Institute of Geophysics, University of TehranJournal of the Earth and Space Physics2538-371X34420090219Determination of natural frequency of soil vibration in Sarcheshmeh copper mine, using microtremor recordsDetermination of natural frequency of soil vibration in Sarcheshmeh copper mine, using microtremor records1127956FAJournal Article20171031It is well known that ground-shaking site effect caused by an earthquake can vary significantly within a small distance. This is because at sites having soft soil and/or topographic and basement undulations, seismic energy gets trapped, leading to amplification of vibration that may cause considerable damage to man-made structures. Theoretical analysis and observational data have shown that each site has a specific resonance frequency at which ground motion gets amplified. Man-made structures having resonance frequency matching that of the site have the maximum likelihood of getting damaged. Hence, in order to construct seismically-safe structures, it is important to know the site response. <br />Various methods are available for the estimation of site response. The best method is to record strong ground motion caused by a large local earthquake. However, fortunately such events are not very frequent in many areas. Hence, for site response analysis, this method is not very practical. Another method is to carry out extensive seismic reflection and/or refraction surveys and geotechnical surveys; this method is extremely expensive and time-consuming. Recently microtremor data have also been widely used for estimation of site response. The advantage of this method is that it takes very little time for data collection. One does not have to wait for an earthquake to occur. Very few instruments are required; the data collection can be handled even with a single instrument. <br />The method involves recording microtremor data from the site to be investigated. It is assumed that signals from a hard rock site are carried equally well at all frequencies. On the other hand, a soft-soil site amplifies the signal at its resonance frequency, which depends on factors such as the soil type, basement configuration, etc. Hence, if the source and the path effects were removed from the spectra of the signal, then we should get a flat spectra at a hard-rock site and spectra showing peaks at resonance frequency at a soft-soil site. <br />In this study microtremor measurements were carried out in the Sarcheshmeh copper mine area at about 12 sites and the natural frequency at each site was estimated considering the main peak in the spectral ratio between the horizontal and the vertical component, the method called the NAKAMURA technique (H/V). Many experimental and theoretical studies have shown the reliability of microtremor measurements in site predominant frequency estimation. At each site traces have been collected 3 or 4 times and for all of them natural frequency based on SESAME project group standards have been analyzed. Then on the basis of natural frequency, site soil types have been determined and average Shear wave velocity for each site has been predicted. All results compared with available field observations.It is well known that ground-shaking site effect caused by an earthquake can vary significantly within a small distance. This is because at sites having soft soil and/or topographic and basement undulations, seismic energy gets trapped, leading to amplification of vibration that may cause considerable damage to man-made structures. Theoretical analysis and observational data have shown that each site has a specific resonance frequency at which ground motion gets amplified. Man-made structures having resonance frequency matching that of the site have the maximum likelihood of getting damaged. Hence, in order to construct seismically-safe structures, it is important to know the site response. <br />Various methods are available for the estimation of site response. The best method is to record strong ground motion caused by a large local earthquake. However, fortunately such events are not very frequent in many areas. Hence, for site response analysis, this method is not very practical. Another method is to carry out extensive seismic reflection and/or refraction surveys and geotechnical surveys; this method is extremely expensive and time-consuming. Recently microtremor data have also been widely used for estimation of site response. The advantage of this method is that it takes very little time for data collection. One does not have to wait for an earthquake to occur. Very few instruments are required; the data collection can be handled even with a single instrument. <br />The method involves recording microtremor data from the site to be investigated. It is assumed that signals from a hard rock site are carried equally well at all frequencies. On the other hand, a soft-soil site amplifies the signal at its resonance frequency, which depends on factors such as the soil type, basement configuration, etc. Hence, if the source and the path effects were removed from the spectra of the signal, then we should get a flat spectra at a hard-rock site and spectra showing peaks at resonance frequency at a soft-soil site. <br />In this study microtremor measurements were carried out in the Sarcheshmeh copper mine area at about 12 sites and the natural frequency at each site was estimated considering the main peak in the spectral ratio between the horizontal and the vertical component, the method called the NAKAMURA technique (H/V). Many experimental and theoretical studies have shown the reliability of microtremor measurements in site predominant frequency estimation. At each site traces have been collected 3 or 4 times and for all of them natural frequency based on SESAME project group standards have been analyzed. Then on the basis of natural frequency, site soil types have been determined and average Shear wave velocity for each site has been predicted. All results compared with available field observations.https://jesphys.ut.ac.ir/article_27956_7016ca09c48d872a3fdef0b751bb3713.pdfInstitute of Geophysics, University of TehranJournal of the Earth and Space Physics2538-371X34420090219Design of digital geo-electrical equipment and its application on noise level reductionDesign of digital geo-electrical equipment and its application on noise level reduction1127957FAJournal Article20171031Electrical resistivity survey is used to explore subsurface layers in hydrology, geology, mining, geotechnical and environmental investigations. The resistivity measurements are normally made by injecting current into the ground through two current electrodes, and measuring the resulting voltage difference at two potential electrodes. Electrical resistivity is a function of injection current, measured with difference potential and geometrical factor of electrodes. From the current (I) and voltage (V) values, an apparent resistivity (?a) value is calculated as ?a=KV/I, where K is the geometrical factor. Increasing the distance between two current electrodes caused an increase in the depth of penetration. To determine the true subsurface resistivity, layers thicknesses, an inversion of the measured apparent resistivity values must be carried out by means of a computer program. The ground resistivity is related to various geological parameters such as the mineral, fluid content, porosity, saline and degree of water saturation in the rock. <br />One of the