مجله ژئوفیزیک ایران
سال نهم شماره 5 (پیاپی 28، Winter 2015)

The generation mechanism of RX-mode waves in the equatorial plasmasphere has not been well understood. The Akebono passing through the storm time geomagnetic equator shows the possibility of the local enhancement of RX-mode waves in association with intense Z-mode waves in the equatorial region. We use the initial parameters inferred from observational data from around the plasmawave generation region obtained by the Akebono satellite. A comparison of linear growth-rate calculations and simulation results is presented. The results of the simulation show two strong peaks related to the Z-mode and RX-mode waves, while the separation of these wave frequencies is equal to one cyclotron frequency. It is shown that electromagnetic Z- and RX-mode waves could be coupled by a nonlinear interaction.

This paper describes the application of three straightforward image-based filtering methods to remove the geological noise effect which masks unexploded ordnances (UXOs) magnetic signals in geophysical surveys. Three image filters comprising of mean, median and Wiener are used to enhance the location of probable UXOs when they are embedded in a dominant background geological noise. The study area consists of three buried UXOs while a geological dyke structure covers the magnetic anomaly of the desired objects. To provide a better representation of the actual locations of UXOs in the observed magnetic anomaly over this area, all image-based filters could appropriately separate the geological dyke effect from the UXOs. These image filters can be good candidates to remove the geological noise effect in UXO detection when encountering a mixed response of multisource magnetic anomaly in contaminated territories with UXOs. An analytic signal map of the separated magnetic anomaly of UXOs was provided to enhance locations of the UXOs in the studied field. Also, a combination of the analytic signal and the Euler deconvolution methods were used to estimate the depth of three buried UXO targets in the study area indicating high sensitivity of the estimated parameter to the noise level.

The Advanced Microwave Sounding Unit-B (AMSU-B) installed on the NOAA-15, 16, and 17 satellites, is the new generation of a series of microwave imagers/sounders that can sense atmospheric moisture and other hydrometeors through clouds. This paper demonstrates the potential of multi-frequency AMSU-B data for classifying different types of hydrometeors. Ten types of these hydrometers have been collected using meteorological data (synoptic reports, radiosonde data and infrared and water vapor images) over Iran. Co-located AMSU-B data were used to perform a quantitative classification of the hydrometers. Three main classes including heavy precipitating, moderate precipitating, and non-precipitating hydrometeors were found based on the multi-frequency brightness temperature signatures. The distinguishing criteria for this type of analysis are: (a) brightness temperature (BT) at 89 GHz frequency, (b) slopes of the BT between 89 and 150 GHz, and (c) crossover of BT curves between 89 and 183 GHz frequencies.

Magnetotelluric (MT) is a natural electromagnetic (EM) technique which is used for geothermal, petroleum, geotechnical, groundwater and mineral exploration. MT is also routinely used for mapping of deep subsurface structures. In this method, the measured regional complex impedance tensor (Z) is substantially distorted by any topographical feature or small-scale near-surface, three-dimensional (3-D) electrical inhomogeneity. The effects of this local galvanic distortion should be separated and removed from the regional response to improve the accuracy and reliability of the data interpretation. In this paper, it is attempted to introduce an effective form of tensor analysis to facilitate the process of GB (Groom-Bailey) tensor decomposition on MT data. This approach was used to recover the regional response of conductivity structures beneath 12 MT sounding sites of a sedimentary basin in South Australia. The results of this study clearly indicate that the regional structures beneath these sites are two-dimensional (2-D) and their strike are mainly in NS (±100) direction which are geologically supported. The obtained results also show that the distortion parameters of the surficial bodies are fairly constant for the whole frequency band or its two or, at most, three subsets. In addition, the low misfit values between the measured impedances and those produced by a hypothetical 3D/2D model confirm that the regional structures beneath all these 12 MT sites are 2-D and some local surficial 3-D features are superimposed on them.

This paper describes an edge detection method based on surface-derived attributes. The surfacederived attributes are widely used in the interpretation of seismic data in two main categories: (1) derivative attributes including the dip angle and the azimuth; (2) derivative attributes including curvature attributes. In general, the magnitude of the normal curvature of a surface (curvature attributes) can be expressed in terms of derivatives of that surface which are called the first and second fundamental forms of the surface. For a quadratic surface which fits to data, it is shown that the dip-angle equation in the interpretation of the seismic data is similar to the horizontal gradient magnitude (HGM) equation in the interpretation of potential field data. Among the infinite number of curvature attributes, a few of them which are suitable for edge detection are shown. The coefficients of a quadratic surface are calculated using the least square method. At a particular point, the attributes are obtained using these coefficients. Zero contours of most positive curvature and the determinant of the curvature matrix delineate the location of the edges of anomalies. The shape index attribute quantitatively reflects the local shape of the surface over sources in terms of cap, dome, ridge, saddle, rut, trough and cup. Here, the maximum curvature is introduced as a new technique to detect the horizontal location of anomalies. First, the introduced attributes were applied to the noise-free synthetic data. Then, the data with the noise added were used to check the stability of the method. In the presence of high level noise, this method was successful in determining boundaries of the anomalies. Zero contours of the most positive curvature, the determinant of curvature matrix and the maximum curvature properly illustrate the linear features within the mapped surface. The results of using surfacederived attributes were compared with tilt angle and HGM filters. This comparison showed that zero contours of the most positive and maximum curvature attributes are approximately matched with zero contours of the tilt angle and maximum values of HGM. Finally, this method was used for real data from Mobrun massive sulfide ore of Canada. MATLAB software was used for programming and calculating the required parameters of this method.

