Journal of the Earth and Space Physics
Year:2005 | Volume:31 | Issue:2|Papers Count:17

In this paper a method for computing offset of zero points of height data with respect to geoid by solving a fixed-free two-boundary value problem (FFTBVP) is addressed. Application of the following observables in the FFTBVP by presenting their corresponding integral equations and the computational algorithm is fully covered. a) Gravitational: gravity potential, modulus of gravity vector, astronomical latitude and astronomical longitude. b) Satellite altimetry. The method presented is applied to compute the offset of zero point of Iranian height datum with respect to geoid. According to the results, height datum of Iran is 0.094 m below the geoid. In this method since the offset of zero points of height data are determined with respect to geoid, height datum unification can be achieved by repeating the same procedure at in different countries. The recommended boundary value problem has the capability of incorporating any type of observations which contain information about the gravity field of the Earth in a simultaneous gravity field modeling, therefore the method by taking advantage of all existing gravity observables can provide the optimum solution for obtaining the offset of the zero points of the height data and/or height datum unification.

To cloud physicists, precipitable water (PW) is one of the most important meteorological quantities. PW is usually measured using radar. However, because of the lack of suitable radar coverage in the Tehran area, the current study is based on radio sound measurements. In this paper a comparison is made between computed PW using thermodynamical diagrams (Skew-T) and precipitation measured by the ground meteorological stations, with a view to constructing a relationship for precipitation forecasting. The PW derived from thermodynamic diagrams for 00 and 12 (GMT) from 1984 to 1998, were analyzed. In addition three sets of relationships for each (GMT) time were found between the measured precipitation and precipitable water for three different precipitable water intervals of (0-15) , (15-30) and (30-45)mm; all relationships were found to be statistically significant. The results show that the 20-25 mm, precipitable water interval could be selected as a suitable threshold for convective cloud seeding decision in the Tehran area. The results confirmed findings by previous cloud physicists. Furthermore it is concluded that there is a possibility of precipitation occurring in the Tehran area when precipitable water exceeds 7mm. The results also show that if the precipitable water estimated at 00 (GMT), exceeds 40mm, then flood may occur in this area.

A gravity survey including 1115 stations incorporating 28 profiles has been performed in order to determine the subsurface geological structures in the Tabas sedimentary basin as a part of a petroleum exploration program. After making a few corrections on the observed data, and eliminating the gravity effects arising from the basement considered as regional effects, a gravity residual anomaly map of the area was then prepared. The residual map indicated some anticline structures, which are of interest in petroleum exploration. One of the most important tasks in the interpretation of gravity data quantitatively is to determine the physical properties such as density and the geometrical para meters of oil traps, including depth, shape as well as surficial extension. To achieve this goal, a commercial computer-based software called Modelvison Pro was used to perform a threedimensional inverse modeling for the indicated anomalies. The results of modeling indicate the presence a few anticlines and synclines. Among them anticline E which is located at an estimated depth of 5196 m, and has the largest surficial extension, can be considered as an important traps for hydrocarbon resources.

In this paper a method for computation of mean gravity value within the Earth between the computation point and the geoid needed for precise orthometric height computations is presented.The method presented is based on following steps: (1) computation of the global and regional gravity effects using ellipsoidal harmonic expansion to degree and order 360 plus the centrifugal acceleration. (2) Computation of the gravitational effect of terrain masses within the radius of 55km around the computational point applying Newton integral in the equal area map projection. (3) Computation of the gravity at two points on the surface of the Earth and on the geoid using steps 1-2, computing mean and standard deviation of the computed gravity values and increasing the number of points within the Earth to meet the predefined standard deviation for the computation of mean gravity within the Earth. (4) Deriving the mean gravity within the Earth from the steps 1-3. The proposed method for the computation of mean gravity within the Earth is checked against the observed gravity values within the Earth in an exploration borehole. The test computations are made in the following two modes: (a) Computation of gravity values at the observation points in the borehole and comparison of the computed values with the observed values (b) Computing mean gravity within the Earth using the proposed method and the observed gravity values. According to the test computations at a depth of 474.7 m computed gravity differs from the observed gravity by 10.768 mGal and the computed mean gravity from the observed mean gravity by 5.56 mGal.

