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基于对合成旅行时道集的分析,P,SH 和 SV 波的各向异性效应对于改变各向异性层之下界面的解释很有意义。因此,如果在地震测线的下面存在着各向异性的话,它对于正确地估算各向异性参数将是重要的。本文讨论了地震波的各向异性效应,并提出了一种利用合成 P 和 SH 波至时间道集来求取横向同性介质的五个弹性常数的方法。当前,该方法仅限于单层模型,但是它的一个主要优点在于能够通过几个方位道集的分析把它扩展到较一般的各向异性对称性中去。为了说明在勘测中可能出现的各向异性效应,计算了单层模型的 P,SH 和 SV 速度随角度的变化。该单层模型包括几种类型的各向异性沉积结构。层理、页岩以及平行乾裂隙的定向排列给出了类似的速度变化。可是,如果各向异性是由另外的裂隙排列所引起,则可按速度变化的符号和周期性来分辨,甚至具有横向同性对称性的结构,速度随角度的变化也可以有宽的范围。合成道集曲线是按选定的速度变化模型作出的。这些曲线之所以偏离于双曲线并给出不正确的深度测定,其关键在于各向异性层的弹性常数上。对于大多数各向异性类型来讲,道集曲线形状和弹性常数之间不具有简单的精确关系。按选定模型的道集数据,采用各向异性层的速度随角度变化的简化的方程式来确定弹性常数是很方便的,此时,速度变化和弹性常数之间呈线性关系。除了很强的各向异性以外,这些方程式对于所有具有两个正交对称面的各向异性介质都是有效的。已推导出表示直接对道集曲线所作的切线和弹性常数之间关系的公式。已用这些公式对单层横向同性模型作了实验,而且可以通过绘制 t~2,x~2平面上的,P 和 SH 合成波至时间道集曲线的三条切线求出五个弹性常数。对于许多不同类型的横向同性结构讲来,把求得的弹性常数和精确的弹性常数相比较,有令人满意的结果。我们按照斜方各向异性层计算了两个方位的道集,对它们的分析也取得了一定的成功。本文所发表的方法和以往的方法比较起来,有更好的通用性。
Based on the analysis of synthetic travel gathers, the anisotropic effects of P, SH and SV waves are of great significance to the interpretation of changing the interface below the anisotropic layer. Therefore, it will be important to correctly estimate anisotropy parameters if anisotropy is present below the seismic line. In this paper, we discuss the anisotropy of seismic waves and propose a method to obtain the five elastic constants of transversely isotropic media by using the synthetic P and SH wave-to-time gathers. Currently, this method is limited to a single-layer model, but one of its major strengths is its ability to extend it to more general anisotropic symmetry through the analysis of several azimuth gathers. To illustrate the possible anisotropic effects in the survey, the P, SH and SV velocities of the monolayer model were calculated as a function of angle. The monolayer model includes several types of anisotropic deposition structures. Directional alignment of bedding, shale and parallel dry fissures gives similar velocity variations. However, if the anisotropy is caused by another fracture arrangement, the structure can be resolved according to the sign of the velocity variation and periodicity, and even the structure with the same direction of transverse symmetry can have a wide range of variations in the velocity with the angle. The synthetic gathers set of curves are made according to the selected speed change model. The reason why these curves deviate from the hyperbola and give an incorrect depth determination lies in the elastic constants of the anisotropic layer. For most types of anisotropy, there is no simple, precise relationship between the gatherer curve shape and the elastic constants. According to the gathers dataset of the selected model, it is convenient to use a simplified equation of the velocity of the anisotropic layer as a function of angle to determine the elastic constant, at which point the velocity variation is linearly related to the elastic constant. In addition to the strong anisotropy, these equations are valid for all anisotropic media with two orthogonal planes of symmetry. A formula has been derived that shows the relationship between the tangent and the spring constant for a straight gathers curve. These formulas have been used to experiment on a single-layer transverse isotropic model and five elastic constants can be derived by plotting the three tangent lines of the P and SH composite waves to the time gathers on t ~ 2, x ~ 2 planes. For many different types of transversely isotropic structures, satisfactory results have been obtained by comparing the obtained elastic constants with the exact elastic constants. We have calculated the azimuth gathers of two azimuths according to the orthorhombic anisotropy, and we also obtained a certain degree of success in their analysis. The method presented in this paper is more versatile than the previous one.