جستجوی مقالات مرتبط با کلیدواژه "anisotropy" در نشریات گروه "زمین شناسی"

Brittleness index is one of the important parameters in rock engineering, which affects the drillability of the rocks in various geotechnical projects such as tunnels, caves, dams, oil and gas wells, road trenches, and building stone quarries. Investigating the drillability of the rocks before drilling operations can significantly help planning and managing the implementation of geotechnical projects, which will save time and cost of exploration operations. In this paper, the drillability of two laminated sandstone samples (with various lamination spacing) has been investigated based on the brittleness index, considering the orientation and spacing of lamination. For this purpose, after preparing cylindrical cores, uniaxial compressive strength and Brazilian tensile strength of sandstone samples at lamination angles (β) of 0°, 30°, 45°, 60°, and 90° were determined. Next, based on the results of these strengths, the brittleness index values were calculated in the various βs. Based on the data analysis, lamination orientation plays a significant role in the drillability, so that the easiest and hardest drilling conditions are obtained at β of 30° and 90°, respectively. Also, the results indicate that the sandstone sample with lesser lamination spacing had more suitable conditions in terms of drillability compared to other sandstone samples. In the end, the findings of the present research can be useful and efficient in practice for geotechnical projects that require drilling in laminated rocks.

Seismic velocity changes and shear wave anisotropy analysis can provide insights into deep structures. In multi-layered structures, cosine moveout pattern of radial component is observed for converted Ps phase. The method to calculate anisotropy parameters in layered structures has been developed by Ruempker et al. (2014). Some seismic stations do not have enough coverage in some azimuths, and we can split parameters (φ, δt) by fitting the best curve on Ps arrivals using the grid-search method. In this study, using synthetic data, the effect of the azimuth coverage is investigated and the sensibility of the results is also examined.

The utilize of Kaiser effect during acoustic emission testing is a novel approach for computing the amount of stress in the rock, which has been tremendously exploited due to the immense reduction in the cost and time of determining the in-situ stress. The Felicity ratio is a precise and appropriate criterion for assessing the accuracy in the evaluation of the Kaiser effect. Numerous factors influence the accuracy of the Kaiser effect. This paper investigates the effect of anisotropy on the Kaiser effect on the rock. For this purpose, acoustic emission effect in the unconfined compression test on the Phyllites specimens with various angles of anisotropy (0, 30, 60, and 90°) has been conducted. The results demonstrate that in all unconfined compression tests, the Felicity ratio of acoustic emission is close to one. In other words, the anisotropy does not have a significant effect on the values of the Felicity ratio. In addition, the results show that the unconfined compression tests are more accurate in determining the Kaiser effect.

This study deal with the effect of anisotropy on strength behavior and failure patterns of Borujerd area slates under Brazilian test conditions. For this purpose, 6 rock blocks were taken from different locations in north and east of Borujerd. After sample preparation and assessment of mineralogical and physical properties, the Brazilian tensile strength test was performed on the disc-shaped samples in 7 anisotropy angles (between the loading direction and anisotropy plane) including 0, 15, 30, 45, 60, 75 and 90°. The results indicate that the maximum BTS is observed at β = 90° for all samples. While, the lowest value of BTS corresponds to anisotropy angles (β) equal to 15° and 30°. The observation of failure patterns of samples after Brazilian test shows that there are three types of dominant failure patterns including parallel to the anisotropy (PA), across the anisotropy (AA) and curved fracture (CF). Transitional angle, which indicates the change in dominant failure pattern from parallel to the anisotropy to across the anisotropy, were also determined. Finally, the relationship between fractures length and Brazilian tensile strength was determined for selected slates.

