The magnetic method involves the measurement and mapping of variations in the earth’s natural magnetic field. Variations in the magnetic field intensity can be due to natural ferrous minerals or the presence of ferrous metals. Magnetometers typically are lightweight and large areas can be investigated quickly. The magnetic method is an effective way to search for small metallic objects, such as buried ordnance, tanks, utilities, and drums, because magnetic anomalies have spatial dimensions much larger than those of the objects themselves.
Common types of magnetic surveys are total field measurement and gradient. The total field survey normally requires base station data to eliminate diurnal and temporal changes in the earth’s magnetic field. Gradient type surveys employ two sensors separated vertically to reduce cultural variations and emphasize the responses from shallow targets.
Anomalies in the earth’s magnetic field are caused by induced or remnant magnetism. Induced magnetic anomalies are the result of secondary magnetization induced in a ferrous body by the earth’s magnetic field. The shape and amplitude of an induced magnetic anomaly is generally a function of the orientation, geometry, size, depth, and magnetic susceptibility of the body as well as the intensity and inclination of the earth’s magnetic field in the survey area.