Gravity readings after applying corrections for instrument drift and Earth tides are referred to as which of the following?

• A. observed gravity
• B. normal gravity
• C. free-air gravity
• D. Bouguer gravity

Answer: (A)

Gravity readings after removing instrument drift and Earth-tide effects are called observed gravity. This value, usually denoted g_obs, represents the gravity measured at the observation time with time-dependent influences accounted for, so measurements are comparable across stations and epochs. It differs from normal gravity, which is the theoretical gravity at a location based on a reference ellipsoid. If you later apply elevation-related corrections (free-air, Bouguer) and other terms, you obtain gravity anomalies, but the immediate corrected measurement is the observed gravity.