The free-air gravity anomaly height correction for every meter above sea level is ______ mgal.

• A. + 0.3086
• B. - 0.3086
• C. + 0.3608
• D. - 0.3608

Answer: (B)

Gravity falls off with height, so when bringing a measurement up from height to sea level you apply a free-air correction that scales with height. Near the surface the vertical gradient is about 0.3086 mGal per meter, and the sign is negative because gravity weakens as you go higher. Therefore, for every meter above sea level, the height correction is -0.3086 mGal. (This negative sign reflects that gravity decreases with height and the correction term used in the reduction is g_corr = g_obs + (-0.3086) × h.)