new developments in recent years is the use of 2-D electrical Imaging/ tomography surveys to map areas with moderately complex geology (Loke & Barker 1996). Chambers et al (1993) use a multi-electrode system and dipole-dipole array for prospecting oil and gas. The results were satisfactory. El-Qady and Ushijima (2001) were used neural networks and inversion of DC resistivity data to interpret deep sounding electric. Busby (2000) has used azimuthally apparent-resistivity measurements for determining fracture strike orientations. Christiansen and Auken (2004) have made a dynamic system for detecting lateral variability. <br />Recording waveform of resistivity data can be done through moderate seismic digitizers. Distinguishing basement type fault by means of this system is very important and significant in seismology, especially in areas of large cities. <br />Electrical noises have an important role in the accuracy of geo-electric data. These noises can originate from geology conditions, self- potential, induction polarization, power electrical cables, and underground railways, pumping engines and electromagnetic fields. In the first step, most noises have been identified and the effects of important noises have been investigated which can be reduced using different field array and processing data. One of the most important ways to decrease the gain of noise is the recording of electrical data in digital way. To do this electrical data have been measured in digital waveforms in different parts of Iran. Then we applied different filtering tools to increase gain of signal to noise ratio. Results of our research showed that using electrical waveforms leads to an increase in the precision of measurements from 1mv (in analogue measurements) to 0.01 mv.Electrical resistivity survey is used to explore subsurface layers in hydrology, geology, mining, geotechnical and environmental investigations. The resistivity measurements are normally made by injecting current into the ground through two current electrodes, and measuring the resulting voltage difference at two potential electrodes. Electrical resistivity is a function of injection current, measured with difference potential and geometrical factor of electrodes. From the current (I) and voltage (V) values, an apparent resistivity (?a) value is calculated as ?a=KV/I, where K is the geometrical factor. Increasing the distance between two current electrodes caused an increase in the depth of penetration. To determine the true subsurface resistivity, layers thicknesses, an inversion of the measured apparent resistivity values must be carried out by means of a computer program. The ground resistivity is related to various geological parameters such as the mineral, fluid content, porosity, saline and degree of water saturation in the rock. <br />One of the new developments in recent years is the use of 2-D electrical Imaging/ tomography surveys to map areas with moderately complex geology (Loke & Barker 1996). Chambers et al (1993) use a multi-electrode system and dipole-dipole array for prospecting oil and gas. The results were satisfactory. El-Qady and Ushijima (2001) were used neural networks and inversion of DC resistivity data to interpret deep sounding electric. Busby (2000) has used azimuthally apparent-resistivity measurements for determining fracture strike orientations. Christiansen and Auken (2004) have made a dynamic system for detecting lateral variability. <br />Recording waveform of resistivity data can be done through moderate seismic digitizers. Distinguishing basement type fault by means of this system is very important and significant in seismology, especially in areas of large cities. <br />Electrical noises have an important role in the accuracy of geo-electric data. These noises can originate from geology conditions, self- potential, induction polarization, power electrical cables, and underground railways, pumping engines and electromagnetic fields. In the first step, most noises have been identified and the effects of important noises have been investigated which can be reduced using different field array and processing data. One of the most important ways to decrease the gain of noise is the recording of electrical data in digital way. To do this electrical data have been measured in digital waveforms in different parts of Iran. Then we applied different filtering tools to increase gain of signal to noise ratio. Results of our research showed that using electrical waveforms leads to an increase in the precision of measurements from 1mv (in analogue measurements) to 0.01 mv.https://jesphys.ut.ac.ir/article_27957_22e2bad38b7038e9e37d1325a688ca0a.pdfInstitute of Geophysics, University of TehranJournal of the Earth and Space Physics2538-371X34420090219Ground roll attenuation in the radial trace domainGround roll attenuation in the radial trace domain1127958FAJournal Article20171031The radial trace transform was introduced by the Stanford Exploration Project many years ago (Ottolini, 1979 and Claerbout, 1983), primarily for use in migration and imaging applications. It has been shown subsequently, because of its particular geometry, to be very useful for wavefield separation (Claerbout, 1983) and coherent noise attenuation (Henley, 1999). The radial trace transform, unlike such transforms as the F-K transform, is a mapping transform which takes each point of the X-T plane into a point of the R-T plane, and vice versa for the inverse. <br />R-T coherent noise techniques rely on the fact that separation of linear noise from reflections can be achieved in the radial trace domain by aligning the transform coordinate trajectories with the linear noise wavefronts in the X-T domain. This causes linear noises which spread across all the traces of an X-T gather to be isolated into small groups of radial traces. In addition, the apparent frequencies of these events are shifted from the seismic band to much lower frequencies (Henley, 1999) by the geometric distortion of the transform. <br />The Radial Trace Transform (RTT), is a simple coordinate transform of normal (t , x) domain seismic gathers; x=vt. The radial coordinate is termed “v” because the RTT sorts the data by apparent velocity. Neglecting dispersion effects, ground roll maps to zero temporal frequency in the RT domain. A linear noise distributed across many traces of an X-T gather maps into relatively few radial traces; and the apparent frequencies of these noise traces shift from the seismic band to sub-seismic frequencies (Henley, 1999). Both these effects of the R-T transform can be used to attenuate the noise relative to reflection signal in the R-T domain. The most straightforward way to attenuate coherent noise in the R-T domain is to apply a high-pass (low-cut) filter to the radial traces, which directly suppresses coherent noises mapped by the R-T transform to sub-seismic frequencies. <br />Filtering seismic data in the radial trace (R-T) domain is an effective technique for attenuating coherent noise on ensembles of seismic traces. In some applications R-T filtering can be more effective than more established methods like F-K filtering. One of the important advantages of the radial transform with respect to F-K transform is the ability to transform non-uniformly sampled data, such as a shot gather with irregular source-receiver offset values.The radial trace transform was introduced by the Stanford Exploration Project many years ago (Ottolini, 