Gravitometers are devices that measure very tiny differences in Earth's gravitational field from one place to another. Gravity measurements are also calculated measuring subtle changes in the paths of Earth orbiting satellites. Orbiting satellites are pulled closer to Earth over regions of higher gravity.
Gravity measurements reveal that there must be denser material deeper in the Earth. Rocks at the surface of the planet are not dense enough to account for the overall higher amount of gravitational attraction that exists between objects on the surface and objects orbiting the planet. Variations in gravitational forces also reveal subtle differences in the density and thickness of the crust in different regions of the world. Figure 3.12 shows variations in the Earth's gravitational field as revealed by satellite gravity measurements. The map shows that older and colder crust, such as under regions in the oceans, is denser (having higher gravitational attraction) than where new ocean crust is forming along mid-ocean ridges. Gravitation is less where the crust is less dense, such as beneath continental regions and where rocks are hotter (associated with regional volcanism).
Figure 3.12. NASA's gravity anomaly map shows subtle differences in the Earth's gravitational field in different portions of the world.
Gravitational forces increase with increasing mass and decreases with distance. The greater the mass between to objects (such as moons or satellites orbiting planets), the greater the gravitational attraction. In addition, the closer two objects (such as moons and planets), the greater the gravitational attraction.