indirect evidence for the earths structure

• Seismology gives us rheological properties, like rigidity, they do not provide compositional details. 
• Density along the earths surface is low and increases with depth. The average density of the planet is 5.5 g/cm3. the core must be high density to balance out the mean density of the planet. 
• Gravity alters the attraction of objects to each other. The greater the mass the greater the attraction/gravity. 
• The earth is not perfectly spherical, due to the spin, and bulges at the equator. The gravity is greater at the poles. 
• The free air anomaly alters the level of gravity, measuring gravity at different altitudes gives different results. when Measuring from altitude you have to add 0.31 milli gals for every 100m above sea level. 
• The bouguer anomaly is the difference in gravity due to rock masses above sea level. 
• Oceans in general, have a positive anomaly. 
• Continents have a negative anomaly. 
• if a gravity value is larger than expected, the anomaly will be positive meaning the mass below the surface is greater than average and the density is high.
• if the gravity value is lower than expected, the anomaly will be negative, meaning the mass below the surface is less than average and the rock density is low.