Acoustic isolation of a monopole logging while drilling tool by combining natural stopbands of pipe extensional waves

For extensional wave propagation along a cylindrical pipe, there exists a natural stopband in the frequency range between the first and second modes. This study explores the feasibility and practicality of building a drill collar acoustic extensional-wave isolator by combining the stopbands of pipes of different thicknesses. Numerical modelling shows that this is indeed possible and a stopband of designated width can be obtained using an optimization procedure. Laboratory measurement on an optimized design further verified this concept. The result provides a viable approach for the acoustic isolation design of a logging while drilling acoustic tool.