Effects of foliation and microcracks on ultrasonic anisotropy in retrograde ultramafic and metamorphic rocks at shallow depths

• Ultrasonic anisotropy is observed in all foliated rock samples tested in this study.
• Ultrasonic wave velocities asymptotically increase with hydrostatic confining pressure.
• The anisotropy monotonically decreases or increases in asymptotic manner.
• The asymptotic variation is inferred as a result of microcrack behavior.
• The interaction between foliation and microcracks results in anisotropic characters.

We have conducted ultrasonic measurements on cylindrical specimens of serpentinite, talc, granite gneiss, biotite schist, and amphibole schist from rock samples taken in the mid-west region of South Korea. Two samples were prepared from each rock block by coring along and across the foliation. Compressional and shear wave velocities were measured along the cylindrical samples at different levels of confining pressure up to 70 MPa at room temperature. Our test results show that the foliation is a primary (or intrinsic) parameter of ultrasonic anisotropy because both velocities along the foliation are considerably higher than those across the foliation, regardless of cell pressure (with the sole exception of talc). The ultrasonic anisotropy varies with confining pressure, and this behavior is the result of the gradual closure of microcracks with increasing confining pressure. The preferred orientation of mechanically induced open cracks is a secondary (or extrinsic) parameter of ultrasonic anisotropy, which disappears when the cracks are completely closed while the primary parameter remains. The secondary parameter affects the lithological anisotropy in one of the two ways: either monotonically raising or monotonically lowering the magnitude of anisotropy with a rise in confining pressure. These two distinct behaviors result from differing angles of the preferred orientation of microcracks with respect to the foliation. These differing angles are possibly related to local variations in the late brittle deformation in the study area.