软地层横波速度的偶极随钻测井反演

Shear velocity can be inversed from the acoustic dipole flexural wave in a slow formation during wireline logging. However, the flexural wave becomes elusive during acoustic logging while drilling (LWD). The existence of the steel collar makes the borehole structure complex and changes the borehole wave field. The flexural wave is mixed with the collar wave, and cannot be used to invert shear wave velocity effectively. Dispersion and excitation analysis show the advantage of the dipole Scholte wave in LWD. In the case of slow formation studied in this paper, Scholte wave is slightly dispersive at low frequency. Its velocity is almost constant value at the high frequency range, and this value is one-to-one correspondence with the shear wave velocity of formation. The velocity of Scholte wave is much smaller than that of other modes, so it can be separated from other mode waves in the time domain. The Scholte wave velocity is much more sensitive to the shear velocity of the formation than all other formation parameters. Thus it can be used for inversion of shear wave in a slow formation.