1979 and Claerbout, 1983), primarily for use in migration and imaging applications. It has been shown subsequently, because of its particular geometry, to be very useful for wavefield separation (Claerbout, 1983) and coherent noise attenuation (Henley, 1999). The radial trace transform, unlike such transforms as the F-K transform, is a mapping transform which takes each point of the X-T plane into a point of the R-T plane, and vice versa for the inverse. <br />R-T coherent noise techniques rely on the fact that separation of linear noise from reflections can be achieved in the radial trace domain by aligning the transform coordinate trajectories with the linear noise wavefronts in the X-T domain. This causes linear noises which spread across all the traces of an X-T gather to be isolated into small groups of radial traces. In addition, the apparent frequencies of these events are shifted from the seismic band to much lower frequencies (Henley, 1999) by the geometric distortion of the transform. <br />The Radial Trace Transform (RTT), is a simple coordinate transform of normal (t , x) domain seismic gathers; x=vt. The radial coordinate is termed “v” because the RTT sorts the data by apparent velocity. Neglecting dispersion effects, ground roll maps to zero temporal frequency in the RT domain. A linear noise distributed across many traces of an X-T gather maps into relatively few radial traces; and the apparent frequencies of these noise traces shift from the seismic band to sub-seismic frequencies (Henley, 1999). Both these effects of the R-T transform can be used to attenuate the noise relative to reflection signal in the R-T domain. The most straightforward way to attenuate coherent noise in the R-T domain is to apply a high-pass (low-cut) filter to the radial traces, which directly suppresses coherent noises mapped by the R-T transform to sub-seismic frequencies. <br />Filtering seismic data in the radial trace (R-T) domain is an effective technique for attenuating coherent noise on ensembles of seismic traces. In some applications R-T filtering can be more effective than more established methods like F-K filtering. One of the important advantages of the radial transform with respect to F-K transform is the ability to transform non-uniformly sampled data, such as a shot gather with irregular source-receiver offset values.https://jesphys.ut.ac.ir/article_27958_e410e4f703d0284bf78a98289838f3e2.pdfInstitute of Geophysics, University of TehranJournal of the Earth and Space Physics2538-371X34420090219Determination of the electrical conductivity and temperature of the upper mantle using geomagnetic quiet-daily variationsDetermination of the electrical conductivity and temperature of the upper mantle using geomagnetic quiet-daily variations1127959FAJournal Article20171031The electrical conductivity of the earth is its physical parameter, which can be studied in global scales. The investigations of the earth's electrical conductivity are based on geomagnetic field variations. Time variations of the geomagnetic field consist of long-term and short-term (transient) variations. External sources induce electric currents into the earth. The induced electric currents give rise, in turn, to an internal component of the magnetic variations observed at the surface. Transient magnetic fields (external sources) pass through electrically conducting earth with amplitude reduction and phase rotation. We are concerned here with the inductive skin-effect of natural geomagnetic variations which they undergo within the earth's interior. These variations are very slowly oscillating and can be regarded as quasi-stationary on a global scale. There is a measure of the penetration of an alternating magnetic field of frequency (or period ) into a conductor of conductivity . The penetration is often expressed by the skin depth S, given by (in terms of kilometers): <br /> (1) <br />Geomagnetic induction studies involve frequencies from a few cycles per minute to fractions of a cycle per day. So the penetration depth at which the main inductive attenuation of geomagnetic variation occurs depends on the period of the variation and electrical conductivity of the earth. <br />The inductive response of the earth's interior to the spectrum of geomagnetic variations and thereby the internal electrical conductivity distribution can be studied by two complementary methods: we can observe (i) the vertical magnetic component Z (magnetic method) or (ii) the tangential electrical component E (magnetotelluric method) of a transient surface field, setting either one of them in relation to the horizontal magnetic component H. The magnetic method is applied in this study. The available geomagnetic data consists of records of three components X, Y, Z (horizontal <br />north, horizontal east and vertical components) or Z, D, H (vertical, declination and horizontal components) of the earth's magnetic field as functions of time recorded at a number of points distributed over the surface of the earth. By separating magnetic variations into parts of internal (i) and external (e) origin, we can determine the electromagnetic response of the earth to a particular input (e). The ratio of the parts of the magnetic field of internal and external origin is a measure of the response and is dependent on both the external current system and the distribution of electrical conductivity within the earth. <br />The geomagnetic quiet-daily variations are the most persistent of all the geomagnetic variations that occur in days when the magnetic activity level is very low. In these days the magnetograms from any (except a high latitude) observatory show smooth pattern with no, or only very small, rapid fluctuations. index is the measure for selecting the quiet day. The geomagnetic bays are the events that occur in geomagnetic quiet days and their periods are between 30 minutes to 3 hours. <br />Once the geomagnetic variations at each observatory have been analyzed in terms of frequency, the spatial behavior can be expressed by expanding each frequency component in a series of spherical harmonics over the surface of the earth. The geomagnetic field outside the earth can be expressed as the gradient of a scalar potential as (Banks 1969): <br /> (2) <br />The potential can be represented as a series of spherical harmonics; in this particular case the harmonics are purely zonal: <br /> (3) <br />The coefficients and , corresponding to the internal and external parts of the field, respectively. The horizontal and vertical components of the geomagnetic field at the earth's surface are derived from as:The electrical conductivity of the earth is its physical parameter, which can be studied in global scales. The investigations of the earth's electrical conductivity are based on geomagnetic field variations. Time variations of the geomagnetic field consist of long-term and short-term (transient) variations. External sources induce electric currents into the earth. The induced electric currents give rise, in turn, to an internal component of the magnetic variations observed at the surface. Transient magnetic fields (external sources) pass through electrically conducting earth with amplitude reduction and phase rotation. We are concerned here with the inductive skin-effect of natural geomagnetic variations which they undergo within the earth's interior. These variations are very slowly oscillating and can be regarded as quasi-stationary