The magnetic method involves the measurement of the intensity of the earth's total magnetic field, a component of the earth's magnetic field, or the horizontal or vertical gradient of the earth's magnetic field.

Anomalies in the earth's magnetic field are caused by induced or remanent 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 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. The magnetic method is an effective way to search for small metallic objects, such as buried ordnance and drums, because magnetic anomalies have spatial dimensions much larger than those of the objects themselves.

Bay uses the magnetic method to

• Locate abandoned steel well casings
• Locate buried tanks and pipes
• Locate pits and trenches containing buried metallic debris
• Detect buried ordnance
• Map old waste sites and landfill boundaries
• Clear drilling locations
• Map basement faults and geology
• Investigate archaeological sites