(∂ nom’ ∂ lē) 1. A deviation from uniformity in physical properties; a perturbation from a normal, uniform, or predictable field. 2. Observed minus theoretical value. 3. A portion of a geophysical survey, such as magnetic or gravitational, that is different in appearance from the survey in general. 4. A gravity measurement that differs from the value predicted by some model, e.g., a Bouguer or free-air anomaly (q.v.). 5. In seismic usage, generally synonymous with structure. Also used for unexplained seismic events. 6. A deviation that is of exploration interest; a feature that may be associated with petroleum accumulation or mineral deposits. 7. An induced-polarization anomaly is usually positive and greater than background (or the normal effect) to be economically interesting. In the frequency domain, an anomalous region has resistivity that decreases with frequency. An interesting resistivity anomaly is generally smaller than background.