Journal of the Earth and Space Physics
Year:2000 | Volume:26 | Issue:1|Papers Count:13

Presence of a conductive body in a magnetic field causes inductive currents to flow in the body. The secondary magnetic field of these currents adds to the primary magnetic field. The receiver then responds to the resultant of the arriving primary and secondary fields, so that the response differs in both phase and amplitude. In the VLF method the primary electromagnetic field has frequency of 5 to 30 KHz, and is received as a plane wave at the measuring point. In this paper we are using the VLF method in the area of a copper mine (Ghaleh Zary copper mine) located 180 km south of the city of Birjand. Measurements were carried out along 21 east-west profiles and the usual methods were used in the interpretation of data. The VLF results agree well with both geological information and the results of IP measurements.

The susceptibility and magnetic anisotropy of the volcanic rocks of Damavand volcano lacated in NE of Tehran. Iran and their relationships with petrofabric, geology and strain have been studied. For this purpose the susceptibility of several samples from six sites of the north Emamzadeh Ali stream which is located in NE of Tehran was measured and then magnetic fabric was computed. Measurement of susceptibility and computation of its anisotropy have been done on the basis of second order tensor which was defined for susceptibility ellipsoid and its anisotropy. The dominant strain of the region is determined by considering the correspondence of the susceptibility ellipsoid with the strain ellipsoid. The direct estimating methods of strain ellipsoid are very hard comparing to this method. Other advantage of the method used in this study is to correlate the geology and petrology maps. The results show a number of linear relationships between various parameters of the magnetic fabric.

This study presents the results of seismic hazard mapping and ground motion assessment for central Alborz region in Iran. The results have been based on historical seismicity, recurrence statistics and probabilistic ground motion analysis. The seismicity parameters were calculated by Weichert Maximum Likelihood Method. Seismic hazard was assessed as maps for 10% and 64% probability of exceedance in 50 years. The estimated values for peak ground acceleration were 0.53g and 0.19 g for the above probability levels respectively in the region 48.5 E to 54.5 E and 34.5 N to 37.5 N. Because of low amount of data and uncertainty of location, time and magnitude of the historical events, estimated value also contains uncertainty. The results of this study show importance of estimating slip rate of faults for more accurate studies in the future.

The cut-off low is a phenomenon which our country often faces with in winter. So, it is attempted here to introducte its synoptic and dynamic aspects from barotropic point of view, hoping to present a better view of this atmospheric phenomenon. The case studied here occurred from 20th of February to 1st of March, 1997, with a life cycle of ten days. The aim of this work is to present a model of the phenomenon and its meteorological effects through careful studies of a number of dynamic parameters, such as relative, absolute and potential vorticites and also vertical velocity. The fundamental part of the work is to produce clear self-descripitive plots of the computed parameters which are related to the observed history of the cut-off low, the following conclusions may be inferred. The case studied here is a polar cut-off low with a very slow motion. The maximum activity of the system is when its spatial axis settels along the north-west to south-east direction. It is this stage of development that system is associated with strong southwesterly winds together with more heavy precipitation. The absolute vorticity solenoids, when formed in the south-west region of trough-line cause the system to be developed with rapid cyclonic rotation. The maximum vertical velocity is about 3cm/s at 500 hPa level according to equivalent model, which decreases in the upper and lower levels. After 6 days from appearance of the system, the barotropic condition becomes dominant (i. e. the spatial maximum gradient of the contour lines.

Thermal conductivity of rocks for accurate estimation of geothermal heat flux is important. Here we introduce the design and construction of a newly built system based on the principle of steady state heat conduction using divided bars method. The system is computer controlled and has been calibrated using standard quartz samples. Typical error in the system is about one percent which is mainly due to the way the sample is positioned in the system. For proper positioning, one must use a conductive greace to generate good contacts between the sample and the rods. Measurements of a few rock samples from Iran, give thermal conductivities which are within the reported values by others. The geothermal heat flux based on these measurements and the geothermal gradients from different parts of Iran, show that its mean value varies between 45 and 70 mW/m2, for these regions. Variations in geothermal gradient along the depth of same wells may also indicate the variations in rock with different thermal conductivities.

Institute of Geophysics has established a digital telemetric seismic network in Tehran region since 1997. Wealth of data from recorded local earthquakes create an opportunity to image crust and upper mantle in the region. We proposed to study velocity and interface structure of the crust and upper mantle in the Tehran region using local earthquakes. As a first step we obtained a 1-D velocity model by analysis of the travel-time curves for nearly 20000 P-wave arrival times from 2900 local earthquakes. Recorded earthquakes were classified along the five separate profiles, and corresponding travel-time curves were analyzed. In addition, a 3-D ray tracing forward modeling code was written in order to refine model parameters. The results indicate a two-layer crust in the study area. Compressional wave velocity of the first layer is 6.05 km/s and that of second layer is 7.01 km/s. Depths for Conrad and Moho discontinuities were obtained 16 and 46 km respectively, which well correlates with the results of the previous gravimetric crustal studies. Since, it was difficult to pick arrival time of the direct shear waves, we could not determined the shear wave velocity for the first layer in the crust, but it found to be 3.90 and 5.20 km/s for the second layer of the crust and the Moho, respectively.

