Acoustics of fluid-filled boreholes with pipe: Guided propagation and radiation

The study of the guided propagation and radiation from radially layered boreholes has applications in monitoring exploratory drilling. Axisymmetric propagation through pipes immersed inside fluid-filled bores in infinite elastic spaces was modeled using full 3-D elastodynamic equations. Three propagating modes are present for frequencies below 1 kHz. These are the lowest order modes and are characterized through their dispersion, attenuation, modeshapes, and particle motions. Mode I is confined mainly to the cross section of the pipe, while modes II and III are present in the fluid layers. Modes I and II are weakly dispersive, while mode III is strongly dispersive. Mode III is the equivalent of the Stoneley mode present in fluid-filled bores without pipe and is most influenced by the properties of the surrounding elastic space. Mode III has a cutoff frequency, below which it radiates, when the shear speed of the surrounding elastic space is sufficiently low.