Massachusetts Institute of Technology. Earth Resources Laboratory
Abstract
Previously formulated scaling laws relating acoustic waveforms in
boreholes to frequency, tool size and borehole diameter were investigated by
repeated logging of the same test interval with different frequencies, and by
logging adjacent boreholes of different diameters with the same logging system.
Acoustic source frequency bands were centered on approximately 15, 20 and 34
kilohertz. Borehole diameters were Band 17 centimeters for test intervals
located at depths ranging from 100 to 400 meters in granite. Test intervals
included zones of homogeneous rock and fracture zones that were
independently characterized with acoustic televiewer logs and core from the B
centimeter borehole. The high frequency transducer produced waveforms
dominated by the tube wave mode in the B centimeter borehole, but by a
complicated interference pattern produced by the superposition of three
normal modes in the 17 centimeter borehole. The two lower frequency
transducers produced waveforms in the 17 centimeter diameter borehole with
power spectra dominated by the first normal mode. Various methods for
picking shear arrivals produced shear velocities in close agreement with known
values for all cases except for the data obtained with the high frequency
transducer in the 17 centimeter diameter borehole. This result was attributed
to the effects of mode superposition and enhanced attenuation of higher
frequencies. Tube wave amplitudes constructed from the data obtained with
the higher frequency transducer in the B centimeter borehole provided the
most unambiguous indication of open fractures. The superior quality of these
amplitude logs was attributed to the strong excitation of tube waves, as
opposed to the primary excitation of the first normal mode by the two lower
frequency transducers in the 17 centimeter borehole