Determination of Shear Wave Velocities in "slow" Formations

Direct determination of formation shear wave travel time is impossible in "slow" formations where the shear wave velocity is lower than the borehole fluid (mud) velocity. However, the Stoneley waves in these formations are very sensitive to changes in formation shear wave properties and can be used to indirectly determine the formation shear velocity, In addition, the P wave packet is highly dependent on the Poisson's ratio and thus can be used to estimate the shear velocity once the P wave velocity is known. These phenomena are demonstrated with both numerical and field examples. INTRODUCTION Full waveform acoustic logs have rapidly become a very important set of borehole measurements. In addition to increasing the radius of investigation by having a larger source-receiver offset than the conventional sonic log, full waveform acoustic logs can' provide measurements of formation shear wave velocity and attenuation (Paillet, 1980: Cheng and Toksoz, 1981; Ingram et al., 1981; Willis and Toksoz. 1982). Direct determination of shear wave travel times is possible only in "fast" or "hard" formations where the shear wave velocities are higher than the mud or borehole fluid velocity. It is impossible to obtain shear wave travel times directly from the full 'waveform acoustic logs in "slow" or "soft" formations, where the shear wave velocities are lower than the mud velocity, because there are no refracted shear wave arrivals. The characteristics of the full waveform acoustic log microseismograms in "slow" or "soft" formations are distinctly different from those in "fast" formations. Specifically, the P wave train appears to be of longer duration, the pseudoRayleigh (or normal mode) waves no longer eXist, and the Stoneley waves are more dispersive and are shifted to lower frequencies. In addition, there exists a direct mud arrival under these conditions. This mud arrival can be qulte prominent in situations where the mud attenuation is low (QJ is high) and can easily be misidentified either as a shear wave arrival or the Stoneley wave arrival. At well logging frequencies, the Stoneley wave velocity in a "slow" formation is significantly lower than the well known tube wave velocity, Unlike the case of a "fast" formation, the Stoneley wave velocity in a "slow" fOJ:mation is very sensitive to changes in formation shear wave velocity. Thus, to measure the shear wave velocity in a "slow" formation, one can first obtain the velocity of the Stoneley wave, and then invert for the shear wave velocity using the