Rock anisotropy plays an important role in engineering behavior of rocks. Slates are anisotropic rocks which have long been used for gable roof, floor tiles, borrow materials, and other purposes. The slates studied in this research are calcareous and have a porphyro-lepidoblastic texture. To determine the role of the anisotropy on the tensile strength and fracture pattern, two variables including ψ (the core axis angle to foliation) and β (the angle between the axis of loading and foliation) in the Brazilian tests were used. The angles were selected at 15° intervals. Thus, for both ψ and β, seven angles of 0˚, 15˚, 30˚, 45˚, 60˚, 75˚, and 90˚ were selected (i.e., there are 43 possible modes). In order to name and examine the failure pattern, 11 models were proposed. The average value of the failure strength for the three stations varies from 3.21 MPa to 20.94 MPa. Based on the obtained results, there is a direct relation between the average tensile strength and density. A comparison between Brazilian test data under dry and saturation conditions shows that the saturated Brazilian tensile strength is 30.8% less than the dry Brazilian tensile strength. Moreover, the changes in fracture length with the changes in ψ and β indicate an inverse relation. Eventually, the average of tensile strength (σt) and strength anisotropy index (Ia) demonstrates that the influence of orientation angle (ψ) is much larger than that of foliation-loading angle (β).

Various interpolation and estimation tools are used to spatially model a regional variable across an area or site. This paper presents a new interpolation method, using the progressive radial basis function network and taking into account the spatial coordinates of the input data. The procedure starts with the study of the spatial structure and anisotropy of the data, to perform interpolation and determining the radiuses and rotation angles based on the directional variography. Next, the neighborhood radius and neighboring points of each node of hidden unit are determined, using the ellipsoidal anisotropy and the covariance matrix. Then, a shape factor is computed based on half the average distance of all the neighboring sample points. The progressive kernel matrix includes the corrected kernel functions and the coordinates of the nodes in the hidden units utilized to solve the weight matrix. The interpolation was finally performed at each point of regular network (unsampled points). The steps of this interpolation algorithm were evaluated by a synthetic data set, having an irregular 3D pattern. The Cross validation between actual and estimated values have a correlation coefficient of about 0.78 and the fitted line passing through the actual and estimated values is close to 45 degrees.

Studying anisotropy properties is a proper procedure to survey the rate of tectonic phenomena in the upper crust and mantle. Therefore, analyzing specific phases of the shear wave, which is intensely sensitive to both the earthquake mechanism and any anisotropy along the path in lithosphere and upper astonosphere, is worthwhile. Consequently, the aim was to determine the relation between the strain and anisotropy, calculation of anisotropic parameters around the world, and determining how much formed by past and present lithospheric deformation and how much by crustal and asthenospheric sources.Most of the researchers consider the anisotropy in the upper crust (10-15 km) as a result of micro crack orientations parallel to minimum main stress. Seismic anisotropy is the variation of seismic wave speed with direction. Whenever a shear wave reaches the anisotropic media, it splits into two directions called fast and slow directions. Parameters which describe the seismic anisotropy are the directions of the polarization of the fast shear wave (φ) and the splitting time between the fast and slow component of shear waves (δt). Two techniques are more commonly used to calculate the anisotropy parameters. The first one is based on the calculation of the cross correlation coefficient of the two horizontal S wave components (Bowman and Ando, 1987). The second one is based on the diagonalization of the covariance matrix of the S waves (Silver and Chan, 1991). Silver and Chan (1991) demonstrated that these two techniques were theoretically equivalent. Zhang and Schwartz (1994) found that the same results were inferred from the application of the two techniques to crustal earthquakes. The methodology in this research is based on the diagonalization of the covariance matrix of the S waves (Silver and Chan, 1991), which has been automated by Teanby et al. (2004). First, a shear-wave analysis window is defined. If anisotropy is present, the particle motion within this window will be elliptical. Second, a grid search over φ and δt is performed. The result that has the lowest second eigenvalue of the corrected particlemotion covariance matrix indicates linear particle motion after correction and is the solution that best corrects for the splitting.In sum, the method consists of three steps: First φ and δt are calculated for a range of start and end times, and a 2D diagram of φ versus δt is plotted. Second, a stable region with tight clusters or desired compaction is specialized by a cluster analysis. Finally, the optimized clusters are applied and the window with the least error in the evaluation of φ and δt is determined (Teanby et al., 2004). In order to select accurate wave forms, an important factor must be taken into consideration: The arrival angle of the S waves must be less than the critical angle. Because whenever S wave hits the surface at an angle (i) greater than the critical angle (ic= sin-1(Vp / Vs)), it can be recorded at the surface as elliptically polarized (Nuttli,1961; Booth and Crampin, 1985). Therefore, the arrival angel of S waves must be less than 35º (i < ic). We applied a casual band pass filter in the frequency range 1-15 Hz to the waveforms to avoid the effects of long period surface waves.This research is based on analyzing shear wave splitting by use of Sg phase to calculate the anisotropy at the upper crust in Mohammadabad Rigan (Southeast of Iran). Data were provided by the temporary seismic network, assembled by Iranian Seismological Center. By analyzing 654 waveforms, we observed an anisotropy at each station and evaluated the direction of the fast component of shear wave in N45 ± 9 °E. The calculated splitting time was 0.18 ± 0.004 (s) on average, which was in accordance with the splitting time in the upper crust. The direction was not consistent with the act of known faults in this region. Therefore, it showed a new fault trend considering the locations of micro earthquakes and the focal mechanism of the main event.

