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多偏移距VSP通常叫作非零偏VSP可以可靠地测量野外各向异性。其结果可以输入到正演程序中以研究各向异性对速度分析、偏移和AVO的影响。合理设计的多偏移距VSP还可以提供在最佳条件下目的层的AVO影响,因为子波恰好记录于反射目的层上方,反射点扩散最小。 在本文中,多偏移距VSP技术扩展到了包括多偏移距方位,并对采集于某一碳酸盐岩油藏上方的多组多方位VSP资料作了P波各向异性和AVO分析。分析了下行至上覆页岩的直达波旅行时和碳酸盐岩的反射时间及振幅。数据分析涉及到考虑非双曲线动校、倾斜和方位各向异性的三项式拟合。结果显示上覆页岩为具有垂直对称轴的横向各向同性(VTI),而碳酸盐岩显示出4-5％的旅行时方位各向异性。波传播快的方向与由压力测井确立的最大水平应力方位一致,同时也与主要断层的走向一致。碳酸盐岩层顶部反射的AVO分析显示在传播快的方向临界角减小,在传播慢的方向梯度最大。这与正演结果吻合,指示出沿波传播慢的方向——垂直于裂隙走向方向振幅灵敏度较大。从原理上,三维观测应当具有更宽的方位覆盖以描述裂隙储层。如果没有这种可能,那么重要的是让方位线覆盖在最小水平应力方向上以优化AVO对裂隙储层的描述。这一方向可以用这里给出的方位P波分析从多方位有偏VSP获得。
Multi-offset VSPs are often referred to as non-zero-offset VSPs to reliably measure field anisotropy. The results can be input into the forward program to study the effect of anisotropy on velocity analysis, offset and AVO. The well-designed multi-offset VSP also provides the AVO effect of the target layer under optimal conditions since the wavelet is recorded exactly above the target of reflection and the reflection point spreads to the minimum. In this paper, the multi-offset VSP technique has been extended to include multi-offset azimuths and P-wave anisotropy and AVO analysis have been performed on multiple sets of multi-directional VSP data collected over a carbonate reservoir. The time and amplitude of the reflection of carbonates during the direct wave travel down to the overlying shale are analyzed. Data analysis involves a triadimetric fitting considering non-hyperbolic maneuvering, tilt and azimuthal anisotropy. The results show that the overlying shales are transversely isotropic (VTI) with a vertical axis of symmetry and carbonate rocks exhibit 4-5% azimuthal anisotropy when traveling. The direction of wave propagation is coincident with the orientation of the maximum horizontal stress established by pressure logging and also coincides with the strike of the main fault. The AVO analysis of the top reflection of the carbonate rock shows that the critical angle decreases in the direction of fast propagation and the maximum gradient in the direction of slow propagation. This coincides with the forward result, indicating that the direction of propagation along the wave is slow - the amplitude sensitivity is larger perpendicular to the strike of the crack. In principle, three-dimensional observations should have wider azimuth coverage to describe fractured reservoirs. If this is not possible, it is important to cover the azimuth line in the direction of the minimum horizontal stress to optimize the AVO description of the fractured reservoir. This direction can be obtained from the multi-directional biased VSP using the azimuth P-wave analysis given here.