Taylor-series method requires some trial-and-error intervention by the user. With a careful choice of smoothing parameters, the Taylor-series method can be designed to perform equally as well as the wavelet method. However, the choice of these parameters is often difficult when the downward continuation distances are large.

In recent years, interest in climatic extreme events (droughts, floods and heat waves) has been growing not only due to their direct impact on many socio-economic sectors, but also as climate change indicators. In this study, the behavior of extreme events is investigated using Extreme Climate Index Software (ECIS) package. The trend analysis of extreme indices is applied to daily series of temperature and precipitation observations from 16 synoptic stations over Iran in the period 1951-2003. Results show that the number of very warm days (T40) has increased while the number of very cool days (ID) has decreased and so the return period (Ťret) of cold extreme indices increased and the Tret of warm extreme indices decreased. Changes in total and extreme precipitation indices vary, depending upon geographic location. Symmetric warming in the tails of most indices is seen overall. On the basis of a systematic analysis of observed changes in indices of climate extremes, a significant proportion of Iran is increasingly affected by a significant change in extreme indices in the ~50-yr study period.

One of the most fundamental reservoir characteristics is the thickness. The analysis of thin bed tuning on seismic reflectivity has been studied extensively by Wides (1973) and Neidel and Pogiaggliomi (1977), who discussed the limits of seismic resolution. During the past decade, the industry has developed a plethora of new attributes in studying thin beds by employing spectral decomposition (Peyton et al., 1998; Partyka et al., 1999), and attributes which are obtained from it (Marfurt and Kirlin, 2001). Spectral decomposition refers to all methods that generate frequency spectrums consisting of amplitude spectrum, phase spectrum, change of phase with frequency and power spectrum in windows with the center of each time sample of a trace. These methods are used in studying geological features, thin beds, hydrocarbon reservoirs and noise attenuation. The most important of these methods are short time Fourier transform (STFT), continuous wavelet transforms, S-transform, Wigner-Ville distribution and matching pursuit decomposition. The result of a trace spectral decomposition is a time-frequency map.In this paper, the STFT method is applied for imaging thin beds in 2D seismic sections. In this method, a short time window with constant size is multiplied by a trace for each of its time samples of it then Fourier transform is applied on it. So, there is a local frequency spectrum for each time sample. The main factor of this method to be considered is the effect of the existence of two boundaries of a thin layer inside a time window. This affects local frequency spectrum and changes it so that by choosing some quantities of local spectrum as attributes it is possible to image the thin bed. The spectral attributes that are studied are the peak frequency, peak amplitude and change of phase with frequency. Detectable layers are related to the size of the time window. In this paper, frequency spectrums of a bed response and their reflection with minimum and zero phase wavelets are considered. Then, the effect of changing of thickness on frequency spectrums is studied. For controlling the main factor of this method, STFT algorithm is applied on a synthetic seismic section. It is shown that with the peak frequency and the peak amplitude obtained from a small portion of each local amplitude spectrum, and local change of phase with frequency in a particular frequency a good conclusion can be obtained. After that the STFT algorithm is applied on a part of a real seismic section and the place of layers with thickness up to 10 ms, between 10 to 18 ms and more than 18 ms are defined. It should be noted that the main idea of detecting thin beds on seismic sections is deference of frequency spectrum of a bed and a boundary. It is necessary to use more than one attribute for achieving more accurate results. It is shown that in particular frequency portions, thin beds are detected better. The minimum thickness that can be detected by this method depends on the frequency content of the seismic wavelet and the time sampling interval. It should be considered that applying STFT in more stages and less difference of size of windows in alternative stages increases accuracy of estimated thickness ranges.

The ultimate goal of the reservoir geophysicist is to determine the reservoir properties (lithology and porosity) and conditions (pore fluid and pressure) from seismic data. This goal can be achieved by applying rock physics transforms to a volume of seismically-derived elastic properties. The basis for establishing rock physics transforms are controlled experiments where the reservoir and elastic sediment properties are measured on the same sample of reservoir under the same conditions. Such experimental data come from well logs and cores. The purpose of this paper is to apply rock physics diagnostic to well log data from the Asmari reservoir in Mansuri oil field and then used to quantify porosity from acoustic impedance inversion volumes. The results show that Nur-Dvorkin (1996) model and Raymer- Greenberg-Castagna (1997) are good for predicting porosity in sandstone and the carbonate zone of the Asmari reservoir respectively.

