مجله ژئوفیزیک ایران
سال پانزدهم شماره 4 (پیاپی 53، Winter 2021)

The seismic ambient noise observation is widely used in site characterization studies to obtain the subsurface velocity structure because of the simplicity and low cost. Different single and array methods have been introduced in this field to study seismic ambient noise characteristics. In this paper, we investigated the effects of distributions of the seismic ambient noise sources on surface wave properties. In this regard, two datasets are simulated regarding omnidirectional and non-omnidirectional distributions of sources. In the former, it is assumed that the waves travel through the array from all even azimuths and for the latter, an arc shape distribution is considered. To process the data, the RTBF (Rayleigh three component beamforming) array signal processing technique is applied to retrieve Rayleigh and Love phase velocity dispersion curves and Rayleigh wave ellipticity. Furthermore, two well-known single station methods HVTFA (horizontal to vertical time frequency analysis) and RayDec (random decrement technique) are implemented to extract the ellipticity curves. For all the applied methods, impacts of the two considered distributions of sources are investigated. Then, the results are compared to theoretical curves which are computed from a specific earth model used to produce synthetic noise data. Moreover, a data recorded in Ramsar (north of Iran), as a typical case of non-omnidirectional distribution of noise sources, selected to justify our assertion in processing synthetic noise data. Benefitting the mentioned array and single station methods, the characteristics of surface waves are studied and the shear wave velocity profile is estimated by joint inversion of retrieved dispersion and ellipticity curves.

The works done in the study area indicate the potential of the existing fault systems in the region to cause large earthquakes, i.e. magnitude close to 7. In addition, an exact understanding of source parameters gives rise to the recognition of the physical process that occurs as a result of an earthquake. This concept is one of the most important parameters that ultimately leads to the identification of both main and auxiliary planes. The purpose of this study is to determine the mechanism of the Mw  5.7 earthquake occurred in the southeastern region of Damghan at 19:23:48 UTC (23:53:48 local time) on 27 August 2010 (as the biggest event of the last few decades in this region) and its largest aftershock. In this work, nodal planes were first obtained by using time-domain moment tensor inversion method; then, the main fault plane was determined using the geometrical method of H-C which can be implemented by obtaining a centroid moment tensor solution and earthquake focal parameters. Furthermore, the stability of the solution was checked by condition number and jackknifing test. According to the results of this research, a branch of the Toroud fault zone with left-lateral strike-slip motion and northeast-southwest strike was the causative fault of this event. Moreover, the strike, dip and rake of these two events were determined 215, 78, 8 and 222, 66, 11 degrees, respectively. Due to the fact that this area has not been significantly active before, careful monitoring and further studies in such areas is necessary to prevent serious damages.

The Gravity Recovery and Climate Experiment (GRACE) mission and its successor, GRACE Follow-On, have been observing the Earth’s static and time-variable gravity field with unprecedented accuracy from 2002, thanks to the precision equipment used, such as very accurate ranging systems, dual-frequency GPS receivers, star cameras and highly sensitive accelerometers. The accelerometers on board of these missions provide high quality measurements of the non-gravitational forces acting on the satellites, provided that they are calibrated. In this paper, a wavelet-based detrending scheme is used to estimate drift and bias of GRACE accelerometer data. This method is applied to a simulated noisy time series and two sets of GRACE accelerometer data (recorded on January 1, 2005 and during March 2015). The results confirm the speed and ease of the proposed method due to the nature of the wavelet-based detrending scheme. The estimated bias and drift parameters have acceptable accuracies because the wavelet-based method does not require any reference value and its results are not affected by uncertainties in gravitational field modeling. Furthermore, some computational problems such as the amplification of noise during the numerical differentiation of satellite positions do not exist. In addition, the accelerometer readouts, provided in the Science Reference Frame (SRF), may be calibrated without applying any coordinate transformation.

Dimensionality and distortion analysis of the measured magnetotelluric (MT) data are challenging procedures to conclude the main properties of regional geo-electric structure (e.g., its strike direction) and quantify the influence of superficial distorting bodies. In this paper, we applied several analysis methods to the same data set from southwest Zagros to determine the dimensionality and degree of the inherent distortion in the data and to recover the regional responses. Results show that the regional 2D responses can be better retrieved in a two-step procedure: First, the dimensionality and distortion are classified by impedance invariants and then, the Groom-Baily (GB) method is applied to remove distortion effects and to estimate the regional strike and principal impedances. An audio-magnetotelluric (AMT) data set acquired at 19 stations along a profile in the southwest of Simply Folded Belt (SFB) of the Zagros Mountains is investigated. The Bahr and WAL invariants confirm regional 1D and 2D structures with local galvanic distortion at most periods. The GB decomposition of MT impedance data reveals period-independent distortion parameters. The strike analysis on all data whose principal phase differences are greater than 5˚ shows a regional trend of about 30˚. 2D inductive effects were also retrieved by removing distortion effects from the measured data.