on a global scale. There is a measure of the penetration of an alternating magnetic field of frequency (or period ) into a conductor of conductivity . The penetration is often expressed by the skin depth S, given by (in terms of kilometers): <br /> (1) <br />Geomagnetic induction studies involve frequencies from a few cycles per minute to fractions of a cycle per day. So the penetration depth at which the main inductive attenuation of geomagnetic variation occurs depends on the period of the variation and electrical conductivity of the earth. <br />The inductive response of the earth's interior to the spectrum of geomagnetic variations and thereby the internal electrical conductivity distribution can be studied by two complementary methods: we can observe (i) the vertical magnetic component Z (magnetic method) or (ii) the tangential electrical component E (magnetotelluric method) of a transient surface field, setting either one of them in relation to the horizontal magnetic component H. The magnetic method is applied in this study. The available geomagnetic data consists of records of three components X, Y, Z (horizontal <br />north, horizontal east and vertical components) or Z, D, H (vertical, declination and horizontal components) of the earth's magnetic field as functions of time recorded at a number of points distributed over the surface of the earth. By separating magnetic variations into parts of internal (i) and external (e) origin, we can determine the electromagnetic response of the earth to a particular input (e). The ratio of the parts of the magnetic field of internal and external origin is a measure of the response and is dependent on both the external current system and the distribution of electrical conductivity within the earth. <br />The geomagnetic quiet-daily variations are the most persistent of all the geomagnetic variations that occur in days when the magnetic activity level is very low. In these days the magnetograms from any (except a high latitude) observatory show smooth pattern with no, or only very small, rapid fluctuations. index is the measure for selecting the quiet day. The geomagnetic bays are the events that occur in geomagnetic quiet days and their periods are between 30 minutes to 3 hours. <br />Once the geomagnetic variations at each observatory have been analyzed in terms of frequency, the spatial behavior can be expressed by expanding each frequency component in a series of spherical harmonics over the surface of the earth. The geomagnetic field outside the earth can be expressed as the gradient of a scalar potential as (Banks 1969): <br /> (2) <br />The potential can be represented as a series of spherical harmonics; in this particular case the harmonics are purely zonal: <br /> (3) <br />The coefficients and , corresponding to the internal and external parts of the field, respectively. The horizontal and vertical components of the geomagnetic field at the earth's surface are derived from as:https://jesphys.ut.ac.ir/article_27959_fe09a9c48f667e514fc6c20c62f2f37d.pdfInstitute of Geophysics, University of TehranJournal of the Earth and Space Physics2538-371X34420090219Least-square estimation of GRACE intersatellite and its time derivativesLeast-square estimation of GRACE intersatellite and its time derivatives1127960FAJournal Article20171031One of the most valuable and important application of artificial satellites in geodetic sciences is recovery of the earth's gravity field. Orbits of satellites either gravimetric or nongravimetric are analyzed to improve the earth's gravity field. Gravimetric satellites are launched at low altitudes to observe gravity field in more detail. GRACE twin satellites are the second spacecraft of gravity field dedicated missions, realizing the high-low (HL) and low-low (LL) satellite-to satellite tracking (SST) concepts. The onboard GPS receivers collect the HL observations while the k-band ranging system (KBR) realizes the LL configurations. Nevertheless, two data sets have different sampling rates. The positions are recorded every 60 seconds whereas the KBR system observes the range changes every 5 seconds. In this article, we propose a new idea for combining these two data sets. We employ Hermite polynomial approximation in Least-squares mode in order to provide a model to interpolate the position by KBR sampling rate. Hermite interpolation is a method closely related to the Newton method of interpolation in numerical analysis, which allows us to consider given derivatives at data points, as well as the data points themselves. We apply a linear Least-squares method of Hermite approximation which fits a polynomial and its derivatives to observations. First, the method was tested on simulated data and the suitable degree of polynomial selected. The algorithm was applied on real data and the suitable polynomial was found. The proposed algorithm leads to finding the best degree of polynomial that estimates range and its derivatives with reasonable accuracy.One of the most valuable and important application of artificial satellites in geodetic sciences is recovery of the earth's gravity field. Orbits of satellites either gravimetric or nongravimetric are analyzed to improve the earth's gravity field. Gravimetric satellites are launched at low altitudes to observe gravity field in more detail. GRACE twin satellites are the second spacecraft of gravity field dedicated missions, realizing the high-low (HL) and low-low (LL) satellite-to satellite tracking (SST) concepts. The onboard GPS receivers collect the HL observations while the k-band ranging system (KBR) realizes the LL configurations. Nevertheless, two data sets have different sampling rates. The positions are recorded every 60 seconds whereas the KBR system observes the range changes every 5 seconds. In this article, we propose a new idea for combining these two data sets. We employ Hermite polynomial approximation in Least-squares mode in order to provide a model to interpolate the position by KBR sampling rate. Hermite interpolation is a method closely related to the Newton method of interpolation in numerical analysis, which allows us to consider given derivatives at data points, as well as the data points themselves. We apply a linear Least-squares method of Hermite approximation which fits a polynomial and its derivatives to observations. First, the method was tested on simulated data and the suitable degree of polynomial selected. The algorithm was applied on real data and the suitable polynomial was found. The proposed algorithm leads to finding the best degree of polynomial that estimates range and its derivatives with reasonable accuracy.https://jesphys.ut.ac.ir/article_27960_c76018ca412395b078eb6aa64fbc7601.pdfInstitute of Geophysics, University of TehranJournal of the Earth and Space Physics2538-371X34420090219Fixed garvimetric-altimetry boundary value problem for geoid determination on islandsFixed garvimetric-altimetry boundary value problem for geoid determination on islands1127961FAJournal Article20171031Precise geoid determination on islands suffers from the lack of accurate gravity data on the open seas. Nowadays, sub-microgal accuracy for the land gravity observations is obtainable. But the sea gravity data which are collected via shipborne techniques, due to the measuring environment at sea area, are usually highly noisy and are contaminated with various systematic errors. <br />On the other hand, satellite altimetry