Abstract

One of the typical anisotropic media is the monoclinic anisotropy (MA) medium, which is formed by embedding two sets of non-orthogonal fracture sets into an isotropic or vertical transverse isotropic (VTI) background medium. Weak anisotropy (WA) parameters and fracture density provide important in situ stress and high-porosity zone information. Estimation of WA parameters and fracture density of MA medium by prestack seismic amplitude inversion is important for shale reservoir characterisation. We derive the expression of generalised WA parameters in a basic reflection coefficients formula of MA medium by incorporating a stiffness matrix of VTI background and disturbance compliance matrix of asymmetric fracture. We then re-express the P-wave reflection coefficients in terms of WA parameters and fracture compliance tensors. To achieve the direct inversion of fracture density, we rewrite the linearised expression of P-wave reflection coefficients related to WA parameters and fracture density. Finally, under the Bayesian framework, the WA parameters and fracture density are estimated by using the amplitude versus offset and azimuth (AVOA) inversion parameters. We use a Monte Carlo simulation to test the effect of uncertainties in the priori information about fracture property parameters. The application of synthetic seismic gathers show that the proposed inversion strategy is reliable within moderate noise. Compared with the results obtained by using inversion based on a rotationally invariant fracture, the test indicates that a fracture model with a simplified shape or wrong assumption will increase calculation error and reduce the inversion accuracy.

中文翻译：

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单斜介质中非对称裂缝的弱各向异性参数和裂缝密度估算

摘要

典型的各向异性介质之一是单斜各向异性（MA）介质，它是通过将两组非正交的裂缝组嵌入到各向同性或垂直横向各向同性（VTI）背景介质中形成的。弱各向异性（WA）参数和裂缝密度提供了重要的原位应力和高孔隙度区域信息。通过叠前地震振幅反演来估算西澳中层介质的WA参数和裂缝密度对页岩储层的表征很重要。通过结合VTI背景的刚度矩阵和非对称断裂的扰动柔度矩阵，得出MA介质的基本反射系数公式中广义WA参数的表达式。然后，我们根据WA参数和裂缝顺应张量重新表达P波反射系数。为了实现裂缝密度的直接反演，我们重写了与WA参数和裂缝密度有关的P波反射系数的线性表达式。最后，在贝叶斯框架下，通过使用振幅对偏移量和方位角（AVOA）反演参数来估算WA参数和裂缝密度。我们使用蒙特卡洛模拟来测试不确定性对有关裂缝特性参数的先验信息的影响。合成地震道集的应用表明，所提出的反演策略在中等噪声范围内是可靠的。与使用基于旋转不变裂缝的反演获得的结果相比，该测试表明，具有简化形状或错误假设的裂缝模型将增加计算误差并降低反演精度。