Laboratory experiments have a specific significance in mining exploration to determine electrical properties of rock specially in the intended parameters in IP procedure. For the time being, IP geophysical procedure receives a specific significance in metallic ore deposits exploration. However, separation of IP anomalies of massive from disseminated mineralization according to the measured IP amounts with common procedures isn't possible. This research has been done on measurement of polarizability in the Taknar copper ore deposits. on the basis of laboratory experiment, polarizability has been measured in three normal directions (A, B, C), and then the mean apparent polarizability in different time after cut off has been calculated. As a result of this operations, a model is presented on which by measuring apparent polarizability and then their mean in two early and late time after cut off (e. g. one 0.5 and the other 17 seconds after electricity cut off and calculation of η̅17/η̅0.5 ratio) We can separate the anomalies of the mineralizations. The ratio specimens in massive mineralization are dramatically larger than disseminated mineralization. This result is used in mining exploration. It's necessary to state that the mean for apparent polarizability in 0.5 second after electricity cut off for massive and disseminated mineralization have a relatively high and approximately near each other amounts and so if only 11 0.5 is measured, separation. Between the apparent polarizability anomalies from the mineralization point of view is not possible.

Compressional and shear wave velocities have been measured in three orthogonal directions in cubic samples of three groups of rocks in both dry and water-saturated states under low axial stress. The three groups of rocks consist of seven granite samples taken from Bandarabbas, six sandstone samples from Chabahar and fifteen andesite samples from Oromieh. Thomson's three dimensionless anisotropy parameters ε, γ and 8 have been calculated for each sample. The results show that for all three groups of rocks the assumption of elliptical anisotropy is justified. Also it is observed that for all of the rocks the elastic constants, Young's moduli, Poisson's ratios and bulk moduli in saturated samples are greater than corresponding values in dry samples.

Ice initiation in clouds, a most important field of meteorological research during the last 50 years, has been of special interest to cloud modification scientists. However, despite many success in this field there remains numerous ambiguities in the ice nucleation process. One of the most important sources of ice production within clouds is Hallett-Mossop (H-M) ice multiplication process. In this research work, The H-M process was analyzed at a relatively constant rime surface temperature (Ts≈-5°C) and liquid water content (LWC≈0.93 gm-3) over a high range of up draught velocities than before (from 1.5 to 12 ms-1) with a rotating rod appratus with radii of either 15 or 25 cm. Experiment detected a local maximum of about 35 to 40 splinters per mg of rime close to 3 ms-I. In addition, for the first time it was detected that there is a maximum rate of 60 to 70 splinters per mg of rime at updraught velocity of 6 ms-1, possibly due to a combination of rime texture change from feathery to kernel and the ice shell-fracture process. It was found that the radial acceleration at constant updraught velocity did not have significant effects on splinter production.

We present numerical solution of the equations of motion of the driven spring-block model with stick-slip dynamics. The system consists of an array of N masses which describes the statistical and dynamical properties of dissipative systems. Here, we mostly emphasise the very low and very high velocity regimes. We show that there is a clear shift in distribution of the slip-force when we move from low velocity regime to intermediate and then to high velocity regime. This changing behaviour is dependent to the behaviour of S type in intermediate velocity and to Inverse-Gaussian distribution in high values of velocity.

In shallow seismic survey source generated coherent noises have a wide range of spectral overlap with seismic signals. Consequently, conventional band-pass filtering could not suppress them. Their elimination requires the implementation of some special techniques that may not commonly used for this purpose. The K-L decomposition technique treats each trace as a data vector, and computes a set of uncorrelated principal component traces from an eigenvalue decomposition of the matrix of zerlag cross-covariance of the given multi-trace input data set. The principal component traces are arranged in order of decreasing energy content, i. e. the signal with the largest variance will appear as first principal component and so on. Subsequently, the input record is reconstructed utilizing only the information contained in a specified selection of the principal component traces, those associated with large eigenvalues. This amounts to reconstruction of the coherent energy present in the input data set. In this study the K-L transformation technique was applied on 96-trace shot records of 3-D seismic survey flattened on the air wave, then the first five of the 96 principal components were used to estimate the air wave components of the records. The technique was found an effective way of eliminating air waves from engineering-scale 3-D seismic shot records. In shallow seismic survey source generated coherent noises have a wide range of spectral overlap with seismic signals. Consequently, conventional band-pass filtering could not suppress them. Their elimination requires the implementation of some special techniques that may not commonly used for this purpose. The K-L decomposition technique treats each trace as a data vector, and computes a set of uncorrelated principal component traces from an eigenvalue decomposition of the matrix of zerlag cross-covariance of the given multi-trace input data set. The principal component traces are arranged in order of decreasing energy content, i. e. the signal with the largest variance will appear as first principal component and so on. Subsequently, the input record is reconstructed utilizing only the information contained in a specified selection of the principal component traces, those associated with large eigenvalues. This amounts to reconstruction of the coherent energy present in the input data set. In this study the K-L transformation technique was applied on 96-trace shot records of 3-D seismic survey flattened on the air wave, then the first five of the 96 principal components were used to estimate the air wave components of the records. The technique was found an effective way of eliminating air waves from engineering-scale 3-D seismic shot records.