横向各向同性地层井孔伪瑞利波、弯曲波、螺旋波的低频极限速度  

The low-frequency asymptotic velocity of pseudo-Rayleigh,flexural,and screw waves in a transversely isotropic formation

作　　者：胡恒山[1] 何晓[1] 

机构地区：[1]哈尔滨工业大学航天科学与力学系,哈尔滨150001

出　　处：《地球物理学报》2009年第7期1873-1880,共8页Chinese Journal of Geophysics

基　　金：国家自然科学基金(40874062);哈尔滨工业大学自然科学研究创新基金(HIT.NSRIF.2008.61)资助.

摘　　要：在多极源声测井中,低频弯曲波或螺旋波被广泛应用于测量地层的横波速度.前人的研究已证明在各向同性地层中井孔伪瑞利波、弯曲波及螺旋波的低频极限速度都等于地层横波速度.大量的数值计算结果似乎表明此结论在横向各向同性(TI)地层情况下也能成立,但缺乏理论证明.本文在井孔平行于TI弹性地层对称轴的模型下,考察了井内声源激发的波在流-固边界上的反射和透射情况,阐述了非泄漏模式导波产生的必要条件并讨论了其速度上限值.我们发现在各向异性参数满足一定条件的TI地层中,导波速度的低频极限值小于沿井孔方向上的横波速度.通过对井孔导波的频散分析以及对时域全波列的数值模拟,论证了在这类地层中进行多极源声测井时,不可能根据弯曲波或螺旋波的到达时间准确获取地层的横波速度值.Acoustic multipole logs have been widely used in the in-situ measurement of formation shear velocity. Previous studies have shown that velocities of the pseudo-Rayleigh, the flexural, and the screw modes equal the shear velocity at low frequency limits in isotropic formations. This conclusion has been taken for granted in anisotropic cases without strict proof. In the present study, we investigate the properties of modes generated by multipole sources in a fluid-filled borehole embedded in a transversely isotropic （TI） medium. Based on Snell＇ s law, the transmission and the reflection on the borehole wall are discussed. The upper limit of a guided wave velocity is found to be dependent on Thomsenrs anisotropic parameters. The velocities of guided waves do not tend to the shear velocity in the borehole direction in TI formations whose parameters satisfy a certain condition. The computation results for dispersion curves and synthetic full waves further confirm that in those formations the shear velocity can never be obtained accurately from the arrival time of the flexural or the screw wave.

关 键 词：各向异性 横波速度 伪瑞利波 弯曲波 螺旋波 

分 类 号：P631[天文地球—地质矿产勘探]