Multipole logging in (multilayered) porous formation

Recent theoretical and experimental studies, and fieldwork have demonstrated the interest of using multipole sources, such as dipole and quadrupole, in full waveform acoustic logging to record shear events in any kind of formation (fast or slow). Multipole sources excite fundamental (surface) modes whose Airy phase are predominant on the records at intermediate frequencies (≅ 3 to 6 kHz). The authors study their dispersion and attenuation in a fluid-filled borehole embedded in a homogeneous fast and slow saturated porous formation. Using the Thomson-Haskell method, the situation of a radially multilayered porous formation is also investigated. The two phase media are modeled using Biot's theory modified in accordance with homogenization theory. Calculation of synthetic full waveform logs is also performed using the discrete wavenumber method. It is only in the presence of fast formations, that the attenuation of the Airy phase of the fundamental modes is representative of the in situ permeability of the inner layer with a permeable borehole wall. In the presence of slow formations, no reliable sensitivity can be detected unless they are saturated by a fluid of low viscosity and high compressibility. Whatever the configuration and the formation, the most reliable information which can be extracted from low frequency parts of the wavetrains are the shear velocity and attenuation of the outermost (i.e., virgin) formation. These parameters are of great interest in the inversion of Stoneley wave velocity and attenuation for the determination of in situ permeability