Shallow density and susceptibility contrasts produce narrow, or high-frequency, anomalies, while deeper contrasts result in broader, low-frequency anomalies. We can use this characteristic of the gravity and magnetic fields to determine depths to sources, both qualitatively and quantitatively. Because gravity signatures come from everywhere, from the air to the Earth's core, such inferences are often not straightforward or reasonably unique. But most magnetic anomalies come from only a few rock types, such as volcanics, intrusives, and basement rocks. Magnetic data therefore can be used to estimate depth to basementa classic use for such data. The illustration shows how the anomalies change from narrow over shallow basement to broad over deeper basement.