Experimental Studies of Multipole Acoustic Logging with Scaled Borehole Models

Abstract

A scaled multipole acoustic tool is built for laboratory measurements at ultrasonic frequencies in borehole models. The source with four separate transducers generate monopole, dipole, and quadrupole waves by different combination of the transducers. Each receiver has two separate transducers and six receivers which records twelve acoustic components of the multipole waves propagating along a borehole. The experimental results in the borehole models show the center frequency of the multipole waves responding in a soft formation is lower than in a hard formation. The monopole system can measure the P-wave and S-wave in hard formation, but no S-wave or slow shear wave in soft formation. In a dipole system, flexural and hexapole modes in both the soft and hard formations can be observed. These dispersive modes propagate at velocities that are slightly slower than the formation shear velocity. The quadrupole system records the screw waves in soft and hard formations, which also propagate slower than the formation shear velocities. in the soft anisotropic formation, no slow screw wave is recorded.