Method and apparatus for indirect determination of shear velocity from guided modes

摘要

Methods and apparatus are described for estimating shear wave slowness of formations traversed by a borehole from guided wave acoustic log data. Acoustic energy is generated at a point in the borehole and received at at least four receiver stations spaced one from the other and from the generating point. Full wave forms of the received signals are produced and windowed to identify guided wavelets. Fourier transforms of the guided wavelets are obtained and from these determined the phase velocity and frequency of the guided wavelet. Values of formation density, drilling fluid density, drilling fluid slowness and borehole diameter are measured and a value of Poissons ratio may be estimated. A multi-dimensional matrix is established comprised of predetermined ranges of assumed values of frequency, Poissons ratio, drilling fluid slowness, drilling fluid density, formation density, phase velocity and borehole diameter. The measured data is fitted to the multi-dimensional matrix to obtain an estimate of shear wave slowness at the point in the borehole, and the estimate is recorded as a function of depth.