Knowing about the stress field is essential to have insight into the seismotectonics and to understand the geodynamics of any study area. Stress tensor inversion is used for obtaining more precise stress information from cluster of earthquakes. Moreover, it is shown that the proper display of earthquake focal mechanism can help to get a better understanding of the seismotectonics of the region. In this study, the maximum horizontal stress directions in Qeshm Island, southern Iran, are extracted by using single earthquake focal mechanism. The Island is very important regarding its oil and gas reservoirs, still being explored, and planned industrial and business developments. Following previous studies in the region, there is an ambiguity in the main stress orientation in the area arising from the cross-cutting directions of the earthquake focal mechanism P axes. The stress tensor inversion of the focal mechanisms related to the 2005 earthquake aftershock sequence in Qeshm Island helped to resolve the observed ambiguity and revealed the correct maximum horizontal stress direction. The stress field is then mapped by implementing a new data visualization method. According to a circular color scale, this method is based on attributing RGB specific values to each rectangular grid cells. For a correct illustration of the directions with a color scale, the opposite directions (SHmax) should be of the same color, which necessitates introducing a new color wheel. The obtained anomaly in the stress direction coincides with the geological features and also InSAR results in the study area. These findings reveal the different stress field of the central parts of the Island, which is uplifted during the 2005 earthquake. Finally, it is proved that the 2005 earthquake and its aftershock sequence perturb the stress field in Qeshm Island.

The existence of coherent and incoherent (random) noises including high-frequency electromagnetic inferences in the Ground Penetration Radar (GPR) signals is inevitable. Therefore, the elimination of noise from GPR data before performing any additional analysis is of great importance to increase the accuracy of the interpretations. We apply the Total Variation De-noising (TVD) and Savitzky-Golay (SG) filter on synthetic and real GPR datasets. For a better perception, the same trace of the data is compared after applying the mentioned methods. The results indicate that the TVD method is more effective than the common adaptive filtering in the time domain for reducing noise such as the SG filter which acts as a low-pass filter for smoothing data based on a polynomial least-squares approximation. However, due to the visibility of staircase artifacts using the TVD method, GPR data is first transferred to the Empirical Mode Decomposition (EMD) frame which is useful for non-linear and non-stationary signal processing, and then the TVD method is applied to it. Finally, noise reduction using TVD is compared in the time and EMD domains. The comparison of the outputs shows that the TVD algorithm in the EMD domain, based on the sequential extraction of the energy belonging to the different intrinsic time scales of the signal, provides better noise attenuation than the other algorithms. In addition, TVD-EMD improves the continuity in sections and preserves the event forms and signal forms.

Proper utilization of 3D seismic data in the upstream oil industry is one of the crucial steps in field developments. Many attributes are used to obtain models of facies distribution in a reservoir. Using prestack inversion methods on 3D seismic data and extracting prestack attributes such as elastic properties play very important roles in precise estimation of reservoir properties. The results of prestack seismic inversion in the oil reservoirs are investigated in this study. Simultaneous prestack inversion provides more details of acoustic impedance section rather than the poststack inversion. Moreover, the compressional acoustic impedance extracted from simultaneous inversion is more accurate than the one obtained from poststack inversion. In addition to the compressive acoustic impedance, shear impedance and density, the elastic parameters can be extracted from simultaneously prestack inversion. The purpose of this research is to transform the prestack inversion products into the geomechanical parameters including Young’s modulus and Poisson’s ratio. Young’s modulus and Poisson's ratio have been used in geophysical algorithms to detect the well drilling trajectory and estimate the stress direction. These parameters can also be used in fluid detection studies.

In this paper, a modified version of strength Pareto evolutionary algorithm SPEA (II) is used as a multi-objective optimization method in gravity data modelling. In this method, a two-dimensional gravity inversion problem is solved by iteratively random creation of forward models. It is shown that it can be used as a fast and effective inversion tool in the depth modelling of two-dimensional layer problems with applications in depth-to-basements, geometry of bedrocks and sedimentary basins modelling cases. Owing to the direct use of the regularization term as a separate objective function, smooth models have a high chance of being selected as final solutions, which makes the results more acceptable and easier to interpret. The most important advantages of this method are that it works independently of the regularization coefficient; thus, there is no need to run the algorithm so many times to find a proper regularization parameter. Furthermore, there is no need to directly deal with inverse formulations, and last but not least, by using a multi-objective algorithm as a global optimization method, convergence to a stable solution does not depend on the initial model, the way classical inversion methods do. For testing the algorithm, a synthetic model is used for layer boundary modelling and to assess the stability of this algorithm, white Gaussian noise is added to the synthetic model. To evaluate the validity of this method, real data from the Recôncavo basin in Brazil is considered for processing and inversion, and the results are compared to the ones from previous studies. All computations have been done in the GNU Octave 5.1.0 environment.

