What is Gravity anomalies?

Let $g$ be the gravity measured at a station of latitude $\varphi$ and height $h$ above sea level.

• A gravity anomaly is the difference between the measured gravity, corrected to a common level, and the normal gravity at the same latitude.
• The free-air correction is $+0.3086\,\mathrm{m}\mathrm{Gal}\,\mathrm{m}^{-1}\times h$: it adds back the decrease of gravity with height. The free-air anomaly is $\Delta g_{\mathrm{FA}} = g + 0.3086\,h - \gamma(\varphi)$.
• The Bouguer correction is $-2\pi G\rho h$: it removes the attraction of the rock between the station and sea level, treated as a slab of density $\rho$, conventionally $2670\,\mathrm{kg}\,\mathrm{m}^{-3}$, which gives $0.112\,\mathrm{m}\mathrm{Gal}$ per metre. The Bouguer anomaly is $\Delta g_{\mathrm B} = \Delta g_{\mathrm{FA}} - 2\pi G\rho h$.