基于平均速度分析的近地表纵波速度反演

Near surface P-wave velocity inversion with average velocity analysis method

面波频散曲线对于横波速度、纵波速度、层厚等近地表地球物理参数的敏感度相差较大,现阶段通过频散曲线可以获得较为精确的近地表横波速度与厚度信息,但无法直接对纵波速度进行反演.研究表明,泊松比对于波长(W)-探测深度(D)关系较为敏感.基于这一发现,本文根据频散曲线与反演获取的横波速度结构计算平均速度,获取W-D关系曲线.但合成数据测试证明,因近地表浅层对应的W-D曲线变化较小,且浅层纵波速度反演不准确会使误差累积,直接反演W-D曲线无法获取可靠的纵波速度剖面.本文改进了基于平均速度分析的近地表纵波速度反演方法,在目标函数中加入了对浅层信息较为敏感的W/D-D信息,同时对W-D曲线与W/D-D曲线进行联合反演.合成数据测试证明可以获取较为准确地浅层与深层纵波速度.将该方法应用于实际地震数据中,联合反演得到的纵波速度剖面与微测井数据较为吻合,证明本文提出的方法可以不借助其他信息,仅通过面波频散信息,获取更为准确地近地表纵波信息.

The sensitivity of surface wave dispersion to different parameters such as P-and S-wave velocity and thickness varies greatly. In surface wave studies, the S-wave velocity and thickness can be easily estimated, but the P-wave velocity cannot be accurately obtained. In engineering and petroleum exploration, the P-wave velocity model is more important than the S-wave velocity model. A previous study has shown that Poisson's ratio is sensitive to the wavelength (W)-depth (D) relationship. Based on this observation, we proposed a procedure to estimate the P-wave velocity model. Once the dispersion curves are extracted and the S-wave model is inverted, the W-D relationship is then obtained according to the average velocity. The synthetic data test proved that direct inversion for the W-D curves cannot produce a reliable P-wave velocity model. The W-D curves are not sensitive to P-wave velocities for superficial layers, which can lead to the failure of the P-wave inversion for the entire profile. We revised the traditional inversion method and added the W/D-D information, which is more sensitive to the superficial layers for the objective function. The synthetic data tests show that the proposed method can produce accurate P-wave velocity for both superficial and deep layers. We then applied it to field data. The resulting P-wave profile is consistent with the drilling data. The results indicate that the proposed method can obtain a good P-wave profile only using the dispersion curves without the help of other information.