Volume 21, Issue 4 (December 1998)
Imaging of Rock Fractures with Low-Frequency Ultrasonic Reflection/Diffraction
This paper documents the development of a method for imaging millimeter-scale fractures in rock with low-frequency ultrasound and seismic algorithms developed originally for kilometer-scale geologic structures. Low-frequency ultrasonic reflection/diffraction imaging provides detailed images of an impedance contrast in terms of two-way travel time from the surface to the fracture, which produces a geometrically distorted representation. Several seismic methods were used to sharpen the images, including migration, which greatly enhances horizontal resolution and inverts the image from the travel time domain back into the depth domain. The resulting images allow quantitative analysis of the geometry and location of the fracture.
Computer-based parametric modeling indicates that velocity changes as small as 5% of the rock matrix velocity can be resolved, and fracture length and thickness less than 1 mm can be resolved. Laboratory-scale tests were run on a cast rock-like block with a known flaw (150-mm disk “sandwich” of oil-separated polyethylene, total thickness 1.5 mm). The processed data allowed quantification of the known flaw as well as revealing a sub-millimeter-thick crack that was created during construction of the block. As a final test, the method successfully imaged the failure crack in a rock beam under static loading.