Report Summary

Abstract

This report contains the results of work completed during the eighth year of the Full Waveform Acoustic Logging Consortium in the Earth Resources Laboratory at M.I.T. During the past year, we were able to accomplish many of the goals we had set out to do. We have concentrated our efforts in understanding logging in fractured and anisotropic media, so as to enable us to infer for the properties of the media from full waveform acoustic logs. For logging in fractured media, we have developed the theory of scattering of Stoneley wave by a horizontal fracture, as well as transmissions through the fracture and attenuation by the fracture. We have also developed the theory for the case when the fractures are small compared to the wavelength of the rock and we have an effective anisotropic medium. In the case of a horizontally layered medium such as a shale, we can invert for the degree of shear wave anisotropy using the Stoneley wave velocity. In the area of logging in permeable formations, we have developed a much sim plified theory that gives similar results as the Biot-Rosenbaum theory. We have also coded the latter for finite-difference modelling. We have developed an inversion algorithm which gives formation permeability from Stoneley wave phase velocity and attenuation. The algorithm gives good results when compared with core permeability measurements. On other related topics, in petrophysics we have a report on the estimation of formation permeability from velocity anisotropy measurements. We have also extended our research away from the borehole to a crosshole situation. We have calculated the near and far field radiation pattern in the formation from a point source inside the borehole. This knowledge is critical in the application of diffraction tomography to crosshole measurements. The following is a summary of the papers in this report.