Electromagnetic waves with different modes, such as Z-, whistler-, LO- and RX- modes are found in different regions of the Earth magnetosphere and in the magnetosphere of other planets. Since whistler-mode waves influence the behavior of the magnetosphere, and they are used as experimental tools to investigate the upper atmosphere, they are important. On the other hand, the mode conversion process can be considered as one of the processes of generating electromagnetic waves that can occur under certain conditions. Usually, propagation waves in an inhomogeneous plasma are a necessary, but not a sufficient condition for a mode conversion process. Snell’s law has an important role in the mode conversion process. Although, this law lets a mode conversion occur from Z- to LO-mode waves, in a case from Z- mode to whistler mode waves, it plays a preventive role. The aim of this paper is to demonstrate the preventive role of Snell’s law in a mode conversion from Z- to Whistler-mode waves in an inhomogeneous magnetoplasma with a low density. We used the dispersion relation in the magnetoplasma with a low density and for an oblique wave normal angle. By applying the Snell’s law, we showed that with the propagation of the Z-mode waves in an inhomogeneous plasma, there is not any matching point between Z- and Whistler mode waves, and for any wave normal angle always an evanescent layer exists between the two modes. In this case, Snell’s law prevents the mode conversion from occurring. It also prevents the transfer of energy from one to another mode waves.

On August 11, 2012, within several minutes, two shallow destructive earthquakes with moment magnitudes of 6.5 and 6.4 occurred in Varzagan, Azerbaijan-e-Sharghi Province, in norhwest of Iran. In this study, the Empirical Green Function (EGF) method was used for strong ground motion simulation to estimate the source parameters and rupture characteristics of the earthquakes. To simulate the first earthquake, two aftershocks with magnitudes of 5.6 and 5.2 were used as the EGFs. In the second event, an aftershock with a magnitude of 5 was used as the small event. The size of the main fault caused by the first event was about 18 km in length and 10 km in width. Also, the size of the asperity in the second earthquake was about 16 km in the strike direction and 11 km in the dip direction. The durations of the ruptures in the first and second events were more than 9 and 10s, respectively. The estimated fault plane solution showed strike-slip faulting for the first earthquake and a reverse mechanism with a strike-slip component for the second one. Strike, dip and rake of a causative fault of the first and second earthquakes were determined as 270, 81 and 175 degrees and 230, 57 and 134 degrees, respectively. In addition, the stress drop in the first and second events was calculated to be about 22 and 34 bar, respectively.

The Total Electron Content (TEC) measured by the Global Positioning System (GPS) is useful for registering the pre-earthquake ionospheric anomalies appearing before a large earthquake. In this paper the TEC value was predicted using the singular value thresholding (SVT) method. Also, the anomaly is detected utilizing this predicted value and the definition of the threshold value, leading to the use of the anomaly as a precursor. The SVT is used in the matrix completion problem, namely the accurate recovery of a matrix from a nearly minimal set of entries. In this study, the SVT has been applied to the ionospheric TEC of the global ionosphere maps (GIM) data on a powerful earthquake in Tohoku on the 11th of March in 2011. In this method, the two-hour TEC observations of this region are converted into a matrix for several consecutive days before and after the occurrence of an earthquake. In this matrix the rows and the columns represent the days and the sequential hours, respectively. The prediction of the non-linear time series is formulated as a method for solving the low-rank recovery problem. Results indicate that under suitable conditions the TEC values can be estimated properly in the aforementioned days and hours by solving a simple optimization problem. In order to show the efficiency of this method in predicting the time series, the results obtained from this research were compared with those from other researches.

Buried channels are one of the stratigraphic hydrocarbon traps. They are often filled with a variety of porous and permeable sediments so they are important in exploration of oil and gas reservoirs. In reflection seismic data, high-frequency components are sensitive to the channel thickness, whereas, low-frequency components are sensitive to the channel infill materials. Therefore, decomposition of seismic data to its spectral components provides useful information about both thickness and infill materials of buried channels. A 4D spectral data is produced by applying spectral decomposition to a 3D seismic data cube which is decomposed into several single frequency 3D cubes. Since different frequencies carry different types of information, each single frequency cube cannot show all subsurface information simultaneously. Therefore, we used color stacking method and constructed RGB plots, which represent more information than single frequency volumes. In this paper, we applied three methods of Deconvolutive Short Time Fourier Transform (DSTFT), S Transform (ST) and Short Time Fourier Transform (STFT) to a land seismic data from an oil field in south-west of Iran. We used the resulting spectral volumes to create RGB color stacking plots for tracing buried channels. According to the results, the RGB plots based on the DSTFT method revealed more details than the ST and STFT methods.