Bikash K. Sinha
Schlumberger--Doll Res., Ridgefield, CT 06877
A low-frequency dipole source excites flexural waves in cylindrical structures. Flexural waves in rods and pipes assume vanishingly small phase velocities at low frequencies. On the other hand, the flexural waves of a borehole surrounded by an infinite formation asymptotically approach formation shear wave velocity at low frequencies. These two individual lowest-order flexural modes interact with each other in a composite structure where a rod or pipe is concentrically placed in a borehole surrounded by an infinite formation with liquid in the annulus. This interaction results in two coupled modes of the composite structure. Depending on how strong this interaction is, these two coupled modes may resemble or drastically differ from the two individual modes of the rod (or pipe) as well as the borehole. The interaction is stronger at frequencies where the two individual modes are closer in phase velocity. Results of experimental and theoretical studies will be reviewed and compared.