Probing Fracture Geometry with Guided-Waves

Natural and induced seismicity are often used to infer the presence of fractures or discontinuities in the subsurface. A key question is whether these signals also contain meaningful data on fracture geometry, namely the spatial and probabilistic distributions of aperture and contact area or minerals that may partially fill a fracture. Here, we present observations from laboratory experiments of wave guiding in idealized fluid-filled fractures.

An idealized fracture was created from two 154.2 mm x 154.2 mm x 154.2 mm acrylic blocks. The fracture aperture is the spacing between the blocks and was set from 3.2 mm to 25.6 mm. The fracture volume was either empty or was filled with an acrylic comb, as an analog of fracture geometry, that consisted of 23 acrylic rods (3.2 mm diameter 150 mm in length) that were spaced center-to-center by 6.4 mm. The samples were submerged in water to saturate the fractures. Piezoelectric, water-coupled transducers (1 MHz) were used to send and receive signals. The source transducer was spherically-focused, and the receiver array contained 21 plane wave transducers.

A time-frequency analysis based on Hilbert wavelet transforms revealed a wealth of shifting acoustic signatures as the fracture aperture increased. With increasing aperture, prompt waves associated with propagation through the acrylic transitioned to guided modes at the acrylic-water interface that transitioned to waveguide modes in the water-filled aperture. When the acrylic comb is inserted into the fracture, major changes occur in the energy partitioning among these waves, and new coda waves arise associated with internal reflections within the comb.

This complex behavior demonstrates that information about the fracture geometry is contained within the frequency dispersion of the full scattered wave field and may be extracted in the future by comparing experiments to forward modeling.

Acknowledgment: This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Geosciences Research Program under Award Number (DE-FG02-09ER16022).