In mid-latitudes, the surface cyclones and anticyclones are mainly generated by the action of mid to upper tropospheric baroclinic waves. To understand the mechanisms of formation and intensification of these weather systems, the study of baroclinic wave packets is essential. In the first part of this two-part paper, using the global data from NOAA for Feb. 2003 provided by Iran Meteorological Organization, a synoptic-dynamic study to detect baroclinic wave packets and determine their evolution is undertaken. Hovmoller diagrams and complex demodulation as applied to various dynamical quantities have been used to detect wave packets and determine their characteristics. For the upper tropospheric wave packets detected in Hovmoller diagrams, group velocity is greater than the phase speed. This characteristic of wave packets is consistent with downstream development of the waves. The results indicate that the blocking action in the east of the Pacific and Atlantic Oceans in the first 10 days of Feb. inhibits the presence of baroclinic wave packets and their related low-level activities. The wave packets detected by Hovmoller and complex demodulation methods have been compared. For the period 10th to 15th of Feb. the two methods consistently give the same packets. Tracking the wave packets by using complex demodulation point to the intensification and weakening of the waves over, respectively, the west and east of the Pacific and Atlantic Oceans, which is studied in more detail in the second part by dynamical analysis considering the energetics of the waves.

Using the global data of NOAA provided by the Iran Meteorological Organization, the energetics of the wave packets detected in part I of this paper are studied in detail. To this end, within the wave packets detected, six troughs with downstream development were selected and for each the eddy kinetic energy as well as the other important terms in the energy budget relation were computed for the life time of the troughs. The ageostrophic flux convergence, total convergence flux, baroclinic conversion, barotropic conversion, and the residual are the terms computed for the study. The energetics results show that in nearly all the cases involving downstream development, baroclinic conversion plays a significant role as either a source in the mature state or a sink in the decaying state of the troughs. The initial perturbation is generated by baroclinic conversion and subsequently initiates the downstream development. The main cause of downstream development is the ageostrophic flux convergence by which energy is radiated from the upstream of the existing perturbation to its downstream. This process leads to the decay of the existing wave and generation of a new wave in its downstream.

Mechanical and electromagnetic perturbations are coupled in a porous medium saturated with liquid. This coupling is such that, a seismic wave propagating through the mentioned medium will cause a relative movement between the solid and liquid. Then this movement will induce an electrical flux. When a seismic pulse propagates through a medium with specific chemical and elastic properties it causes an unbalance in the electrical flux. The unbalance in the electrical charge will separates the dipoles and multi-poles an both sides of an interface and that makes it possible to record the electromagnetic perturbations at the earth’s surface. In this paper, electroseismic wave propagation has been studied and simulated in a layered and porous medium saturated with liquid. For this purpose the governing equations of Pride (1994) and the coupled equations of the Biot and Maxwell were used. To calculate the electroseismic traces, the generalized reflection and transmission matrix method was applied.

Due to the non-stationary properties of seismic signals, it is convenient to use multi-scale and multi- resolution transform to analyze seismic data. S transform is an extension of the ideas of the continuous wavelet transform (CWT), and is based on a moving and scalable localizing Gaussian window. The S transform is unique in that it provides frequency-dependent resolution while maintaining a direct relationship with the Fourier spectrum. In this study we took advantage of the Stransform to design time and frequency dependent filters to attenuate ground rolls and random noises from seismic data. We used single frequency seismic sections in order to identify the low frequency shadows on seismic sections, which is the direct indicator of hydrocarbon.

