A method for detecting defects in a ferromagnetic material , the method comprising: - disposing a plurality of magnetometers about a surface of the ferromagnetic material , wherein each magnetometer of the plurality of magnetometers is fixed in position, relative to the ferromagnetic material ; - using the plurality of magnetometers to sense a magnetic field generated by the ferromagnetic material 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 and representing strength of the sensed magnetic field proximate the location at the first point in time; - using the plurality of magnetometers to sense the magnetic field generated by the ferromagnetic material 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 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 that corresponds to the plurality of data points. - disposing a plurality of magnetometers about a surface of the ferromagnetic material , wherein each magnetometer of the plurality of magnetometers is fixed in position, relative to the ferromagnetic material ; - using the plurality of magnetometers to sense a magnetic field generated by the ferromagnetic material 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 and representing strength of the sensed magnetic field proximate the location at the first point in time; - using the plurality of magnetometers to sense the magnetic field generated by the ferromagnetic material 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 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 that corresponds to the plurality of data points. <EOS>