The known dependence of the seismic velocities measured by an acoustic logging system on the mechanical properties of rocks in situ indicates such measurements could be used to characterize the extent of fracturing adjacent to the borehole. Acoustic full wave-form logs are used to provide a qualitative measurement of the mechanical properties of formations, although fracture interpretation from these logs is subject to a significant degree of ambiguity. The most effective methods of direct fracture interpretation from full wave-form logs involve recognition of distinctive reflection patterns indicative of open fractures adjacent to the borehole. Quantitative interpretations of fracture properties using acoustic full wave-form logs are based on specific indices constructed from waveform amplitude data. Some of the earliest of these approaches were based on correlation of compressional or shear amplitudes with fracture aperture or permeability. However, the most reliable and frequently cited of these methods is based on the consistent correlation between measurements of tube-wave or Stoneley-wave attenuation and independent estimates of fracture permeability. Tube-wave attenuation is related to fracture permeability by calibrating the percent decrease in energy of tube waves propagating across fractures in terms of the total fracture transmissivity over the depth interval between the acoustic source and receiver. The correlation holds for fracture orientations ranging from horizontal to vertical and is insensitive to transducer frequencies > 5 kHz as long as tube waves are generated, imposing an effective frequency window on tube-wave interpretation applications that ranges from 5 to 15 kHz, depending upon borehole diameter and seismic velocities. Tube-wave reflection methods and the adaptation of tube-wave interpretation to frequency-dependent formulations provide additional means for evaluating fractures intersecting the borehole using frequencies < 5 kHz. Although the theory of these applications is still being worked out, the longer wavelengths offer the possibility of investigating larger, more permeable fractures in sedimentary rocks where tube waves are poorly excited at higher frequencies.

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