One of the transient variations is named Geomagnetic bay, which looks like a map of an indentation in a coastline or Gaussian curve. This type of variation (and all variations) induces electric currents in the earth. The induced electric currents give rise in turn to an internal component of the magnetic variations observed at the surface. 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 the electrical conductivity within the earth. Using this method, the electrical conductivity of the earth can be determined for various depths according to the period of the magnetic variations. The duration of bay can be from a few minutes to a few hours. The bays generally occur on solar quiet days accompanied with Pi1 and Pi2 micropulsations. The bays are two-stage events; the early stage is the trigger bay and the second is the main bay. In this paper, a statistical study of bays recorded at Tehran Geomagnetic observatory for the period of 1971-80 is made according to the time of their occurrence, intensity and direction. We have also taken into consideration the Geomagnetic seasons. Maximum numbers of bays occur at midnight with moderate intensity and positive direction. Most of the bays take place during the Equinox and December solstice. Return currents are predominant current system. The delay time between the onsets of the two stages lie the range of 10-30 minutes. The relation between the numbers of the sunspots and the bays is inversely proportional

Geomagnetic observatories are constructed to continuously record the earth’s Geomagnetic field. The importance of such buildings has resulted in an increasing construction of Geomagnetic observatories all over the world. Although Geomagnetic observatories are sometimes unknown to most people and even some scientists, these very important data centers have been measuring the Geomagnetic field for about 500 years. The number of standard observatories is now approaching 150, but there are still very few standard Geomagnetic observatories in the Middle East. Despite the large area of Iran, Tehran Geomagnetic observatory is the only observatory in the country. It was constructed by the Institute of Geophysics in 1961. Unfortunately, in the recent decade, due to the old instruments used in this observatory and considering the expansion of Tehran city, the data obtained at this Geomagnetic observatory are subjected to many noise sources. Therefore, its data are not reliable anymore and Tehran observatory is removed from the list of world’s standard Geomagnetic observatories. In this paper, which is aimed as an informative research note, the necessity of constructing new Geomagnetic observatories in Iran is discussed from various aspects. The most important advantages of constructing a Geomagnetic observatory include: providing a data center for the correction of magnetic exploration data, prediction of magnetic storms and providing a constant monitoring of the Geomagnetic field variations as one of the probable earthquake precursors. Considering the various advantages of continuous recording of the earth’s Geomagnetic field, it is completely necessary to construct new Geomagnetic observatories in Iran. The first step to construct a standard Geomagnetic observatory that can record highly reliable data, is to select an optimum area for the construction of the observatory. From the very early observations of the Geomagnetic field, it was understood that the observatories must be far enough from natural fluctuations resulted from volcanic rocks or mineral deposits. It was also discovered that artificial noises can disturb the data recorded in a Geomagnetic observatory. However, there is still no comprehensive report discussing the various criteria to be studied while selecting a site for the construction of these observatories. This paper presents different parameters that should be carefully investigated to select optimum sites for the construction of Geomagnetic observatories. Considering the type of data that should be recorded in Geomagnetic observatories, knowledge of the criteria affecting the Geomagnetic data is critical. Various criteria such as the magnetic intensity, variations of the electrical conductivity of subsurface soils, artificial sources of Geomagnetic changes, development of cities, geology, topography, access roads, and underground possible economical deposits can affect selection of the optimum sites for the construction of Geomagnetic observatories. Keeping a proper distance away from magnetic anomalies and controlling the homogeneity of the electrical conductivity of subsurface soils fall among the most important factors to be considered at the first stage. Then, artificial sources of the Geomagnetic disturbance and urban development patterns must be carefully considered. It should be noted that finding the areas that can fully satisfy all the criteria might be impossible, but the minimum requirements should be satisfied to construct an observatory. Closure or relocation of several Geomagnetic observatories all over the world is an experience showing how carefully the site selection of these structures should be carried out. Having understood the necessity of constructing new Geomagnetic observatories in Iran, Kerman Province is considered as one of the favorable areas for the construction of an observatory. A comprehensive research is being carried out by authors to carefully acquire and interpret all the required data to find the best sites for this purpose.

