Analysis of borehole guided waves for geotechnical application

The reliability of Stoneley waves (SWs) is discussed for the characterisation of the mechanical properties of soft and hard rock in borehole seismic techniques using source on the surface and hydrophones as receivers. The SWs propagate along the fluid-filled borehole; the propagation is affected by the mechanical and hydraulic properties of the fluid and the surrounding medium. At low frequencies, in a non-diffusive medium (impermeable formation), the wave velocity depends on the density, the wave velocity of the fluid and the shear modulus of the formation. The models adopted to infer the wave velocity in elastic formations in uncased and cased boreholes are discussed. We discuss two examples to check the discrepancies between the theoretical and the experimental evidence. The presence of casing in soft rock greatly reduces the sensitivity of the SW propagation to the mechanical properties of the medium. In hard rock, the scattering of the primary wavefields could be adopted to detect the presence of rock mass discontinuities (fractures).