The free-air anomaly correction per meter above sea level is +0.3086 mgal. The corresponding datum in the correction is which of the following?

• A. geoid, +0.3086
• B. sea level, +0.3086
• C. sea level, -0.3086
• D. geoid, -0.3086

Answer: (B)

The free-air correction converts gravity measured at height to the value it would have at a reference surface. Here that reference surface is sea level, so the datum is sea level. Because gravity decreases with altitude, the correction is added: +0.3086 mGal per meter of height above sea level. So at h meters above sea level, you’d add 0.3086 × h mGal to bring the value to sea-level gravity. The geoid is related to mean sea level but the standard free-air correction uses sea level as the reference surface here, and the positive sign reflects gravity weakening with height.