Shear-wave velocity anisotropy. (a) Shear wave with a single polarization (plane of motion perpendicular to ray path) is split by fractured medium into two waves, one parallel to cracks (fast) and one perpendicular (slow). Emerging shear wave therefore contains two arrivals; fractional difference in arrival times (velocity anisotropy) is approximately equal to crack density. (b) Map view of laterally propagating shear waves in vertically cracked rocks, showing mathematical separation into fast (fracture-parallel) and slow (fracture-perpendicular) directions.