has provided a new source of information for geoid determination at sea area. It should be noted that satellite altimetry has accuracy at centimeter level which reaches decimeters at coastal. Such accuracy in geometric space is equivalent to microgal in gravity space, which is equivalent to microgal in gravity space. Therefore, one can see the altimetry data as a relatively accurate source of information for gravity applications. <br />With satellite altimetry observations at the sea area and accurate gravity data on the islands, we can define a gravimetric-altimery boundary value problem. Geometry of the oceanic part of the Earth’surface is given by the altimetric data. Ergo the problem at the oceanic part is a fixed boundary value problem. At the continental part, now, GPS is operable. The availability of the GPS coordinates means the geometry of the continental part can be considered as known. Ergo, we deal with a fixed gravimetric-altimetry boundary value problem. <br />By applying variational techniques to the fixed gravimetric-altimetry boundary <br />value problem the existence and uniqueness of its weak solution can be proved (Keller, 1996). <br />In this paper, using satellite altimetry observations on the open sea and gravity <br />from gravimetry on the island, a fixed gravimetric-altimetry boundary value problem <br />for geoid computations at islands has been developed. The problem is defined as <br />follows: <br /> <br />Where is the gravity potential of the Earth, the norm of the gravity vector on the island, geoid from satellite altimetry observations, mass density, the angular velocity and geoid potential. <br />The first step towards the solution of the proposed fixed-free two-boundary value problem is the linearization of the problem. After linearization we obtained an oblique boundary condition on the island and a Dirichlet condition on the sea area. <br />The algorithmic steps of the solution of the fixed garvimetric-altimetry boundary value problem for geoid computations at islands are as follows: <br />- Application of the ellipsoidal harmonic expansion complete up to degree and order 360 and of the ellipsoidal centrifugal field for removal of the effect of the global gravity from gravity intensity at the surface of the island. <br />- The removal from the gravity intensity at the surface of the Earth the effect of residual masses at the radius of up to 55 km from the computational point. <br />- Derivation marine geoid from satellite altimetry data. <br />- Application of the ellipsoidal harmonic expansion complete up to degree and order 360 and of ellipsoidal centrifugal field for removal of from the geoidal undulations derived from satellite altimetry the effect of the global gravity. <br />- The removal from geoidal undulations derived from satellite altimetry of the effect of water masses at the radius of up to 55 km from the computational point. <br />- Application of Koch and Kusche algorithm (Koch and Kusche, 2002) for derivation of disturbing gravity potential at the surface of the reference ellipsoid from residual gravity intensity and residual gravity potential of satellite altimetry data. <br />- Restoration of the removed effects on the surface of the reference ellipsoid. <br />- Application of ellipsoidal Bruns formula in order to compute geoid undulations. <br />- Computation of the geoid of Qeshm Island of Iran has successfully tested this methodology.Precise geoid determination on islands suffers from the lack of accurate gravity data on the open seas. Nowadays, sub-microgal accuracy for the land gravity observations is obtainable. But the sea gravity data which are collected via shipborne techniques, due to the measuring environment at sea area, are usually highly noisy and are contaminated with various systematic errors. <br />On the other hand, satellite altimetry has provided a new source of information for geoid determination at sea area. It should be noted that satellite altimetry has accuracy at centimeter level which reaches decimeters at coastal. Such accuracy in geometric space is equivalent to microgal in gravity space, which is equivalent to microgal in gravity space. Therefore, one can see the altimetry data as a relatively accurate source of information for gravity applications. <br />With satellite altimetry observations at the sea area and accurate gravity data on the islands, we can define a gravimetric-altimery boundary value problem. Geometry of the oceanic part of the Earth’surface is given by the altimetric data. Ergo the problem at the oceanic part is a fixed boundary value problem. At the continental part, now, GPS is operable. The availability of the GPS coordinates means the geometry of the continental part can be considered as known. Ergo, we deal with a fixed gravimetric-altimetry boundary value problem. <br />By applying variational techniques to the fixed gravimetric-altimetry boundary <br />value problem the existence and uniqueness of its weak solution can be proved (Keller, 1996). <br />In this paper, using satellite altimetry observations on the open sea and gravity <br />from gravimetry on the island, a fixed gravimetric-altimetry boundary value problem <br />for geoid computations at islands has been developed. The problem is defined as <br />follows: <br /> <br />Where is the gravity potential of the Earth, the norm of the gravity vector on the island, geoid from satellite altimetry observations, mass density, the angular velocity and geoid potential. <br />The first step towards the solution of the proposed fixed-free two-boundary value problem is the linearization of the problem. After linearization we obtained an oblique boundary condition on the island and a Dirichlet condition on the sea area. <br />The algorithmic steps of the solution of the fixed garvimetric-altimetry boundary value problem for geoid computations at islands are as follows: <br />- Application of the ellipsoidal harmonic expansion complete up to degree and order 360 and of the ellipsoidal centrifugal field for removal of the effect of the global gravity from gravity intensity at the surface of the island. <br />- The removal from the gravity intensity at the surface of the Earth the effect of residual masses at the radius of up to 55 km from the computational point. <br />- Derivation marine geoid from satellite altimetry data. <br />- Application of the ellipsoidal harmonic expansion complete up to degree and order 360 and of ellipsoidal centrifugal field for removal of from the geoidal undulations derived from satellite altimetry the effect of the global gravity. <br />- The removal from geoidal undulations derived from satellite altimetry of the effect of water masses at the radius of up to 55 km from the computational point. <br />- Application of Koch and Kusche algorithm (Koch and Kusche, 2002) for derivation of disturbing gravity potential at the surface of the reference ellipsoid from residual gravity intensity and residual gravity potential of satellite altimetry data. <br />- Restoration of the removed effects on the surface of the reference ellipsoid. <br />- Application of ellipsoidal Bruns formula in order to compute geoid undulations. <br />- Computation of the geoid of Qeshm Island of Iran has successfully tested this methodology.https://jesphys.ut.ac.ir/article_27961_d77bf3179c1f7851489a32c774272e04.pdfInstitute of Geophysics, University of TehranJournal of the Earth and Space