branch of geophysics concerned with all aspects of the Earth's magnetic field, including its origin, variation through time, and manifestations in the form of magnetic poles, the remanent magnetization of rocks, and local or regional magnetic anomalies. The latter reflect the difference between theoretical and observed magnetic intensities at points of measurement with a magnetometer, and, when plotted on a magnetic map (called an aeromagnetic map if the magnetometer was flown across the area), the anomalies provide the basis for inferences about probable subsurface structure and composition.