The free-air anomaly is defined as the difference between observed gravity and which gravity value at a point?

• A. observed
• B. unknown
• C. theoretical
• D. fixed

Answer: (C)

The main idea is that the free-air anomaly measures how much the real gravity differs from what a smooth, standard gravity model would predict at the exact location and height. It uses the theoretical gravity value from a reference Earth model (the gravity at that place on the reference ellipsoid, accounting for its height) as the baseline. You then subtract this theoretical value from the observed gravity. If the measured gravity is stronger than the model, the anomaly is positive; if weaker, it's negative. This baseline value is not the measured value itself, not something unknown, and it's not a fixed constant—it's the predicted gravity from the reference model at that specific location and height.