Seismic velocities are related to density. The continents, being light and thick, float well above the denser, thinner ocean crust. This elevation equilibrium is called isostacy . It means that continents, and mountain ranges within them, have deep roots of lighter material. Isostacy also menas that gravity should be equal over the surface of the globe.
Gravity is measured by means of a gravimeter.
Gravity anomalies may be observed over areas that are out of equilibrium. In addition to geophysical constraints, we also have geochemical constraints on the compositions of the Earth's interior. The Earth is presumed to have formed from particles that were similar in composition to primitive meteorites. These meteorites are called chondrites . The Earth's upper mantle must be of a composition that can produce basalt by partial melting.