IRANIAN JOURNAL OF GEOPHYSICS
Year:2020 | Volume:14 | Issue:1 |Papers Count:8

Due to the approximately spherical nature of the earth and the complex nature of atmospheric and oceanic flows, numerical solution of corresponding governing equations requires using an appropriate coordinate system on the spherical geometry. All spherical grids defined for the spherical surface or shell, have their own advantages and disadvantages generally. The Yin– Yang grid belongs to the family of overset grids. This coordinate is composed of two grid components named Yin and Yang with partial overlapping at their boundaries. Some advantages of the Yin– Yang grids are as follows: 1-Yin and Yang grid components are both orthogonal and based on the conventional latitude– longitude grid; 2-The singular points are absent; 3-The metric factors of the both grid components are analytically known; 4-The grid lengths are uniform approximately; 5-It requires less grid points than for the conventional latitude– longitude grid; 6-We can adapt the existing latitude– longitude discretizations and codes for the use with the Yin– Yang grids. In this research, three types of the Yin– Yang grid are compared: the rectangular (basic), modified and modified with identical components. It is worth noting that the Yin– Ying grid with identical components is introduced for the first time in the present study. The central second-order finite difference scheme is applied to solve the two-dimensional advection equation on three types of Yin– Yang grid for a well-known test case. In addition, the fourth-order Runge– Kutta method is used to advance the governing equation in time. Results show that using the Yin– Ying grids to solve the advection equation is highly effective in reducing the computational cost compared to the conventional latitude– longitude grid. However, the use of rectangular and modified Yin– Yang grids entails a lower computational cost than the modified Yin– Yang grid with identical components. In addition, global errors are computed using the absolute, square and infinite norms. By calculating the errors using these norms, it can be seen that there is a slight increase in the errors in all three grids compared to the conventional latitude– longitude grid. Another point to note is a little higher accuracy of modified Yin– Yang grid with identical components relative to rectangular and modified Yin– Yang grids in the same resolution; though, the higher accuracy is associated with a relative increase in computational cost. In the considered algorithm, reduction of the accuracy in using Yin-Yang grids is likely due to interpolation. However, interpolation is an essential part of numerical solving process for various oceanic and atmospheric equations on Yin-Yang grids in spherical geometry.

The chaotic nature of the atmosphere, inexact equations of motions, and gaps in specifying the initial state of the atmosphere result in some uncertainties in the weather forecasting. One of the operational methods that has been employed to overcome this difficulty and improve the accuracy of weather forecasts is ensemble forecasting which gains popularity as a technique in numerical weather prediction due to its improved forecast results. There are some different approaches to develope an ensemble forecasting system including using multi-numerical weather prediction models with different numerics, utilizing different physical parametrization schemes, and perturbing initial and boundary conditions. The aim of the present work is to develope a multi-physics ensemble forecasting system for the Weather Research and Forecasting (WRF) model and to evaluate its performance in forecasting of precipitation as a low predictability parameter. To construct the ensemble members, seven microphysics, two planetary boundary layers (PBL) and six cumulus parameterization schemes are employed. These choices are used to generate 84 memebers of the ensemble system. For running the model, a three-nested domain consisting of 36, 12, and 4 km spatial resolution was used to simulate the precipitation for 12 days during 2015-2016 winter in central region of Iran. The one degree FNL data of National Center for Environmental Prediction (NCEP) are utilized as the initial condition for running WRF. The model outputs are evaluated against observations using some standard metrics of forecasting verification including categorical, continuous and probabilistic indices. The observed 24-h accumulated precipitation measured at 132 synoptic stations of national meteorological organization of Iran was used for verification of simulations. In this study, the general performance of the different members of ensemble system is discussed. The members employing MYJ planetary boundary layer scheme, KF and GF cumulus schemes and WSM6, Goddard and New Thompson schemes are found to be appropriate more than the others for accumulated precipitation simulation in the study region. Furthermore, a multi categorical verification using three threshold of 1, 10, and 50 mm/day showed the suitable appraisal of the total performance of ensemble system. Moreover, increasing in ensemble size results in better outputs. However, it seems that expanding the ensemble members might be limited at an optimum number. It means that the skill of ensemble system doesn’ t change significantly when the ensemble members are more than the optimum number. In addition, the results show that using a cumulus parametrization scheme for the spatial resolution of the finest domain of this study (i. e., 4 km) is useful to improve the precipitation simulations. It is worth to mention that this finding is in agreement with reports of other studies.