Physics2538-371X34420090219Application of wavelet transformation in fracture detection and fracture modeling in one of the Iranian Gas fieldsApplication of wavelet transformation in fracture detection and fracture modeling in one of the Iranian Gas fields1127962FAJournal Article20171031Nowadays, by complication of production mechanisms from reservoirs and depletion of most of the reservoirs in Iran, secondary production has become more important. Regarding the nature of Iranian reservoirs and carbonate inherencies and diagenetic and tectonic effects on these reservoirs, we see heterogeneities in them which cause uncertainty in predictions and performance of these reservoirs. So, the importance of fracture study of these reservoirs would become more obvious. <br />The way in which we determine fractures in wells and in further steps, incorporate geological factors in modeling of reservoirs and fractures modeling, is the main objective of this study. In this study, we selected one of the Iranian fields and conducted our study on 10 wells of this field. <br />After selection of the wells, we found from direct and indirect information, the location of fractures through the wells. Determination of fracture location in drilled wells by using limited imagery logs or using of core data on surface conditions is a complicated and problematic work. On the other hand, because of the availability of petrophysical logs in most of the wells, we may use them as valuable information for fracture study. We chose the wavelet transformation as a helpful tool in the detection of fractures from conventional logs. <br />Finally, by the distribution of fracture frequency and the preparation of the model of this parameter and its dependant parameters (e.g. fracture porosity) this model would be ready for further studies and export to flow simulation processes.Nowadays, by complication of production mechanisms from reservoirs and depletion of most of the reservoirs in Iran, secondary production has become more important. Regarding the nature of Iranian reservoirs and carbonate inherencies and diagenetic and tectonic effects on these reservoirs, we see heterogeneities in them which cause uncertainty in predictions and performance of these reservoirs. So, the importance of fracture study of these reservoirs would become more obvious. <br />The way in which we determine fractures in wells and in further steps, incorporate geological factors in modeling of reservoirs and fractures modeling, is the main objective of this study. In this study, we selected one of the Iranian fields and conducted our study on 10 wells of this field. <br />After selection of the wells, we found from direct and indirect information, the location of fractures through the wells. Determination of fracture location in drilled wells by using limited imagery logs or using of core data on surface conditions is a complicated and problematic work. On the other hand, because of the availability of petrophysical logs in most of the wells, we may use them as valuable information for fracture study. We chose the wavelet transformation as a helpful tool in the detection of fractures from conventional logs. <br />Finally, by the distribution of fracture frequency and the preparation of the model of this parameter and its dependant parameters (e.g. fracture porosity) this model would be ready for further studies and export to flow simulation processes.https://jesphys.ut.ac.ir/article_27962_f3e4be0a6ea0de5433d824bff15fdefe.pdfInstitute of Geophysics, University of TehranJournal of the Earth and Space Physics2538-371X34420090219Comparing congruency robust method and L1 norm minimization in micro geodesy networksComparing congruency robust method and L1 norm minimization in micro geodesy networks1127963FAJournal Article20171031For calculation of the displacements of points in micro geodesy networks, it is essential to discover stable and unstable points. Without knowing stable points, calculated displacements are due to datum deficiency. In this case, calculated displacements are not valid. There are two methods to discover stable and unstable points: <br />a-Congruency robust method <br />b- L1 Norm minimization <br />In this study the two mentioned methods are compared and the advantages and disadvantages of both are studied. For this reason, the two methods are programmed and several networks tested by them. The results of comparing these two methods appear below: <br /> <br />1- The two methods similarly detect all the points moved eighteen percent. L1 norm minimization results are better than the congruency robust method by seventy four percent in detecting points moved. On the other hand, the congruency robust method detects moved points better than the other method by eight percent. <br />2- In the networks whose displacements of points are about a few millimeters, L1 norm minimization detects moved points much better than the other method. Some of the samples are available in the tables below. These two methods discover all points when the displacements of moved points are a few centimeters and both methods are reliable. Thus, either L1 norm minimization or congruency robust method can be used in order to detect moved points. <br />3- The congruency robust method is not reliable when all points or all points except one or two are moved because it cannot find all moved points in this situation. On the contrary, all points are detected by the L1 norm minimization method. Neither the norm minimization nor the congruency robust method could find moved points when we have all points moved. Generally, if we have at least two unmoved points in the network, the results are reliable. In spit of this, deformation tensors should be applied. <br />4- The algorithm of norm minimization is simpler and its programming is easier than that of congruency robust method. <br />In order to discover moved and unmoved points in the network, the study suggests that the norm minimization method should be applied. Of course it is proposed that both methods be considered and the unmoved points obtained from them considered altogether as stable points. Moved points that are erroneously detected as unmoved points are discovered by a statistical test applied after calculating the displacements of unmoved points. These points are considered as unmoved points.For calculation of the displacements of points in micro geodesy networks, it is essential to discover stable and unstable points. Without knowing stable points, calculated displacements are due to datum deficiency. In this case, calculated displacements are not valid. There are two methods to discover stable and unstable points: <br />a-Congruency robust method <br />b- L1 Norm minimization <br />In this study the two mentioned methods are compared and the advantages and disadvantages of both are studied. For this reason, the two methods are programmed and several networks tested by them. The results of comparing these two methods appear below: <br /> <br />1- The two methods similarly detect all the points moved eighteen percent. L1 norm minimization results are better than the congruency robust method by seventy four percent in detecting points moved. On the other hand, the congruency robust method detects moved points better than the other method by eight percent. <br />2- In the networks whose displacements of points are about a few millimeters, L1 norm minimization detects moved points much better than the