A method for detecting defects (102, 104, 106, 108, 110, 700) in a ferromagnetic material (100, 200, 702, 800, 900,1102,1202, 1302, 1402, 1602), the method comprising: - disposing a plurality of magnetometers (1106, 1404, 1406, 1408, 1504, 1506, 1508) about a surface of the ferromagnetic material (100, 200, 702, 800, 900, 1102,1202,1302,1402,1602), wherein each magnetometer of the plurality of magnetometers (1106, 1404, 1406, 1408, 1504, 1506, 1508) is fixed in position, relative to the ferromagnetic material (100, 200, 702, 800, 900,1102,1202, 1302,1402, 1602); - using the plurality of magnetometers (1106, 1404, 1406, 1408, 1504, 1506, 1508) to sense a magnetic field generated by the ferromagnetic material (100, 200, 702, 800, 900,1102,1202,1302, 1402, 1602) at a first point in time; - generating data points of a first two-dimensional map from the sensed magnetic field, each data point corresponding to a respective location on the surface of the ferromagnetic material (100, 200, 702, 800, 900, 1102, 1202, 1302, 1402,1602) and representing strength of the sensed magnetic field proximate the location at the first point in time; - using the plurality of magnetometers (1106, 1404, 1406, 1408, 1504, 1506, 1508) to sense the magnetic field generated by the ferromagnetic material (100, 200, 702, 800, 900, 1102, 1202, 1302, 1402, 1602) at a second point in time, later than the first point in time, - generating data points of a second two-dimensional map from the sensed magnetic field, each data point corresponding to a respective location on the surface of the ferromagnetic material (100, 200, 702, 800, 900, 1102, 1202, 1302, 1402,1602) and representing strength of the sensed magnetic field proximate the location at the second point in time; - subtracting the data points of the second two-dimensional map from spatially corresponding data points of the first two-dimensional map, thereby yielding a difference two-dimensional map; - identifying, in the difference two-dimensional map, a plurality of data points that conform to a predefined spatial pattern of magnetic field strength; and - outputting a location proximate the surface of the ferromagnetic material (100, 200, 702, 800, 900, 1102, 1202, 1302, 1402, 1602) that corresponds to the plurality of data points. - disposing a plurality of magnetometers (1106, 1404, 1406, 1408, 1504, 1506, 1508) about a surface of the ferromagnetic material (100, 200, 702, 800, 900, 1102,1202,1302,1402,1602), wherein each magnetometer of the plurality of magnetometers (1106, 1404, 1406, 1408, 1504, 1506, 1508) is fixed in position, relative to the ferromagnetic material (100, 200, 702, 800, 900,1102,1202, 1302,1402, 1602); - using the plurality of magnetometers (1106, 1404, 1406, 1408, 1504, 1506, 1508) to sense a magnetic field generated by the ferromagnetic material (100, 200, 702, 800, 900,1102,1202,1302, 1402, 1602) at a first point in time; - generating data points of a first two-dimensional map from the sensed magnetic field, each data point corresponding to a respective location on the surface of the ferromagnetic material (100, 200, 702, 800, 900, 1102, 1202, 1302, 1402,1602) and representing strength of the sensed magnetic field proximate the location at the first point in time; - using the plurality of magnetometers (1106, 1404, 1406, 1408, 1504, 1506, 1508) to sense the magnetic field generated by the ferromagnetic material (100, 200, 702, 800, 900, 1102, 1202, 1302, 1402, 1602) at a second point in time, later than the first point in time, - generating data points of a second two-dimensional map from the sensed magnetic field, each data point corresponding to a respective location on the surface of the ferromagnetic material (100, 200, 702, 800, 900, 1102, 1202, 1302, 1402,1602) and representing strength of the sensed magnetic field proximate the location at the second point in time; - subtracting the data points of the second two-dimensional map from spatially corresponding data points of the first two-dimensional map, thereby yielding a difference two-dimensional map; - identifying, in the difference two-dimensional map, a plurality of data points that conform to a predefined spatial pattern of magnetic field strength; and - outputting a location proximate the surface of the ferromagnetic material (100, 200, 702, 800, 900, 1102, 1202, 1302, 1402, 1602) that corresponds to the plurality of data points.