Aligned fractures/cracks in rocks are a primary source of elastic anisotropy. In an azimuthally anisotropic formation surrounding a borehole, shear-waves (S-waves) split into fast and slow waves that propagate along the borehole and are recorded by a borehole logging tool. However, when the formation has conjugate fractures with orthogonal strike directions, the azimuthal anisotropy vanishes. Hence, azimuthal anisotropy measurements may not be adequate to detect orthogonal fracture sets. We have developed a method for obtaining azimuthal and radial S-wave anisotropy parameters simultaneously from 4C array waveforms. The method uses a velocity tomogram around the borehole. The azimuthal and radial anisotropy were determined by integrating a shear velocity radiation profile along the radial direction at different azimuthal angles. Results indicate that this approach is reliable for estimating anisotropy properties in aligned crack systems. The advantage of this interpretation method is shown in multiple conjugate crack systems. Field data processing examples are used to verify the application of the proposed technique. Comparison of results against those obtained with a conventional procedure shows that the new method can not only provide estimates of azimuthal anisotropy, but also of the radial anisotropy parameter, which is important in fracture network evaluation.

中文翻译：

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基于井眼周围弹性波速度变化的3D剪切波各向异性评估

岩石中对齐的裂缝/裂缝是弹性各向异性的主要来源。在围绕井眼的方位各向异性地层中，剪切波（S波）分裂成沿井眼传播并由井眼测井仪记录的快波和慢波。但是，当地层具有走向方向正交的共轭裂缝时，方位各向异性就消失了。因此，方位各向异性测量可能不足以检测正交裂缝组。我们已经开发了一种同时从4C阵列波形中获取方位角和径向S波各向异性参数的方法。该方法使用井眼周围的速度层析X线照片。方位角和径向各向异性是通过在不同方位角沿径向方向合并剪切速度辐射轮廓来确定的。结果表明，该方法对于估计对准裂纹系统的各向异性是可靠的。这种解释方法的优势在多个共轭裂纹系统中得以体现。现场数据处理示例用于验证所提出技术的应用。将结果与常规方法获得的结果进行比较表明，该新方法不仅可以提供方位各向异性的估计值，而且还可以提供径向各向异性参数的估计值，这在裂缝网络评估中很重要。