A fluctuating electric current flowing in the Earth’s atmosphere causes corresponding electric currents to flow in the conducting Earth below the source current. The depth of penetration of the induced currents is determined by the characteristics of the source currents as well as the distribution of electrically conducting materials in the Earth. At the Earth’s surface observatories measure the composite of external (source) and internal (induced) field components from the currents? The quiet daily field variations, called Sq for 'solar quiet-time', provide a natural signal source with frequencies appropriate to upper mantle conductivity studies. This paper is concerned with the quiet-day field variations, their separation into external and internal contributions, and the use of this separation to profile the electrical conductivity of the Earth's upper mantle. For the situation in which field measurements are available about a spherical surface that separates the source from the induced currents (and a current doesn’t flow across this surface), Gauss (1838) devised a special solution of the differential electromagnetic field equations that is separable in the spherical coordinates r, q and f In Gauss’s solution, the field terms that represent radial dependence appear as two series-one with increasing powers of the sphere radius, r, and one with increasing powers of 1/r. As the value of r increases (outward from the sphere) the first series produces increased field strength, as if approaching external current sources. As the value of r decreases (toward the sphere center) the second series of 1/r terms indicate increased field strength, as if approaching internal current sources. Gauss had devised the way to separately represent the currents that were external and internal to his analysis sphere. For these external and internal series, there are individual spherical harmonic analysis (SHA) polynomial (Legendre) terms, each having two indices, degree m and order n. The full field is then represented as paired (external and internal Legendre terms) elemental parts, each satisfying the physical laws.Gauss applied the SHA method to the global field observations and verified that most of the Earth’s main field originated from internal sources. By the turn of the century, the method was also used to show that the daily, quiet-time geomagnetic field variations came mostly from sources of current external to the Earth (Schuster, 1889 and 1908); this finding led to the discovery of the ionosphere. We will analyze the quiet fields with this SHA to separate the Sq ionospheric source currents from the induced currents within the Earth.On days undisturbed by solar-terrestrial field and particle activity the geomagnetic records from a surface observatory display a smooth variation of field during the daylight hours. These variations are dominated by 24-, 12-, 8-, and 6-hr spectral components that change slowly from day to day through the seasons. Some authors prefer to select quiet days by a limiting value of the day's Ap (e.g., Ap=10). Others take a fixed number (e.g., five) of the quietest days for a given month, whatever the values may be. We have preferred to select those days for which no Kp index exceeded a limiting level (e.g., 2+) because active records can cause unrealistic conductivity computations.The 19 North American observatories are selected in this study. There are 60 days in 1997 in which the global geomagnetic disturbance index, Kp, have all 8 daily values less than 2+. These days are taken as preliminary "quiet day" recordings. All observatories have 60-min sample records. The original recordings of field are in Universal Time (UT) as orthogonal north, east, and into-the-earth components of field as X, Y, and Z. The data for each component are Fourier analyzed for each quiet day.In the method outlined by Schmucker (1970) for profiling the Earth's substructure, formulas are developed that provide the depth (d) and conductivity (s) of apparent layers (substitute conductors) that would produce surface-field relationships similar to the observed components. These profile values, Dn,m =z*- p*/2 (km)                                                                                                                     (1)And sn,m = 5.4x104 / m (pp*/2)2                   (s/m)                                                                               (2)need to be determined for each n, m set of SHA coefficients using the real z * andimaginary P*/2 parts of a complex induction transfer function, Cmn, given as:Cmn=z*-ixp*/2                                                                                                                             (3)Schmucker (1970) showed that the transfer function is obtained from the ratio of the field components, for a given n, m, as:(Z/X)n,m =i[n(n+1)Pmn (q)/ R(dpmn/dq]Cnm                                                                                    (4)(Z/Y)n,m =i[n(n+1)sinq / mR]Cnm                                                                                                  (5)where X, Y and Z are the northward, eastward, and into the Earth field components in gammas, R is the Earth's radius in kilometers, q is the colatitude at the field measurement location, and m Pmn (cos q) is the Schmidt normalized associated Legendre polynomial. To use the above conductivity-depth formulation, it is now necessary to find the individual n, m SHA terms of the three field components at the field-measurement location. With the fitted potential function (V) given as:V(q,f)=å4m=1 å12n=m R[Amn cos (mf)+bmnsin (mf)]pmn (cos q)                                                          (6)in which the cosine (A) and sine (B) coefficients of the expansion for the external (ex) and internal (in) parts are taken to be:Amn=[a(ex)mn+a(in)mn]                                                                                                                 (7)Bmn=[b(ex)mn+b(in)mn]Campbell and Anderssen (1983) showed that Equations (1) and (2) may be determined directly from the SHA coefficients with the expressionsZ*=r / n(n+1) {Amn [na(ex)mn-(n+1)a (in)mn+bmn[nb (ex)mn-(n+1) b(in)mn]/(Amn)2+(Bmn)2                     (8)P*/2=R/n(n+1){amn[nb(ex)mn-(n+1)b(in)mn]-bmn[na(ex)mn-(n+1)a(in)mn / (amn)2+(bmn)2                      (9)Electrical conductivity properties of the upper mantle for a North American sector of the Earth have been determined using the 24-, 12-, 8-, and 6-hr spectral components of the quiet-day geomagnetic variations. Spherical harmonic coefficients obtained from an analysis of the three components of the quiet daily variation (Sq) field for the solar-quiet year of 1997 were applied to a modeling procedure that was modified from Schmucher's (1970) publication. From a depth of about 100 to 650 km, the conductivity, s(S / m) , may be represented by s=0.0038e0.0074d with d is the depth in kilometers. Small perturbations of conductivity indicating some layering at 100 to 400 and 400 to 650 km correspond to the similar behavior of the Earth's density in these regions. From temperature-depth models we infer that the multiphase bulk properties of the expected silicates in these regions behave approximately as s=1.42x103 e -1.86x104/T with T as the temperature in Kelvin.