Vessel traveling at sea causes hydrodynamic anomalies in sea water called Kelvin waves. Motion of conductive sea water due to Kelvin waves in natural Earth's magnetic field makes these waves visible using magnetic transducers. Geomagnetic anomaly induced by the motion of vessels may extend several kilometers and stay up long hours under certain conditions which consider this Geomagnetic anomaly as a good candidate in marine surveillance. From remote sensing point of view in this work, we proposed a method for detecting vessel Geomagnetic anomaly using an airborne magnetic sensor. Analytical formulations are derived and shown that physical properties of hydrodynamic Kelvin waves in shallow water are directly related to vessel parameters such as speed and traveling direction. Numerical simulations are used to demonstrate the applicability of our results. Vessel detection is performed in frequency domain. It is shown that the proposed detection method is noise robust and can be used to detect a vessel in a noisy ambient. Effect of vessel parameters such as velocity and traveling direction in frequency spectrum of received signal are investigated.

Wake is hydrodynamic footprint of a vessel at sea. The Earth's magnetic field makes these footprints visible using magnetic sensors. Geomagnetic anomalies induced by the motion of vessels may extend several kilometers and stay up long hours under certain conditions. Physical properties of this Geomagnetic anomaly are studied in this paper. A mathematical model is derived to simulate the anomaly in shallow waters. It is shown that the peak of Geomagnetic anomaly spectrum is located in the range of frequencies where the corresponding value of the ambient noise spectrum is less significant. In addition, it is shown that there is an optimum depth in which the Geomagnetic anomaly has a maximum. Variation of the Geomagnetic anomaly with vessel speed is studied and it is shown that higher speed vessels make stronger anomalies in Geomagnetic field.

Summary Global and regional Geomagnetic field models give the components of the Geomagnetic field as functions of place and time. Most of these models utilize polynomials or Fourier series to map the input variables to the Geomagnetic field values. The only temporal variation in these models is the long term secular variation. However, there is an increasing need amongst certain users for the models that can provide shorter term temporal variations, such as the Geomagnetic daily variation. In this research, we have constructed an empirical model of the quiet daily Geomagnetic field variation based on functional fitting...

In recent years, scientific advances in navigation systems and technological development of low-power consumption and high-precision in magnetic sensors have made researchers to realize that earth’ s magnetic field can be applied for locating purposes. Earth’ s magnetic field is applied in the navigation method where the required data from earth’ s magnetic field can be read from high accuracy magnetic sensors. It is possible to determine the location by comparing the data with the reference maps through adaption of algorithms and/or filtering. Generally, in this method of locating, the inertia system is used to determine the velocity and condition, and the magnetic navigation system represents navigational assistance. In the first step toward obtaining a magnetic locating system, a reference magnetic map must be created; so, it is required to carefully analyze the earth’ s magnetic field, the quantity, and quality of the field variations over different time and places. In this paper, the possibility of obtaining the geographical location of an observatory by extracting available data of a magnetic observatory has been investigated and, then, the effect of the displacement of geographical location on the magnitude of the earth’ s magnetic field has been examined by an experimental test. The results of simulation and data collection confirm the fact that geographic location for a variety of vehicles can be attainable just using earth’ s magnetic field data and there is no need to use any other navigation sensors.

One of the most important goals in Geomagnetic investigations is detecting local anomaly locations. Regional anomaly can be simulating as a trend surface, and local anomalies will be detected by comparison of measured data and simulated trend surface. The problem is trying to find best coefficients of trend surface model using inverse methods based on modern optimization techniques, which are faster and more accurate than common methods. The main idea of inverse method based on modern optimization approach is to search for a model, which gives its predicted values that are as close as possible to the observed ones. Extensive advances in computational techniques allowed researchers to develop new search strategies for use in optimization problems.Genetic Algorithm is one of the evolutionary optimization algorithms, based on the population of chromosomes, which is widely used in engineering optimization problems. Evolutionary algorithms are developed based on swarm intelligence and social behavior of individuals in nature. Besides, the populations in evolutionary algorithms called agents affected by neighbor agents and the best agent. At the end, optimum solution will be specified with respect to optimize objective function. In this paper, genetic algorithm is used for minimizing the differences between real and simulated data. In order to study Geomagnetic anomaly, first, forward model should be developed and then, using inverse method based on GA, regional anomaly trend surface will be simulated. The objective function is define as , where, and are positions of the field study locations that are measured by GPS and is the magnetic value of the positions. Also, A, B, C, D, E and F are unknown coefficients that will be determined using inverse method. According to the objective function, a two-dimensional equation is proposed for simulating regional anomaly trend surface. Two-dimensional equations are better than one-dimensional and three-dimensional or higher dimensional equations. One-dimensional equations do not guarantee to cover all aspects of data. Besides, three or higher dimensional equations are also not recommended for modeling data, because, over fitting to the data may be occurred. Therefore, the two-dimensional equation is the best model for simulating the regional anomaly trend surface. It is important to note that the optimization technique will usually perform well in nonlinear forward models. The unknown coefficients of trend surface on regional magnetic anomaly in Doroh area in southeast of Iran were optimized using inverse analysis, and finally the local anomalies were detected. In order to find locations of local Geomagnetic anomalies, total anomaly trend is subtracted from regional anomaly trend and then, the potential locations for drilling investigation are recognized. Our experimental results demonstrate very promising results of the optimization technique for solving inverse problems using GA for detecting local Geomagnetic anomaly trend surface, which is validating through drilling investigations. Besides, upward and reduce to pole filters and combination of them, which are common filters for detecting local Geomagnetic anomaly locations, are used for conformation our results.

