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Bouguer Anomaly Calculator

Enter observed and theoretical gravity, elevation, and crustal density.

How it is calculated

The tool removes the effect of height from a gravity reading: BA = g_obs − g_theo + FAC − BC. The observed and theoretical (normal) gravity are both in mGal, the elevation is in metres and the crustal density is in g/cm³. The free-air correction is FAC = 0.3086 × elevation mGal and the Bouguer slab correction is BC = 0.0419 × density × elevation mGal. The theoretical gravity is not worked out from latitude, you must supply it. Results show three decimals in mGal. No terrain correction for nearby hills or valleys and no tidal or drift correction is applied, and a negative elevation reverses the signs of both corrections.

Worked example

A station at 500 m elevation reads 979,850 mGal, while the normal gravity for its latitude is 979,900 mGal, and you assume a crustal density of 2.67 g/cm³. Observed minus theoretical is −50 mGal. Free-air correction: 0.3086 × 500 = 154.300 mGal. Bouguer correction: 0.0419 × 2.67 × 500 = 55.9365, shown as 55.937 mGal. The anomaly is −50 + 154.3 − 55.9365 = 48.364 mGal, a positive anomaly that suggests denser rock than assumed beneath the station.

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Common mistakes

  • Entering gravity in m/s² or µGal: the tool expects mGal, so 9.8 m/s² is about 980,000 mGal and the corrections would be out of scale.
  • Entering density in kg/m³ such as 2670: the field is in g/cm³, so 2670 would make the Bouguer correction one thousand times too large.
  • Entering the elevation in feet or the height above ground instead of height above sea level (the reference level): both corrections scale directly with this number.
  • Entering the observed gravity where the theoretical value belongs, or forgetting that the theoretical value is for the station's latitude: the tool does not compute it, and a wrong latitude value shifts the whole anomaly.

About this calculator

Raw gravity measurements at the Earth's surface vary not just because of what's underground but because of elevation and the mass of rock sitting between the measurement point and sea level — effects that have nothing to do with subsurface geology and need to be removed before the data is useful. The Bouguer anomaly does this by applying a free-air correction (accounting for the point's height above a reference level) and a Bouguer correction (accounting for the gravitational pull of the rock slab beneath that elevation, based on an assumed crustal density) to the difference between observed and theoretical gravity.

This calculator computes that anomaly from observed gravity and theoretical gravity (in mGal), elevation (in metres), and crustal density (in g/cm³). It applies only these two corrections, using a simple flat-slab Bouguer term, and does not add a terrain correction, so in rugged country that adjustment has to be made separately. It's a core calculation in geophysics and mineral/oil exploration, where the resulting anomaly map reveals subsurface density variations — such as ore bodies, salt domes, or basin structures — that would otherwise be masked by elevation effects alone.

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Comments (1)

  • Editorial team

    Elevation matters more than most inputs. The two corrections partly cancel, so each metre of height changes the anomaly by 0.3086 minus 0.0419 times the density, about 0.197 mGal per metre at 2.67. A 100 m height error therefore moves the result by 19.7 mGal. Switching density from 2.67 to 2.3 at 1,000 m shifts it by 15.5 mGal. Which density do you use for the rock in your survey area, and where does it come from?

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