Physical modeling of the full acoustic waveform in a fractured, fluid‐filled borehole
Three concrete models were constructed, one each with a fracture oriented at 90, 45, and 10 degrees to the axis of the borehole. These were used to simulate physically the propagation of the full acoustic waveform through a fluid‐filled borehole in crystalline rock and to ascertain the effects of fracture aperture and orientation of fluid‐filled fractures on the waveform. The tube‐wave mode of the waveform was most indicative of the magnitude of fracture aperture. Normalized tube‐wave amplitude decreased as a negative exponential function of aperture over the range of fracture apertures studied (closed to 0.66 cm). The 90 degree fracture orientation caused greater tube‐wave amplitude reduction than the 45 degree fracture. We hypothesize that this reduction can be attributed to the borehole wall’s guiding the wave across the 45 degree fracture. However, the 10 degree model gave ambiguous results, which are believed to be related to the low ratio of tube‐wave wavelength to aperture as measured parallel to the borehole axis, i.e., axial aperture.