______ gravity anomaly refers to small regional variations in the Earth's gravity field resulting from density variations in underlying rocks.

The concept of a Bouguer gravity anomaly pertains to the measurement of small regional variations in the Earth's gravity field that arise from differences in density in the underlying geological formations. This anomaly is calculated by correcting observed gravity values for the effects of terrain, as well as for the gravitational attraction of the mass of the rock layers above the measurement point.

Bouguer gravity anomalies are crucial in geophysical studies because they allow for the identification of subsurface structures based on density variations. When these anomalies are analyzed, they can indicate the presence of mineral deposits or variations in geological formations, providing essential information about the Earth's crust.

In contrast, the other terms relate to different types of gravity corrections or measurements. For instance, free-air corrections account for variations in elevation above sea level but do not consider subsurface density changes directly. Helmert gravity anomalies, on the other hand, involve further modifications to the gravity data and may not specifically isolate density differences. Stokes' method relates to potential theory used in geodesy but is distinct from the direct measurement and implications of Bouguer anomalies.