The objective of this study is the assessment survey of short-term wave prediction in the southern Caspian Sea by using the WAM wave model. For the run of the WAM model, a 35×45 rectangularity grid is used, covering 36.5º to 47.5º N in latitude and 46.5º to 55º E in longitude with 15 grid interval in both directions. In order to survey the ability of the model in forecasting wave, two different atmospheric conditions, one with weak and the other with strong wind, are considered. The required wave model input data are the Caspian Sea topography and 10 meter wind field. The weak wind input data are extracted from the transmitted GTS synoptic data of regional centers to the Forecasting Center of Iran Meteorological Organization, the storm wind input data are extracted from the output of MM5 mesoscale model which is run by using the AVN data of NOAA Center and the topography input data are extracted from the Caspian Sea Hydrography map of Iran, Armed Forces Geographical Organization. Before transforming input data into the WAM model format, they were interpolated into the Caspian Sea grid. Making use of written programs in FORTRAN, GrADS and MATLAB languages, the wave model output is transformed into suitable graphics including contour map of wave height, wind field, diagram of Custer, 1-D frequency spectrum and 2-D angular spectrum. According to wave model outputs of the Caspian Sea, the results of this research are as follows: -Because of existence of evident changes in measurement and gradient of wave height in relation to different atmospheric conditions that are descriptive of model sensibility to wind input data and very much consistent with wind and wave field that is specification of prediction wave in closed basins, the WAM wave model is able to predict wave characteristics as short-term in the Caspian Sea. -Because of the existence of conformity wave double-peaked frequency spectra of the Caspian Sea with Strekalov and Massels’ researches about wave frequency spectra of the Caspian Sea, the WAM wave model is able to predict in the short-term and display accurate pattern of wave frequency spectra in the Caspian Sea. -Because of the existence of right skewness of wave frequency spectra of the southern Caspian Sea, number of waves taking part in propagating wave spectral energy is related to frequency domain higher than peak frequency, but integrally the maximum amount of wave spectral energy is propagated by the waves with frequency lower than peak frequency. -Because of the existence of good relativity agreement between the WAM model output and the wave data of the Anzali bouy when they are compared in order to survey model verification, it is found that the WAM model is able to predict wave characteristics in the short-term in the southern Caspian Sea with high relativity accuracy.

Based on the transfer function characteristics a semi daynamical real time flood-forecasting model has been derived which is mathematically stable. Furthermore an user-friendly software has been developed. Due to the incapability of models to perfectly portray complex natural systems, and due to faulty model input data, every forecast is subject to an error. Consequently, it is necessary to correct (update) the forecast in the light of recent model performance to minimize forecast error. Generally speaking three simulated volume (∝), shape (ε) and time (γ) factors influence simulated runoff. None of them could cause the model to become unstable or fluctuating. By adjusting these three factors it is possible to make the updated model more accurate than the static one. The model is made in the form of a software package. This program needs the input files including the rainfall and runoff data. In this investigation, eight events at Behbahan station in the Maroon basin have been used in calibration phase. The order and parameters of the model should be determined so that the realistic impulse response can be obtained. The volume, shape and time parameters can be updated automatically using recursive least squares method. If these parameters are not suitable, they can be chosen manually to achieve better results. The last part of program allocates the flood forecasting. Again, four separate events at the Behbahan station have been chosen to investigate the capability of the model at the forecasting phase.

One-dimensional fast Fourier transform (1D FFT) is used to solve the ellipsoidal Stokes integral (Martinec and Grafarend, 1997) in an ellipsoidal cap around the computational point (near-zone contribution) numerically. For the far-zone contribution the spherical harmonic expansion can be applied. The geoidal height computation through direct numerical solution of the integral and 1D FFT will be compared for an area in Canada. The comparison shows relatively a great difference due to the application of FFT to the original ellipsoidal Stokes integral.

The function defined in MATLAB for minimizing the unconstrained multivariable function (fminunc) based on the quasi Newton method is used to minimize the square differences between calculated and observed data (misfit function). The computer code provided is applied for estimating the average depth of n-sided polygons as synthetic models. The method is also used for real data.