Acoustic Propagation In The Vicinity Of Fractures Which Intersect A Fluid-Filled Borehole

Nearly all reviews of technological breakthroughs required for the long term storage of high level radioactive wastes have cited the in situ determination of fracture hydrology as a primary objective. Resolution and depth of penetration arguments suggest that conventional acoustic logging systems which record complete waveforms can provide an effective means for sampling rock bulk properties, and may be used to characterize fractures which have been previously identified with the acoustic televiewer. This hypothesis has been tested by analyzing the digitally recorded waveforms from a series of boreholes up to 1000 m deep in the Lac DuBonnet Batholith of southeastern Manitoba, Canada. Analysis of the data from massive unfractured intervals shows remarkably consistent waveforms with compression and shear velocities of 5.85 + 0.10 and 3.35 + 0.10 km/s respectively, in close agreement with the mean of core measurements. Synthetic waveforms generated by numerical evaluation of the separation of variables solution confirmed the identity of energy arriving in the 3.35 km/s window as a shear head wave. Test hole geometry here restricts normal modes to a single component, usually referred to as a "tube wave". Because careful analysis of the waveforms from unfractured intervals confirmed the uniform nature of the unfractured rock, variations in waveform character could generally be related to propagation across fractured zones. The most diagnostic feature of waveform character in the vicinity of fractures which appear open on the acoustic televiewer and television logs is a 20 to50 percent reduction in tube wave amplitude, with nearly complete tube wave amplitude reduction adjacent to several intensely fractured zones. The presence of open fractures also strongly affect wave energy arriving at approximately shear velocity. These effects are attributed to shear attenuation across fluid-filled fractures and mode conversion effects when the sonde receiveris located directly adjacent to an open fracture.