Inversion of basement relief of sedimentary basins is an important application among the non-linear modeling techniques. Particularly in sedimentary basins with hydrocarbon source potential, the thickness of sediments is one of the primary factors in determining the thermal maturation of these basins. Gravity methods have been vastly used to estimate the base of sedimentary basins. The aim of this research is two-dimensional modeling of the basement geometry of a sedimentary basin using the inversion of the gravimetry data. A common way to approach this problem is discretizing the basin using polygons (or other geometries), and solving the non-linear inverse problem by local optimization iteratively. This procedure provides a solution which highly depends on the initial model and the used prior information. Besides, due to the non-linearity of this inverse problem, local optimization methods will fail whenever there is no reliable initial model. The global optimization method is a promising alternative to classical inversion methods because the quality of their solutions does not depend on the initial model. Also, they do not use the derivatives of the objective function. Ant colony algorithm (ACO) is one of the kinds of important swarm intelligence algorithms which have been successfully applied in many fields such as inversion of geophysical data. This research, in two separate stages, investigates the design and implementation of the ACO as a powerful tool for two-dimensional non-linear modeling of gravity data. ACO can be a substitution for the local response methods such as Marquardt-Levenberg and Gauss-Newton. To apply this algorithm in the problem under consideration, it was validated with the data obtained from a synthetic model and then, reverse modeling of the real data was performed. For evaluating the validation of this developed algorithm, it was tested by the synthetic model. Data from the synthetic models were modeled by using the developed algorithm, and acceptable results were obtained. By using this approach, the topography of the basement in the synthetic model was obtained with acceptable accuracy. In this study, the effect of ACO algorithm on different values of probable noises was investigated. The results indicate that this algorithm is suitably stable against the Gaussian white noise with relatively high amplitudes. In modeling for high noise percentage, the root mean square error of the data calculated with the original data didn't exceed 1. 64 mGal and that obtained with the original model at most was 131. 4 m. The results of modeling show acceptable agreement with the original model even in the case of data contaminated with 10% Gaussian white noise. The reliability of the proposed method to the inversion of a real gravity data was confirmed by applying it on a real gravity profile in the Moghan sedimentary basin. The results of this modeling are compatible with previously published works in this area.

The prestack seismic inversion converts seismic data to the physical properties of a rock such as sonic and shear impedance and density. It provides accurate information for predicting lithology changes and fluid types. In this paper, well logging data is used to construct synthetic seismogram. In the final stage, by transforming offset domain to angle domain and using the well information to the prestack seismic inversion, the extracted petro-physical parameters are discussed. The applied average wavelets of 7 wells in prestack inversion were in the angles of 5-9, 9-13, 13-17, 17-21 and 21-25 degrees. After wavelet extraction, low frequency acoustic impedance models and shear impedance and density were made as one of the inversion inputs. We built these, low frequency initial models using sonic log, shear impedance log and density log from well data. There are two techniques for doing the pre-stack seismic inversion: simultaneous and elastic inversion. These technics require wavelets and background model. Simultaneous pre-stacking inversion is defined by seismic trace angle, logarithms of P-impedance and S-impedance, and extracted wavelet but Elastic inversion uses a transformation of the Zeoppritz equations In other words, by performing the prestack inversion, the sonic and shear impedance and density are calculated using above mentioned equations. In this paper, prestack seismic inversion method was carried out in one of Iranian oil fields in Ghar-Asmari Reservoir and Jahrum Reservoir formations. The results showed that the presence of oil and gas in the Ghar-Asmari zone caused the reduction of the sonic and shear impedance and density. From Ghar-Asmari zone to Jahrum, the amount of the sonic and shear impedance and density increased. Also, the results of sonic impedance cross-plots versus the ratio of sonic-to-shear wave velocity were determined based on porosity variations and water saturation changes. In Ghar-Asmari zone, porosity is high and water saturation is low because of the presence of gas and oil in this section. From Ghar-Asmari zone to Jahrum, water saturation increases and porosity decreases. Hence, using simultaneous inversion, the hydrocarbon reservoir was identified.

