Open cracks and cavities play important roles in fluid transport. Underground water penetration induces microcrack activity, which can lead to rock failure and earthquake. Fluids in cracks can affect earthquake generation mechanisms through physical and physicochemical effects. Methods for characterizing the crack shape and water saturation of underground rock are needed for many scientific and industrial applications. The ability to estimate the status of cracks by using readily observable data such as elastic-wave velocities would be beneficial. We have demonstrated a laboratory method for estimating the crack status inside a cylindrical rock sample based on a vertically cracked transversely isotropic solid model by using measured P- and S-wave velocities and porosity derived from strain data. During injection of water to induce failure of a stressed rock sample, the crack aspect ratio changed from 1/400 to 1/160 and the degree of water saturation increased from 0 to 0.6. This laboratory-derived method can be applied to well-planned observations in field experiments. The in situ monitoring of cracks in rock is useful for industrial and scientific applications such as the sequestration of carbon dioxide and other waste, induced seismicity, and measuring the regional stress field.