Free-air anomaly is the difference between the _____ gravity and theoretical gravity at a point.

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

Answer: (A)

Free-air anomaly is about comparing what gravity actually is at a location with what gravity theory says it should be there. The gravity you measure with a gravimeter is the observed gravity. The gravity predicted by a standard Earth model at the same point—taking into account latitude and height on the reference surface—is the theoretical (normal) gravity. By adjusting the observed value for height (the free-air correction) and then comparing to the normal gravity, you get the free-air anomaly. The key point is that the anomaly stems from the difference between the measured gravity and the predicted gravity, revealing regional mass-density variations near the surface. The other options don’t describe the quantity used in defining the anomaly, since it’s about the actual measured gravity, not something unknown, assumed, or fixed.