Zero-porosity basalt has a velocity of 6.7 km/s. Surface extrusive processes introduce pore space which decrease this velocity. The method of Christensen and Shaw, [1970] was used to estimate the density assuming that velocitiy differences are due to pore space.
Another possible explaination for velocity differences, particularly at depth, is chemical composition. Domed structures may have a higher mantle component. High velocities can be considered a mix of granitic (velocity=2.7 km/s, density=2.7 g/cm3) and upper mantle material, 70% olivine, (velocity=8.2 km/s, density of 3.3 g/cm3).