Inversion of formation shear speed from dipole Scholte wave during logging while drilling in slow formations

Abstract

Shear speed can be inverted from the acoustic dipole flexural wave during wireline logging. The flexural wave, however, does not appear in dipole acoustic logging while drilling in a soft formation. This study demonstrates the possibility of inverting the shear speed in a slow formation from the Scholte wave near the fluid-formation interface. The wave velocity of this wave is shown to be almost constant at frequency higher than 4 kHz. The first order collar wave mode does not have an Airy phase and arrives earlier than the Scholte wave. Although the Airy phases of higher order collar wave modes have lower group velocities than that of the Scholte wave, they will not be excited at a frequency lower than 15 kHz. When the source frequency is chosen between the above two frequencies, the Scholte wave arrives later than all other wave modes and can be detected in the time domain. The Scholte wave speed is shown to be much more sensitive to the shear speed of the formation than all other parameters. Thus it can be used to invert the shear wave with required accuracy.