The Bouguer correction is a first-order correction to account for the excess mass underlying observation points located at elevations above the elevation datum and is deficient when points are below the datum. What is the term for this correction?

• A. Bouguer correction
• B. Free-air correction
• C. Eötvös correction
• D. Instrument drift correction

Answer: (A)

Bouguer correction accounts for the gravitational effect of the mass between the observation point and the reference datum plane, assuming a uniform density slab. This first-order adjustment removes the gravity contribution of that mass so the remaining value reflects subsurface density variations rather than the topography itself. For points above the datum, removing this mass brings the measurement toward what would be observed at the datum level. When points are below the datum, the simple plane-parallel approach becomes insufficient because the mass beneath the datum also influences gravity in a way the basic Bouguer model doesn’t fully capture, so additional modeling or corrections are needed.