This equation was derived from Maxwell's equations for the case of a uniform time-harmonic electromagnetic field interacting with a semi-infinite slab with a flat interface. Although this equation was derived for this specific case, it is often used more generally to estimate the thickness of material where effects from induced eddy current will begin to be significant.
Magnetic Field B(z) = B(surface) x exp(-z/skin depth) x sin((2pi x frequency) - z/skin depth)
Eddy Current Density J(z) = J(surface) x exp(-z/skin depth) x sin((2pi x frequency x time) - z/skin depth)
where the "z" is the distance into the material from the interface surface and the
electromagnetic skin depth = sqrt(2/(2 x pi x frequency x electrical conductivity x magnetic permeability))