Laboratory experiments were conducted to measure compressional wave velocities under hydrostatic pressure in Shirahama and Tako sandstone with a porosity of 12 and 24%, respectively. In dry samples under hydrostatic pressure, the compressional wave velocity increased significantly in Shirahama sandstone, as a result of the closure of micropores with low aspect ratio, while the velocity in Tako sandstone was less affected. The pressure dependence of velocity is ascribed to the difference in micropore structure. Velocities were also measured to map the movement of the injected CO2 within a water-flooded sample during CO2 flooding. In the water-flooded sample, velocity changes caused by the CO2 flooding, provided at the bottom end of the sample, are typically on the order of -6%. The compressional wave velocities decreased simultaneously along horizontal paths in two orthogonal directions. (c) 2004 Elsevier Ltd. All rights reserved.