Transversely Isotropic Saturated Porous Formations

The wavefields generated by monopole and dipole sources in a fluid filled borehole embedded in multilayered transversely isotropic saturated porous formations are studied. The layers are modeled following Biot theory modified in accordance with homogenization theory. It allows to take into account a transversely isotropic skeleton and/or a transversely isotropic complex permeability tensor. Their axes of symmetry are assumed to coincide, parallel to the vertical axis of the borehole. A general formulation, valid for any order of multipole source and based on the Thomson Haskell method, allows to take into account any combimi.tion of elastic and saturated porous layers, either isotropic or transversely isotropic. The presence of an external fluid layer is also possible. The study focuses on the modes behavior. It is achieved through the computation of dispersion and attenuation curves, sensitivity coefficients with respect to the stiffness constants of the skeleton(s), and full waveform synthetic microseismograms using the discrete wavenumber method. In the simple hole model with an impermeable borehole wall, whatever the type of the formation (fast or slow), the behavior of the modes is analogous to that in the presence of simple elastic formations with body wave attenuations added. The phase velocity of the Stoneley wave generated by a monopole source is sensitive to the horizontally propagating SH-wave velocity. Such a coupling decreases with increasing frequency and stiffness of the formation. The low frequency part of the zero-th order (Le., flexural) mode generated by a multi(di)pole sourc~ measures the vertically propagating SV-wave velocity. The shear wave anisotropy may then be evaluated. With a fast formation, the vertically propagating SV-wave velocity can also be obtained "'Now at Etudes et Productions Schlumberger, 26 Rue de la Cavee, 92142 Clamart, France.