- A gravity anomaly is the difference between the observed gravitational acceleration at a specific location on Earth and the theoretical value predicted by an idealized reference model, such as the International Gravity Formula. This discrepancy arises because the Earth is not a perfect sphere of uniform density; instead, it has an irregular shape (the geoid) and variations in subsurface mass distribution. These variations affect the local gravitational field, causing it to deviate from the smooth, predicted profile.
- Gravity anomalies are typically classified into free-air anomalies, Bouguer anomalies, and isostatic anomalies, each serving different purposes in geophysical interpretation. A free-air anomaly corrects the observed gravity for elevation above sea level but not for mass between the observation point and the reference ellipsoid. A Bouguer anomaly goes further by removing the gravitational effect of the rock mass between the measurement point and sea level, producing a clearer signal of subsurface density variations. Isostatic anomalies account for the theoretical balance of crustal masses in the mantle, providing insights into deep crustal and mantle structures.
- These anomalies arise from density contrasts in the subsurface. For example, a dense ore body or basaltic intrusion can create a positive gravity anomaly because the higher density produces a stronger local gravitational pull. Conversely, low-density features such as sedimentary basins, salt domes, or voids produce negative gravity anomalies. Such measurements are invaluable in mineral exploration, petroleum prospecting, and understanding tectonic processes, as they reveal otherwise hidden geological features.
- Interpreting gravity anomalies requires careful data correction and modeling. Observations must be adjusted for latitude (due to Earth’s rotation and equatorial bulge), elevation, tidal effects, and instrument drift. Once corrected, anomaly maps can highlight subtle variations that may indicate fault systems, volcanic roots, or crustal thickness changes. On a global scale, satellite missions like GRACE (Gravity Recovery and Climate Experiment) have extended gravity anomaly mapping to ocean basins and polar regions, providing essential data for geodesy, oceanography, and climate science.
