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The ratios of elastic anisotropy in cohesionless soils are always of substantial importance in respective analyses to the geotechnical and geological engineering projects. These ratios are raising from the available discrepancies in anisotropic elastic parameters ascribed to the different directions and planes of soil mass. The major objective of this study is to recognize the variations range of anisotropy ratios resulting from anisotropic shear and Young’s moduli for a variety of cohesionless soils followed by assessing the potential relations among these two anisotropies. To this end, by assuming the transversely isotropy in cohesionless soils, the anisotropic elastic constants from 266 conducted laboratory tests on 37 various soil specimens relating to 10 different sands were derived from conventional triaxial and seismic waves laboratory tests coupled with the numerical testing results in literature. By sorting the collected data and subsequently their analyses, at the first stage, the values of shear and Young’s moduli anisotropy ratios were calculated for the studied soils. Furthermore, by plotting the anisotropy ratios in several joint panels and performing a series of regression analyses on the resulting values, the possible dependencies were inspected between these two anisotropies. At last, the indicative equations among shear and Young’s moduli anisotropies were developed with insistence on use of which instead of the former similar relations in literature.

The ratios of elastic shear stiffness anisotropy and fabric anisotropy in granular soils are of very important characteristics in soil mechanics, which can influence directly lots of geotechnical engineering attributes. The shear stiffness anisotropy in a soil mass is directly related to the soil fabric anisotropy, which in turn has a fundamental contribution in variations model of shear stiffness anisotropy ratio. The main objective of this study is to evaluate the variations ranges of shear stiffness and fabric anisotropy ratios in granular soils by developing a novel approach for estimating fabric anisotropy ratio from soil grading and particles shape properties. By presuming cross-anisotropy, the anisotropic shear stiffness values of 1042 conducted tests on 200 distinct sandy and gravelly soil specimens from 43 various soil types of diverse sites throughout the world were acquired from literature. Those were then integrated with their associated void ratios, stress conditions, grading parameters and particles shape specifications to produce a comprehensive database of anisotropic shear moduli with respect to testing conditions. The collected data were analyzed, from which the shear stiffness and fabric anisotropy ratios could be calculated for examined geomaterials. The resulting values for fabric anisotropy ratio were then depicted versus grading and particles shape information to inspect the level of dependences through deriving the respective correlations. The findings of this study may serve as a suitable technique to obtain first-order approximations for fabric and shear stiffness anisotropies from soil grading and particles shape characteristics.

Fabric anisotropy plays a crucial role in the variations model of anisotropy degree of soil elastic coefficients. This study aims to quantify this anisotropy in sandy soils, examining its variations in terms of the available stress conditions, and assessing its dependence on grading, shape and soil particles weight characteristics. To this end, by presuming the transverse isotropy in the soil medium, the elastic coefficients obtained from 419 laboratory tests on 28 different samples of 17 different sands were collected from literature. Indeed, those were attained from the seismic velocity measurements and supplemented with corresponding data on void ratio, stress state, grading, shape and weight properties. Based using on a famous experimental equation, the collected data was analyzed, and the fabric anisotropy ratio was computed for each soil sample. By drawing these results versus the respective experienced stresses, the variability model and the amplitude of the ratio were identified with respect to the applied stresses. Finally, by performing a series of simple and multiple regression analyses, the potential correlations between the fabric anisotropy ratio and the representative characteristics of grading, shape and particles weight were investigated resulting in a couple of mutual relationships.

In oil exploration, because of the anisotropy of the earth, the velocity of the waves in horizontal nd vertical directions are not uniform; however, with a good accuracy in exploration procedures, e can assume that in a layer, velocity changes are limited as a results of slow variations in ensity as well as the elastic properties of the layers in these horizontal directions. In general, ariations of the above mentioned parameters inhorizontal directions are much slower than in ertical ones. For this reason, the acquisition area is often divided into smaller areas; horizontal variations are neglected while the same vertical velocity distributions are applied in any sub-area. There are basically two methods in calculation of the bin size and migration aperture in a 3-D seismic survey design. The first method is based on using a constant velocity model which is not compatible with real conditions. In this model, we assume that the medium between the surface of the earth and the target layer is replaced with supposed layer and ascribe a constant amount velocity to this layer that is equal to the average velocity in medium between the surface and the target layer. The second method uses the model wherein the velocity changes with depth and therefore a linear velocity model is assumed which is more compatible with reality in comparison with the previous method. Whereas the linear velocity method can include all important wave propagation effects, it involves a certain circular logic. This method, involves building a detailed subsurface velocity model and uses ray tracing or other simulation techniques to customize the survey for the local subsurface. In Ahwaz Oil Field, the main target was Asmari Formation and the deep target was Fahlian Formation. The 3-D seismic survey design of Ahwaz Oil Field was performed on the main target located in the depth of 2900 m and the deep target located in the depth of 5000 m from the mean sea level. Ground level was about 15 to 40 m higher than the sea level in this area. By considering the check shot, VSP and sonic log data from 14 well logs, the image area was divided into 14 parts, so that the variations of the horizontal velocity could be neglected in each part and the constant contribution for the vertical velocity could be used. Finally, using the velocity values at the desired vertical depth to the reflection point (target depth), the dip angles of the target horizon (dip of reflector at the reflection Point) and the maximum frequency reflected from the main target, we were able to calculate the bin size and migration aperture in each part. At last, we could select a value for the bin size in this project. In this study, we examined the parameters of the velocity-dependent 3-D seismic survey design. These parameters included the bin size and migration aperture. Conventional formula for the bin size and migration aperture for Ahwaz Oil Field was carried out based on the linear model between the velocity and depth. As an intermediate between constant velocity and interval velocity model, we have given expressions valid for a linear velocity function. By using the linear velocity model, the design parameters incorporated first-order ray bending. Hence, this method was adjusted to the reality and led to better results compared to a constant velocity model. Linear V(z) is an attractive approximation for three reasons. First, this kind of velocity variation captures the first-order effect of the pressure and the temperature increases with depth. It does not require detailed knowledge of the subsurface velocities. Second, analytical expressions are available for the ray path geometry and travel times in such a medium. Third, the linear V(z) propagation allows turning waves which have potential for imaging dips beyond 90 degrees. Migration aperture is overestimated by constant velocity calculations, whereas the bin size is underestimated and this results in an increase in costs. On the other hand, calculations based on a linear velocity model require a less migration aperture and a larger bin size. The bin size and migration aperture are two sensitive economy parameters. Hence, using a larger bin size and a smaller migration aperture obtained from a linear velocity model, the cost of a 3-D seismic survey design will be decreased.

