Hello Amie,

If reliable ocean depth data are available, one may choose between employing the Bouguer anomaly or the Free Air anomaly for geophysical modelling of an offshore basin.

While the Free Air anomaly ignores the mass of the rocks between the observation site and the geoid, the Bouguer anomaly does. This implies that reliable and thorough ocean depth information is needed to calculate the Bouguer anomaly. If such information is unavailable, free-air gravity anomaly interpretations and maps are more often seen over marine regions.

In 1969 I proposed a combined procedure, see Berkman, E., and LaFehr, T.E., 1969, "Bouguer Reduction Technique for Surface-Ship Gravity Meter Data", Pacific Coast Section, SEG.

Conference Paper

Full-text available on ResearchGate

• January 1969
• 1969, Pacific Coast Section, SEG.
• Ernest Berkman
• T. R. Lafehr

For geophysical modeling of offshore basins, the free-air anomaly is generally more suitable than the Bouguer anomaly. There are a few reasons for this:

Bouguer anomalies are often used in areas where the topography and near-surface geology are complex, such as mountainous regions or areas with significant variations in sediment thickness. In offshore basins, Bouguer anomalies are often preferred over Free Air anomalies because they provide a better representation of the density distribution in the subsurface, which can be used to infer information about the structure and composition of the crust.

The Bouguer correction attempts to remove the gravitational effect of the rock between the measurement point and a reference elevation (often sea level). Offshore, there is no well-defined reference elevation, so the Bouguer correction becomes uncertain.

In contrast, the free-air anomaly does not require a Bouguer correction. It simply compares the measured gravity to the theoretical gravity at the measurement point. This makes it well-defined and accurate for offshore measurements.

Offshore basins generally have thick sedimentary fills overlying denser basement rock. The contrast in densities between sediments and basement is the main source of gravity anomalies, not variations in terrain/topography. The free-air anomaly, which is not corrected for terrain, is, therefore, more representative of the geology.

The free-air anomaly is directly proportional to the thickness and density contrast of subsurface bodies, making it more useful for geological modeling.

Reducing the data to free-air anomaly allows results from both offshore and onshore surveys to be combined more easily, providing a more complete geophysical model of the basin.