Wave components of acoustic logs in transversely isotropic porous formations
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摘要: 采用横观各向同性的流体饱和孔隙介质地层模型,模拟了井中由点声源激发的声场. 计算出了井孔声场函数的全部极点,发现最低阶泄漏模式波极点的虚部与正常模式波极点的虚部大小属于同一数量级,因而是不可忽略的. 在算例中还发现, 伪瑞利波相速度的低频极限值不一定等于地层横波速度. 计算了由极点留数对应的斯通利波、伪瑞利波和两种泄漏模式波的激发频谱和时域波形, 以及由割线积分给出的激发频谱和时域波形. 通过与全波的比较,发现在一些情况下仅由准横波割线积分不能获得准横波首波,必须将泄漏模式波与准横波割线积分波叠加后才能得到准横波首波,泄漏模式对全波的贡献不能忽略.Abstract: The acoutic field excited by a monopole point-source wasnumerically investigated with a model of a transversely isotropicfluid-saturated poroelastic formation. The wavefields inside and outside theborehole are formulated in the frequency-wavenumber domain. All the poles ofthe acoustic function are obtained to correspond to several modes of guidewaves: the normal modes (including Stoneley wave and pseudo-Rayleigh wave)and two leaky modes. The numerical results showed that the contribution ofthe lowest order leaky mode can not be ignored since its imaginary part hasthe same magnitude order as that of the normal mode. Moreover, the velocityof pseudo-Rayleigh wave at the low-frequency limit is not always equal tothe shear wave velocity in transversely isotropic formations. Meanwhile, theexcitation spectra and waveforms in the time-space domain are acquired forStoneley wave, pseudo-Rayleigh wave and two-leaky-modes wave, contributed bythe poles and two branchcuts (the quasi-shear and the quasi-fast-compressional branchcuts). The results showed that the contribution ofleaky-mode poles can not be omitted in the full waveforms.
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Key words:
- anisotropy /
- porous formations /
- acoustic logging /
- leaky mode /
- excitation intensity /
- acoustic field simulation
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