Any 3D seismic survey can have an acquisition footprint. Acquisition footprint is an expression of the surface geometry (most common on land data) that leaves an imprint on the stack of 3D seismic data. Often we recognize it as amplitude and phase variations on time slices, which of course display the amplitudes within our data set at a specified two way time (Cordsen, 2004). On the other hand, acquisition footprint is often used to describe amplitude stripes that appear in time slices or horizon slices produced from 3D seismic data volumes. Although acquisition design of a 3D survey has a major influence on the nature and severity of a footprint, improper data processing techniques such as the use of incorrect normal moveout (NMO) velocities can also create footprint (Cordsen, et al., 2000). More seriously, on horizon slices, footprint can interfere with and confuse stratigraphic patterns. Many different contributions to the generation of acquisition footprint are possible. These can be divided into two main categories: (1) geometry effects: line spacing, fold variations, wide versus narrow patch geometry, source generated noise and variations of offset and azimuth distribution. (2) non-geometry effects: topography, culture, weathers, surface conditions and processing artifacts. In this article we study the effects of these parameters for 3D seismic survey in AHWAZ oil field and calculate acquisition footprint noise in this field. Most of the time the acquisition footprint is based on the source and receiver line spacing and orientations. The larger the line spacing, the more sever the footprint. In land situations where access is very open and, therefore, the lines are very regularly spaced, we may be able to recognize the footprint very clearly. Because the geometry is regular, the footprint also will have the same periodicity. Fold variation themselves are the simplest form of an acquisition footprint. Fold changes with offset (or rather mute distance from the source point); each offset range, therefore, has differing fold contributions (Cordsen, 1995). Because each individual bin of a 3D survey has changing offset distributions, the CMP stack of all traces in a bin will display bin-to-bin amplitude variations. This variation in itself can produce an acquisition footprint. Generally it has been thought that acquisition footprint is far worse in the shallow part of the seismic and therefore, of course, the geological section, mainly because the fold is lower, and amplitude variations necessarily are far more dramatic. Offset limited fold variations alone may produce a recognizable footprint. The higher the fold, the better the signal to noise ratio; therefore, less footprint is evident. Wide recording patch geometries are far more accepted these days than narrow patch geometries (Cordsen, et al., 2000). The reasons are numerous and ranges from reduction in acquisition footprint (particularly that due to back-scattered shot noise) to improved statics solutions and the availability of large channel capacities on seismic recording crews (also leading to higher fold). In addition to the impact of the fold variations, acquisition footprint is made worse by source generated noise trains that penetrate our data sets. The lower the signal to noise ratio is, the worse the footprint will be. Unfortunately, the noise typically has a low frequency content that is much less affected by attenuation. Therefore the noise becomes more prominent relative to the signal content deeper in the section. Our experiences have shown that acquisition footprint problems can be just as prevalent in the deep section as they are in the shallower section. If surface access is poor because of topography variations, tree cover, towns, etc., we irregularize the geometry by moving source points to locations of easier access, and therefore mask the acquisition footprint. It is still present, however. The footprint is just so much harder to identify. Weather and surface conditions may also impact the recorded amplitudes. One can model an acquisition footprint by creating a stack response on either synthetic or real data. We stack the data in a 3-D cube and display the resulting seismic data over a small time window. The best input is a single NMO and static corrected, offset sorted 2D (or 3D) CMP gather. These traces will be applied to each CMP Bin in the recording geometry. In summary, we should attempt to minimize footprints by employing proper seismic acquisition and processing techniques, but if a footprint persists in the stacked data, there are ways to filter the data and mitigate its effect on geological interpretation. In this article we optimized acquisition parameters in order to minimize acquisition footprint noise for 3D seismic survey in AHWAZ oil field and finally with 3D modeling by OMNI software we saw the intensity of this noise in our seismic sections.