MANDREL EFFECTS ON THE DIPOLE FLEXURAL MODE IN A BOREHOLE

The flexural mode is the lowest-order borehole mode with dipole excita tion. The low-frequency asymptote of the flexural wave speed is used t o estimate the formation shear speed in well logging. The borehole fle xural mode measurement system consists of a mandrel that holds the tra nsmitter and an array of receivers. This mandrel is, generally, design ed to minimize the interference between any mandrel and the borehole f lexural modes. A slotted sleeve housing that makes the sleeve arrival significantly slower than the formation arrival enables the processing of the recorded waveforms based on a model of fluid-filled boreholes without any mandrel. The objective of this work is to investigate the basic physics of mandrel effects on borehole flexural mode for the con dition that the mandrel is in the form of a rod or pipe that is not ne cessarily slower than the formation arrivals. To this end, we describe an experimental and theoretical study of flexural modes in a borehole with a concentric mandrel and water annulus. The mandrels are of simp le geometry such as a rod or a pipe, not intended to model any particu lar sonic tool in its entirety. However, the presence of a rod or a pi pe can approximately represent the waveguide nature of the mandrel and its possible influence on borehole flexural modes. A fluid-filled (wi thout any tool) borehole and a mandrel immersed in an infinite fluid ( without any borehole) are two independent systems. Each supports a fle xural mode that can be excited by a dipole source. These two uncoupled modes have distinct velocity dispersions. In a coupled mandrel-in-bor ehole system, two flexural modes are generated with a dipole source. T hese two coupled modes may resemble or significantly differ from the t wo uncoupled modes depending on the proximity of dispersions of these two uncoupled modes. (C) 1998 Acoustical Society of Al,lei-ica. [S0001 -4966(98)05910-4]