Density and Gravity
As the topography varies, the column of rock thins and thickens; this changes the gravity field.
Changes in rock type, hence rock density, cause variations in the gravity field.
The Earth's surface varies: we have mountains and valleys to wide plains and ocean trenches.
To understand anomalies, let's assume that the Earth is completely smooth and featureless.
Some regions of the planet—such as mountains—rise above the assumed surface creating a positive anomaly. Red-yellow areas show where these anomalies exist.
Some regions of the planet—such as oceans—sink below the assumed surface creating a negative anomaly. Blue-green areas show where these anomalies exist.