Gravity anomaly
A small difference in the pull of gravity from place to place, caused by denser or lighter rock underground. Surveys map them to see hidden structures.
A gravity anomaly is a place where the pull of gravity is very slightly stronger or weaker than expected. Gravity depends on mass, so dense rock buried beneath your feet pulls a tiny bit harder, and light, broken or porous rock pulls a tiny bit less.
Geophysicists measure it with a gravimeter, essentially an extremely sensitive spring balance. Oil companies have used gravity surveys since the early twentieth century to find buried domes and basins where oil can collect.
The differences are minute. Gravity anomalies are measured in milligals: one milligal is about one millionth of the pull of gravity at the Earth's surface. Over the buried Chicxulub crater in Mexico, the gravity low is only a few tens of milligals, yet it draws a clear ring on a map.
A common misconception is that gravity is the same everywhere on Earth. It is not: it varies with height, latitude and the rock below, which is exactly what makes it useful for seeing underground.