other method. Some of the samples are available in the tables below. These two methods discover all points when the displacements of moved points are a few centimeters and both methods are reliable. Thus, either L1 norm minimization or congruency robust method can be used in order to detect moved points. <br />3- The congruency robust method is not reliable when all points or all points except one or two are moved because it cannot find all moved points in this situation. On the contrary, all points are detected by the L1 norm minimization method. Neither the norm minimization nor the congruency robust method could find moved points when we have all points moved. Generally, if we have at least two unmoved points in the network, the results are reliable. In spit of this, deformation tensors should be applied. <br />4- The algorithm of norm minimization is simpler and its programming is easier than that of congruency robust method. <br />In order to discover moved and unmoved points in the network, the study suggests that the norm minimization method should be applied. Of course it is proposed that both methods be considered and the unmoved points obtained from them considered altogether as stable points. Moved points that are erroneously detected as unmoved points are discovered by a statistical test applied after calculating the displacements of unmoved points. These points are considered as unmoved points.https://jesphys.ut.ac.ir/article_27963_8edbd600a70657eae57466385c3e28f9.pdfInstitute of Geophysics, University of TehranJournal of the Earth and Space Physics2538-371X34420090219Field study and physical modeling of a few downbursts in the Tehran areaField study and physical modeling of a few downbursts in the Tehran area1127964FAJournal Article20171031Downbursts are small-scale, short-lived and energetic events in the atmosphere. They usually occur from the beginning of spring to summer when the weather is more humid and the convective weather conditions are suitable. Because of the small-scale property of the downburst, it has sometimes been observed only in a meteorological station and other stations have not observed considerable changes. <br />The structure of the downburst outflow is examined by means of surface meteorological data at the Institude of Geophysics and Mehrabad Airport stations and skewt charts. Distinctive features associated with downburst are: a sudden drop in temperature, sudden increase in wind speed, wind shift, pressure rise and humidity increment. Strong vertical and horizontal shears are also observed according to sodar data. <br />In this study the characteristics of 12 outflow samples between 2003 and 2005 using surface meteorological data, and 5 outflow samples using skewt charts and one downburst sample using sodar data, are considered. Using sodar data for the downburst, the horizontal and vertical speed and wind direction relatiue to altitude have been analysed before and after the downburst occurauce. Using these profiles, and changes and maximums have been calculated. Skewt charts have been used for 5 instances, the temperature laps rate in the environment and buoyancy force have been calculated for each one, and compared to other similar instances worldwide. Downbursts have occurred when laps rates of temperature in the environment had been more than 9 (around dry adiabatic lapse rate). When the temperature laps rate in the environment was closer to the dry adiabatic lapse rate, the downdrafts were found to be more powerful and is independent from humidity mixing ratio. <br />The best downburst detectors are Doppler radars and lidars. Because of the unavailability of such instruments a physical model of downburst was setup in the laboratory. Vertical release of a known volume of dense fluid is simulated in an isotropic and in a density stratified environment. As soon as the released fluid reaches the surface, it forms the shape of a vortex ring and spreads out in all directions like a gravity current. Typical Froude numbers of the flow in 0.7. The structure of the flow is monitored by conductivity probes showing inhomogeneous turbulent structure. Because vertically moving flow in the atmosphere can lead to danger to flying aircraft near the ground it must be studied and monitored near the airports using fast response monitoring systems. This study may be a commencement for gathering downburst climatology data in the Tehran district.Downbursts are small-scale, short-lived and energetic events in the atmosphere. They usually occur from the beginning of spring to summer when the weather is more humid and the convective weather conditions are suitable. Because of the small-scale property of the downburst, it has sometimes been observed only in a meteorological station and other stations have not observed considerable changes. <br />The structure of the downburst outflow is examined by means of surface meteorological data at the Institude of Geophysics and Mehrabad Airport stations and skewt charts. Distinctive features associated with downburst are: a sudden drop in temperature, sudden increase in wind speed, wind shift, pressure rise and humidity increment. Strong vertical and horizontal shears are also observed according to sodar data. <br />In this study the characteristics of 12 outflow samples between 2003 and 2005 using surface meteorological data, and 5 outflow samples using skewt charts and one downburst sample using sodar data, are considered. Using sodar data for the downburst, the horizontal and vertical speed and wind direction relatiue to altitude have been analysed before and after the downburst occurauce. Using these profiles, and changes and maximums have been calculated. Skewt charts have been used for 5 instances, the temperature laps rate in the environment and buoyancy force have been calculated for each one, and compared to other similar instances worldwide. Downbursts have occurred when laps rates of temperature in the environment had been more than 9 (around dry adiabatic lapse rate). When the temperature laps rate in the environment was closer to the dry adiabatic lapse rate, the downdrafts were found to be more powerful and is independent from humidity mixing ratio. <br />The best downburst detectors are Doppler radars and lidars. Because of the unavailability of such instruments a physical model of downburst was setup in the laboratory. Vertical release of a known volume of dense fluid is simulated in an isotropic and in a density stratified environment. As soon as the released fluid reaches the surface, it forms the shape of a vortex ring and spreads out in all directions like a gravity current. Typical Froude numbers of the flow in 0.7. The structure of the flow is monitored by conductivity probes showing inhomogeneous turbulent structure. Because vertically moving flow in the atmosphere can lead to danger to flying aircraft near the ground it must be studied and monitored near the airports using fast response monitoring systems. This study may be a commencement for gathering downburst climatology data in the Tehran district.https://jesphys.ut.ac.ir/article_27964_bc222a4bf89e8bad2d42a67af60861cc.pdfInstitute of Geophysics, University of TehranJournal of the Earth and Space Physics2538-371X34420090219Simulation of snow storm of 6-12 February 2005 in Tehran with Advance Regional Prediction System (ARPS)Simulation of snow storm of 6-12 February 2005 in Tehran with Advance Regional Prediction System (ARPS)1127965FAJournal Article20171031During February of 2005 the north of Iran witnessed the development and activity of a variety of severe meteorological systems accompanied with unusual heavy precipitation, mostly snow and hail. The rate and amount of snow in the northern coastal region was so heavy that it made a new record for that region with a report of more than 3m of snow in Rasht, a city located in that area. Also the damage it caused in the area was great, with heavy human and economic losses. In this region with a dominant subtropical and sub-Saharan climate such an event is very rare and the yearly average of precipitation is much less than the global average, therefore, occurrence of such systems should be treated as a very motivating case for meteorologist and atmospheric researchers. The rate of precipitation is not the only factor that makes it a particular case but also the duration (more than 2 weeks) and type of precipitation (frequent snow accompanied with hail-storms) was unusual. The domain of activity of these systems is also a challenge, covering one third of the north of Iran and the east of Turkey and the west of Afghanistan, a domain in the Middle East where the prediction of atmospheric events is highly risky because of its topological existence factors influencing weather and precipitation. This system was activating in the area from Feb. 5-12 and because of its intense precipitation was chosen as the case to be simulated. <br />Our purpose in carrying out this study is to explore through sensitivity experiments, the possible success of the applied numerical forecasting model. The results of these experiments are of interest for what they reveal both the specific storm and the predictability of precipitation within storms. Predictability of large scale meteorological parameters, especially the precipitation, is limited to within 2 weeks, but for mesoscale (where precipitation is one of the most interesting and most challenging parameter for a model) prediction, it is not clear how long and how successful it will be. These questions might be asked from 2 points of view, first the previous research and simulations of systems in this area (the Middle East) and second, the ability and quality of the numerical model for simulation of such systems. In this study the Advance Regional Prediction System (ARPS) model is run with horizontal resolution of 4, 10 and 30km in three phases for 6-12th Feb. 2005 and the results have been analyzed. In 30km horizontal resolution running at the surface level and 500 hPa levels have been compared with observation. Generally, in sea surface maps, the pressure simulated by the model is less than the actual one. But there is a reasonable similarity between the 2 map patterns. The simulated geopotential height in 500hPa level is in agreement with observations. For the 10km simulation, the isoprecipitation maps of the northern half of Iran have been analyzed. Generally, the precipitation patterns reveal maximum amount of precipitation on the coasts of the Caspian Sea regions in agreement with actual precipitation data. For the Tehran region the isoprecipitation pattern shows the oscillating precipitation similar to the actual precipitation data. For obtaining more accurate results and better simulation, 4km horizontal resolution has been tried and quantitatively the precipitation values have been compared in some stations in the Tehran area. This comparison revealed that the precipitation values in most cases are more than the actual ones. In addition the comparison of thermodynamical graphs in Tehran, Mehrabad airport indicates the relative success of the model in simulating vertical profile of the atmosphere.During February of 2005 the north of Iran witnessed the development and activity of a variety of severe meteorological systems accompanied with unusual heavy precipitation, mostly snow and hail. The rate and amount of snow in the northern coastal region was so heavy that it made a new record for that region with a report of more than 3m of snow in Rasht, a city located in that area. Also the damage it caused in the area was great, with heavy human and economic losses. In this region with a dominant subtropical and sub-Saharan climate such an event is very rare and the yearly average of precipitation is much less than the global average, therefore, occurrence of such systems should be treated as a very motivating case for meteorologist and atmospheric researchers. The rate of precipitation is not the only factor that makes it a particular case but also the duration (more than 2 weeks) and type of precipitation (frequent snow accompanied with hail-storms) was unusual. The domain of activity of these systems is also a challenge, covering one third of the north of Iran and the east of Turkey and the west of Afghanistan, a domain in the Middle East where the prediction of atmospheric events is highly risky because of its topological existence factors influencing weather and precipitation. This system was activating in the area from Feb. 5-12 and because of its intense precipitation was chosen as the case to be simulated. <br />Our purpose in carrying out this study is to explore through sensitivity experiments, the possible success of the applied numerical forecasting model. The results of these experiments are of interest for what they reveal both the specific storm and the predictability of precipitation within storms. Predictability of large scale meteorological parameters, especially the precipitation, is limited to within 2 weeks, but for mesoscale (where precipitation is one of the most interesting and most challenging parameter for a model) prediction, it is not clear how long and how successful it will be. These questions might be asked from 2 points of view, first the previous research and simulations of systems in this area (the Middle East) and second, the ability and quality of the numerical model for simulation of such systems. In this study the Advance Regional Prediction System (ARPS) model is run with horizontal resolution of 4, 10 and 30km in three phases for 6-12th Feb. 2005 and the results have been analyzed. In 30km horizontal resolution running at the surface level and 500 hPa levels have been compared with observation. Generally, in sea surface maps, the pressure simulated by the model is less than the actual one. But there is a reasonable similarity between the 2 map patterns. The simulated geopotential height in 500hPa level is in agreement with observations. For the 10km simulation, the isoprecipitation maps of the northern half of Iran have been analyzed. Generally, the precipitation patterns reveal maximum amount of precipitation on the coasts of the Caspian Sea regions in agreement with actual precipitation data. For the Tehran region the isoprecipitation pattern shows the oscillating precipitation similar to the actual precipitation data. For obtaining more accurate results and better simulation, 4km horizontal resolution has been tried and quantitatively the precipitation values have been compared in some stations in the Tehran area. This comparison revealed that the precipitation values in most cases are more than the actual ones. In addition the comparison of thermodynamical graphs in Tehran, Mehrabad airport indicates the relative success of the model in simulating vertical profile of the atmosphere.https://jesphys.ut.ac.ir/article_27965_e33d02314c68f0413d9861511e8d95e0.pdf