Recently, the surface magnetic field measurement (SMFM) technique for the detection and sizing of surface breaking cracks in ferromagnetic metals was introduced [1–5]. In this technique a rectangular coil or a set of U-shaped wires excited by a high frequency ac current produces the eddy current in the area under inspection. The location, depth and width of a crack in the test area are obtained by interpreting the measurements on a tangential component of the magnetic field at the metal surface using a mathematical modeling. The mathematical modeling assumes that in the absence of crack, the magnetic field at the metal surface has magnitude and phase distributions equivalent to those produced at the surface of a perfect conductor. This assumption allows the eddy current incident at a crack to be simply calculated using the negative image technique. The technique is well-known and involves replacing the metal by the negative image of the inducer and finding the resultant field from the inducer and its image [6]. The reason for the applicability of the negative image technique to perfect conductors lies in the fact that an ac current produces an infinitely thin skin current in the metal and as a result no normal field component can exist at the surface of the conductor. For practical non-ferromagnetic conductors, the negative image technique is valid for all frequencies for which the current skin depth is small (a fraction of a millimeter), leading to a negligible normal component at the metal surface. For ferrous metals, in spite of small current skin depth at low frequencies (e.g., skin depth in mild steel is about 0.2 mm at 1.6 kHz), the accuracy of the negative image technique can become questionable. The cause of the ambiguity is the large permeability of ferrous metals.