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  1. Gravimetric Surveys

    Linked via "FAC"

    Free-Air Correction (FAC)
    The FAC accounts for the change in gravity due to elevation ($h$) above the reference ellipsoid. It assumes that the mass between the station and the reference surface is absent (a vacuum):
    $$\text{FAC} = +0.3086 h \quad (\text{mGal per meter})$$

  2. Gravitational Anomaly

    Linked via "free-air correction"

    Bouguer Anomalies
    The Bouguer anomaly ($\Delta gB$) is the most common measurement in terrestrial gravity surveys. It incorporates the free-air correction (accounting for altitude) and the terrain correction, but assumes a constant density ($\rhoc$) for the intervening crustal material, typically set at $2,670 \text{ kg/m}^3$ (the density of standard [granite](/en…