A new coupled system of the aerosol HAM model and the Weather Research and Forecasting (WRF) model is introduced and tested in this paper. Unlike the two other aerosol schemes currently coupled in the WRF model (MADE/Sorgam and MOSAIC), the HAM applies a new "pseudo modal" approach for the representation of the aerosol particles. The aerosol population in this model is represented by the superposition of seven modes based on size and solubility. The particle size is assumed to be distributed log-normally. The seven modes are categorized into four geometrical size classes, ranging from the nucleation, Aitken and accumulation modes to coarse modes. The aerosols are also divided into two types of internally mixed and water soluble particles (four modes), and externally mixed and insoluble particles (three modes). This classification makes possible the prediction of the hygroscopic properties of initially insoluble aerosol compounds which controls their atmospheric lifetimes and also their interactions with clouds. The WRF-HAM model includes the various microphysical processes of condensation of sulfuric acid, nucleation and new particle formation, coagulation of aerosol particles, and the thermo dynamical equilibrium of aerosols with the water vapor. The main removal processes for the aerosol particles in the coupled WRF-HAM model are gravitational sedimentation and dry deposition. The model also considers the in-cloud scavenging of aerosol particles by precipitation within the convective cumulus clouds. The main global aerosol compounds including sulfate, black carbon, particulate organic carbon (POM), sea salt and mineral dust have been considered in this study. The emission fluxes of different aerosol compounds are based on the prescribed Emission Inventory for the Aerosol Model Inter-comparison Experiment B, AEROCOM representative for the year 2000. The simulations are carried out for a 6-day simulation period from 6 to 12 May 2006 over a domain with 30-km grid spacing, covering south-western Asia, North Africa and some parts of Europe. The diurnal variation of the simulated hourly PM10 mass concentration at Tehran is qualitatively close to the hourly observations made by the Air Quality Control Company (AQCC) of the Municipality of Tehran. The model captures diurnal cycle and the magnitude of the observed PM10 concentration during most of the simulation period. Coupling WRF model with HAM aerosol scheme improves the hourly PM10 mass concentration compared to the case where WRF was coupled with MADE scheme. A negative radiative forcing and cooling of the atmosphere are found mainly over the regions of high emission of mineral dust. The absorption of shortwave radiation by black carbon causes warming effects in some regions with positive radiative forcing. The inclusion of aerosol feedback in the shortwave radiation scheme improves the simulated daily mean shortwave radiation fluxes in Tehran. The difference between the simulated and observed mean daily downward shortwave radiation, temperature and surface pressure by the HAM model is smaller than by the aerosol MADE scheme. Compared to WRF-MADE, using the coupled WRF-HAM model improves the simulation of downward shortwave radiation by up to 40 Wm-2 . The spatial variation in the simulated mean optical depth of aerosols at 500 nm wavelength by the WRF-HAM model is qualitatively close to the measurements by MODIS instrument. Furthermore, the comparison of the simulated aerosol optical depth by HAM and MADE aerosol schemes and the observations in Solar Village site of the global AERONET Network shows that the HAM model highly outperforms MADE.