Geomagnetism is one of the most applied techniques of geophysics in geology. Today, this method is applied in different disciplines such as Magnetic fabric of rocks, palaeomagnetism and environmental magnetism. Each of these magnetic methods is suitable for a particular lithology.Therefore, the potential of sandstones of Shemshak Group, in central Alborz Mountain range were examined to record their response to the application of some conventional magnetic methods. This rock unit is formed during two major tectonic events of early and middle Cimmerian and has great coverage not only in the study area but also in a vast majority of Iran. The total number of 135 oriented core samples was taken from 18 stations. In this study combination of magnetic and petrographic data are examined. The result of magnetic mineralogy analysis of all samples (except samples from stations 10 and 15) show irreversibility i.e. minerals such as hematite and magnetite, have formed during the heating stage. The results of thermal demagnetization analysis showed that ferromagnetic minerals present in the samples (except samples from stations 10, 14 and 15) have demagnetized below 400oC and by proceeding heating, samples show zigzag pattern or show abnormal increase in susceptibility. This indicates that, the magnetic minerals are formed during heating in one stage and due to unstability lose its susceptibility in other stage. The magnetic susceptibility (Km) vary from 200-400×10-6 SI for more stations of study area, indicating abundance of paramagnetic mineral in this rock unit. From petrography point of view all the sandstone samples are classified as arenites and ratio of quartz to lithic fragment and feldspar is relatively low, which may indicate low mineral maturity. Comparison of magnetic mineralogy and thermal demagnetization data for two stations S10 and S15 show that there is an inverse potential relationship between amount of stable ferromagnetic mineral in a sample and amount of its alteration during heating stage. The results of this study reveal the poor nature of Shemshak Group sandstones for palaeomagnetic studies due to their low mineral maturity and water percolation which increase the possibility of acquisition of chemical remanent magnetization (CRM). The high sedimentation rate of sandstones cause magnetic inclination and declination error, and low ferromagnetic mineral fraction in samples, make them unsuitable to record magnetic directionsduring formation of rocks. However, abundance of paramagnetic minerals such as biotite in these rocks, proves their applicability for magnetic fabric studies.

Background and Objective: Stroke has a high prevalence and is associated with a high premature mortality rate. Thus, it is important to identify the effective factors in increasing the incidence of stroke. The aim of this study was to investigate the role of seasonal changes and environmental factors in the occurrence of stroke. Methods: This descriptive-analytical study was done on 3, 639 patients including 3, 102 (85. 3%) with ischemic stroke, 472 (12. 9%) with intracerebral hemorrhage and 65 (1. 9%) with subarachnoid hemorrhage patients. After measuring environmental factors such as air temperature, air pressure, humidity and Geomagnetic field, admission data of stroke patients between 2015-2019 were extracted from the Sayyad Shirazi Hospital information system. Results: The mean occurrence of intracerebral hemorrhage was highest in winter, especially in January and March, and lowest in summer, especially in July and August. Intracerebral hemorrhage had a positive significant correlation with air pressure (P=0. 001, r=0. 452) and a negative correlation with ambient temperature (P<0. 001, r=-0. 457). Subarachnoid hemorrhage and ischemia were not related to the season or the month. It was also found that ischemic stroke had a significant positive correlation with Geomagnetic field fluctuations. Subarachnoid hemorrhage was not associated with any of the variables, but intracerebral hemorrhage was significantly positively associated with air pressure and inversely associated with temperature. Conclusion: This study illustrated that winter, low temperature, high air pressure and high Geomagnetic fluctuations are associated with a higher risk of stroke. Therefore, the underlying physiopathological causes of this relationship should be carefully investigated in future studies.