When an earthquake occurs, a part of the released energy is propagated in the form of elastic waves at a speed that depends on the nature of the propagation environment. The radiated energy from earthquake focus in all directions is called radiation pattern and depends on the fault mechanism. The environment response to longitudinal waves appears as compression and expansion of the environment. This can affect on water levels in aquifers, wells and springs before, during and after the event. The changes of the groundwater levels caused by an earthquake are important with respect to other induced parameters; they may generate crustal deformations, affect on water supplies and production of oil wells, initiate liquefaction, control the distribution of aftershocks, trigger earthquakes and mud volcano eruptions. In this study, the effect of April 2013 Kaki earthquake (Mw6. 3) on water levels in wells located in a range of 50 km radius around the earthquake epicenter was compared in successive months in the preceding years, during and after the event. Regarding the earthquake focal mechanism and using the estimated tensile and compressive zones in the study area, the correlation of these areas with changes in water levels and discharge of wells and springs was examined. The results of evaluations indicate that the water levels in wells rose and the discharge of the wells and springs located within the radiation range of the pressure waves increased after the earthquake. The wells located in the direction of compressive radiation showed a level of elevation. In contrast, wells located along the shear strain either experienced a drop in water level or had no significant changes. The results show that there is compatibility between the pattern of seismic waves and the changes in the water levels in wells and springs. Because of the difference in characteristics of geological formations, the response of the wells located in the same seismic pattern may not be the same. On the other hand, since the earthquake occurred in the low rainfall time, changes in water levels cannot be caused by rainfall. Field data shows other recorded effects such as the recovery of the springs of Kale and Seyyed Ali which indicates the increase in discharge of water in the region. The liquefaction created in the area is a reason for the passage of a compressive wave that has led to increase of water levels in the wells. The amount of decrease or increase in water level depends on the magnitude and distance to hypocenter of the earthquake and geological characteristics of the well. The water level in the well located in Shonbe village (28. 34° N, 51. 76° E) increased 1. 6 m after the earthquake.

In this research, in order to explain the drought and wet year periods of Iran and their time overlap with teleconnection cycles of North Atlantic Oscillation (NAO), two sets of data were used. The first set is the data related to monthly precipitation of 63 synoptic weather stations of Iran during 1988-2015, obtained from Iranian Meteorology Organization, and the second one is the values of North Atlantic Oscillation index for the same period obtained from National Center of Environmental/National Center for Atmospheric Research (NCEP/NCAR) affiliated with US National Oceanic and Atmospheric Administration (NASA). Moreover, Standardized Precipitation Index (SPI) was used to calculate and identify the droughts and wet years of studied stations. By calculation of SPI for all studied stations in a monthly scale, their time scales were prepared and set separately for all the studied months, in station scale and regional scale. In addition to these two time series, the North Atlantic Oscillations were also obtained for these months. Finally, in order to identify the dominant periods existing in the time series, the spectral analysis was done using Fourier method. The results of spectral analysis on NAO time series show that in each month, different periods from short periods to long periods are observable and the most dominance is related to short periods (less than 10 years). Investigation of the overlap of these periods with periods related to droughts and wet years of Iran in studied stations shows that except May, more than 65% of stations had at least one period identical to that of NAO. This can indicate the effectiveness of this climate fluctuation on conditions of droughts and wet years happening in Iran.