Discontinuities have properties such as orientation، number of set and frequency that can affect the rock strength. Rock specimens having one، two and three cross- sets of discontinuities، various frequencies and orientations of 0، 30، 45، 60 and 90 degrees were prepared. The numbers of rock pieces increased progressively with an increase of frequency and set of discontinuities. As specimens having three sets of discontinuities that one of their sets had four number of parallel discontinuities were consisted 20 rock pieces and they represented jointed rock mass. Joint factor، uniaxial compressive strength and friction angle along the discontinuity surface in direct shear were determined. The uniaxial compressive strength of specimens having one، two and three sets of discontinuities in horizontal and vertical direction was less than the uniaxial compressive strength of intact rock. The uniaxial compressive strength of specimens approached approximately to zero value particularly when the orientation of discontinuities was 60 degrees. This considerable decrease of strength was occurred also for specimens having two and three sets of discontinuities at orientation of 30 degrees. The analysis of results showed that the relationship between ratio of uniaxial compressive strength of jointed specimens to the uniaxial compressive strength of intact rock specimens (anisotropy factor) and joint factor of this research is considerably different with the suggested relationship by Ramamurthy. Properties of discontinuities have altogether essential role on the strength of rock mass.

Zoning is an important practice in earth sciences. In zonation, the study area is divided into separate parts and by compiling the results of these parts, a unique model is obtained. In this study, clustering methods are applied for zoning of Semilan dam site. Optimal number of clusters are measured based on geotechnical parameters (lugeon, RQD), the importance of various dam structures and lithology indicators. By ranking of 7 clustering validity indexes, the optimum number of clusters found to be 4. In this paper, clustering was performed by faults locations and self-organizing neural network. In the former case, the study area was divided into four zones based on faults. This two dimensional zoning is independed of the third dimension (depth) and each sample belonged to a cluster. In the later case, a self-organizing map (SOM), which is a kind of neural network capable of clustering, was used. The SOM input data consists of, three dimensional parameters (X,Y,Z), geotechnical parameters (lugeon, RQD) and finally indicators of importance of various dam structures and lithology. Then, 7 input parameters were normalized between 0 to 1 and entered the network for training.The output data were allocated to four zones (clusters). For RQD spatial distribution realization, variography and anisotropy parameters for all four zones were calculated for both cases, Based on the main principal of clustering method which is maximum difference between clusters and maximum similarity between members of each cluster, performance and validation of two cases of clustering, RQD data were defined. Clustering quality index defined as sum of mean differences between two clusters divided by sum of standard deviation of clusters. Maximizing of this index is optimal solution. This study showed that clustering by SOM gives more accurate results than clustering by faults.

