The boundary element method is used to analyze the ACPD (alternating current potential drop) field perturbed by a semi-elliptical surface flaw in a metallic sheet. The depth of the penetration of the alternating current is assumed to be small as compared to the crack depth. This allows us to adopt the "unfolding" technique developed by Dover et al. [ASTM STP 722, 401-427 (1981)], which reduces the problem to a two-dimensional surface Laplacian field. This surface potential field is determined for different surface cracks with crack aspect ratios ranging from 0.2 to 2. To facilitate the use of these results, an empirical formula is developed based on the numerical results. A simple inverse analysis methodology is also developed for determining the crack depth for semi-elliptical surface flaws of known surface length with measured potential drop readings.