Electric current will flow around on open crack in a conductor and give rise to very abrupt variations in the field. If the crack has a negligible opening it acts as a surface barrier where the field is virtually discontinuous. Effectively the crack is then equivalent to a layer of current dipoles with the dipole orientation normal to the surface and pointing upstream. An integral equation for the dipole density has been derived for an idealised subsurface crack using the Green’s function method [1]. Numerical solutions have been found by assuming a piecewise constant dipole density and satisfying boundary conditions on the crack at a finite number of points. Here we shall develop the theory further, making use of a knowledge of the dipole distribution for a given incident field, to calculate probe impedance changes ΔZ, due to subsurface cracks.
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