Anisotropy has an important role in exploration and reservoir characterization. Inpractice, the determination of seismic anisotropy is not easy, but it has importantconsequences in enhancement of seismic data recording and processing. Anisotropyinteracts with reflection seismology, acquisition, processing and interpretation. Ignoringanisotropy can lead to poor seismic imaging, misleading the seismic reflector responses,inaccurate location of well-ties, and incorrect interpretation of seismic arrival times andamplitudes for the determination of lithology and fluid content.Shear wave velocity anisotropy is commonly referred to as shear wave splitting,because a shear wave traveling in an heteregeneous medium splits into two shear waves.At a given receiver, shear waves are characterized by their orthogonal polarizationdirections (fast and slow) and a delay between their arrival times.The most common anisotropic models have been related to the framework oftransverse isotropy or a hexagonal isotropy system. When the symmetry axis is alignedwith the vertical axis, the model is called vertical transverse isotropy or VTI. For a VTImedium, there are five stiffness coefficients and three independent phase velocities.Thomsen (1986) replaced these stiffness coefficients with two vertical velocities (Vp0 andVS0) and three dimensionless anisotropy parameters (namely, ε, γ and δ). Anisotropyparameters can be determined in several ways, including velocity measurements on coresamples in a laboratory or from field data in a VSP experiment. A common form ofanisotropy observed in many geological area (thinly horizontal layers or fractures). Thisinvolves the reference axis of symmetry being normal to the bedding surfaces. Thomsen(1986) introduced three anisotropic parameters (ε, γ and δ) to describe weak anisotropy,which is believed to be the simple model of anisotropy. Thomsen parameters can becomputed with the stiffness tensor considered in anisotropic media. Alkhalifah andTsvankin (1995) showed that, for P-wave Moveout, there exists a range of kinematicallyequivalent models which are governed by the stacking velocity and introduced by theparameter η.The Dipole Shear Sonic Imager (DSI) is an example of devices that are used to obtainand analyze sonic measurements of formations surrounding a borehole. The DSI Imagercan measure the components of shear slownesses in many directions in a planeperpendicular to a borehole axis. The DSI tool is a full waveform acoustic tool thatdelivers measurements of sonic waves in a wide variety of formations. In theconventional DSI logging tool, one can present compressional slowness, Δtc, shearslownesses, Δts, and Stoneley slowness, Δtst, each as a function of depth. The DSI tool canestimate the orientation and magnitude of stress from velocity dispersion. By invertingthe dispersion curves from DSI logs, one can estimate the horizontal stresses. One type ofthese special dipole modes enables the recording of both inline and crossline(perpendicular) waveforms. These modes, both called cross receivers (BCR) which areused for anisotropy evaluation.In this paper, one of the anisotropy parameters of Thomsen (γ) was determined by theuse of S-wave velocities and their relationship with the DSI tool used in the Kangan andDalan gas zones of the South Pars field. Subsequently, the γ parameter was comparedwith the Gamma Ray log in depth. The results show anisotropy behavior in shaly zones ofKangan and Dalan Formations. It is found that the average of the γ parameter for theKangan and Dalan Formations are 0.015 and 0.02, respectively. Also, this parameter wascompared with slowness based on anisotropy. A good correlation was observed betweenanisotropy parameter γ and the slowness based on anisotropy (slowness vector).

SKS phase passes through the mantle as an S wave; it converts to a P wave at the core-mantle boundary (CMB) beneath the source, travels through the fluid outer core, and then converts back to an S wave at the CMB on the receiver side. When a shear wave (SKS) propagates through an anisotropic media, it is partitioned into orthogonally polarized fast and slow shear waves. This property is referred to as shear wave splitting. In this study we used shear wave splitting of SKS phase from teleseismic events to identify and estimate the shear wave anisotropy in the upper mantle of the Alborz region. In this study, to find stable splitting measurement with small error, we selected the automate analysis window by performing a grid search over 210 windows. The selected teleseismic events (and) were recorded by 3 broadband seismic stations, DAMV, CHTH and THKV (belongs to IIEES network) and by 5 short period seismic stations, FIR, DMV, AFJ, MHD and GZV (belongs to IGTU network). They used to determine anisotropy parameters in upper mantle and crust beneath the Alborz region. In this study the average direction of anisotropy in mantle and crust of the Alborz region is estimated about 48±12 degree by study on shear wave splitting of SKS phase. Thus, the fast azimuth of anisotropy oriented approximately in NE-SW direction too. Moreover our results showed that the magnitude of anisotropy is about 1.5±0.4 seconds.