We obtained a three-dimensional (3D) shear wave velocity model of the upper crust of the Iranian Plateau, based on the inversion of fundamental mode Rayleigh wave group velocity. The surface wave group velocity measurements for the period range 5-25 s were extracted from two seismic data sets: ambient noise cross-correlations and regional earthquakes. The low shear wave velocity (Vs) anomalies of the upper crust correspond to regions of thick sediments. The surrounding basins of the Plateau, the South Caspian Basin (SCB) in the north and the Zagros Fold-Thrust Belt (ZFTB) and the Makran accretionary wedge in the south form the thickest sedimentary covers of the region exceeding 20 km. The thickest parts of inland basins such as the Jazmurian depression and the Dasht-e Kavir are ~10 km. The Vs structure of southern Zagros is almost homogeneous at all levels of the crust, but the low velocity anomaly beneath the southern Lorestan Arc separates the central Zagros from the northernmost Zagros. The volcanic belt of the Makran Subduction Zone forms another large/deep low velocity zone, where the observed low Vs possibly results from magma migration and heat transfer from the mantle wedge of the subduction zone. High velocity regions in the upper crust are observed in the Sanandaj-Sirjan Zone (SSZ), the Urmia-Dokhtar Magmatic Assemblage (UDMA), and in the south of the Lut Block.

Since many sophisticated Global Navigation Satellite System (GNSS) applications require satellite precise ephemeris (orbit and clock products), in recent years, many organizations have been responsible to provide users with information on GNSS. With the advent of these products, a positioning method known as Precise Point Positioning (PPP) was introduced. This technique is based on only using the observations of one receiver. Therefore, it is possible to determine the position using the observation of code and phase. In the last decade, due to the advent of this technique, achieving high accuracy, the need of only one receiver instead of differential observations, as well as reducing the cost of operation is possible by users for scientific and commercial research. This technique makes it possible to determine the position with a precision of centimetres to decimetres for static and kinematic applications. According to the results presented in the review of two case studies, comparison of estimated results with the actual values provides centimeters-level accuracy. For this reason, these services can be used to access the position components using PPP. In this research, the accuracy of the PPP method has been evaluated. The observations of four stations have been used globally. The collected data is processed using online services and open source software. Finally, the relevant results have been carefully examined. Based on the results presented in the first part and comparison with the final characteristics of the known points in the study area, on average, a difference of 3 mm has been reported when using 24 hours of observation. Considering the set of observations related to ten points in six-time intervals, the accuracies provided for the X, Y and Z coordinate components in the first period are 42, 31 and 20 mm, respectively and in the sixth period, the values of 9, 11 and 7 mm are reported.

This paper presents the recent advances made in Iran using the radio wave technique for ionospheric studies. The characteristics of the VLF signal transmitted from Turkey (26.7 kHz) and India (18.2 kHz) and received in Tehran are studied in detail to verify the behavior of signal and implement that for earthquake prediction and ionospheric remote sensing purposes. While several theories are proposed regarding the propagation of VLF, this paper aims to investigate the received signal along with the electron density variation and magnetic field strength along the propagation path. The observational data for 40 days is averaged over 24 hours with a 1-minute time resolution. The propagation of the VLF signal in the earth-ionosphere waveguide is studied by considering the electron density variation along the propagation path at several selected locations using the IRI (International Reference Ionosphere) model throughout the day. The IGRF (International Geomagnetic Reference Field) model is used to study the earth’s magnetic field and the associated conductivities in the ionosphere. The excitation of secondary VLF waves in the lower ionospheric E region and propagation along the maximum conductivity is compared with the reflection from the ionosphere and propagation in the earth-ionosphere waveguide as the two different propagation mechanisms for the VLF signal. A conclusion on the propagation characteristic of the signal is made and several variation characteristics of the radio signal during the day are determined.

Fréchet derivatives calculation or sensitivity matrix is an integral part of every non-linear inversion process. The sensitivity values indicate the variation of the forward response with respect to the variation of model parameters. Sensitivity patterns are also a criterion to assess the reliability of inverted models and to design optimum resistivity surveys. In this study, a numerical approach based on the forward matrix calculation in the framework of the 2.5D finite difference electrical resistivity forward modeling is presented. First, using the potential distribution in the Fourier space obtained from the forward calculation and the derivatives of the coupling coefficients with respect to the conductivity distribution, the sensitivity values in the wavenumber domain are computed. Then, these values are transformed into the space domain using an inverse Fourier technique. To verify and analyze the proposed numerical method, the sensitivity distributions assuming the homogeneous and inhomogeneous media for commonly used electrical resistivity tomography configurations (e.g. pole-pole, pole-dipole, dipole-dipole, and the Wenner arrays) are computed. The numerical experiments reveal that the sensitivity patterns vary spatially throughout the model depending not only on the resistivity distribution but also on the electrode configuration. It is also concluded that the sensitivity analysis can be used as a supplementary tool for any optimum electrical tomography survey design.