TUBE WAVES AND MANDREL MODES - EXPERIMENT AND THEORY

The characteristics of tube waves in a borehole, with and without a so lid cylindrical mandrel, which may be either elastic or pore-elastic, are compared. With an elastic mandrel, the tube waves are slower, more dispersive, and more sensitive to the formation shear modulus than wi thout. Similarly extensional mode characteristics are compared in the presence of a formation. In the presence of an elastic formation the e xtensional mode is faster, more dispersive than without, and it is onl y slightly sensitive to the formation shear properties. These calculat ed characteristics are in excellent agreement with our measured data. Additionally, the characteristics of tube waves and extensional modes are studied in a borehole in the presence of a concentric liquid-satur ated porous mandrel whose acoustic properties are calculated using the Blot theory. The coupling of the tube wave propagating in the annulus with the slow wave propagating in the porous mandrel introduces atten uation and additional dispersion to the tube waves; the effect on the extensional mode is slight. Theoretically calculated and experimentall y measured dispersion and attenuation are in excellent agreement with each other, with no adjustable parameters. Whether the mandrel is poro us or not, there are two modes which are weakly dispersive in the quas istatic limit. Very roughly, the two modes consist of an extensional m ode in the rod and a fluid-based mode in the annulus which are coupled together. (C) 1997 Acoustical Society of America. [S0001-4966(97)0641 2-6].