A closed form formula for determining the depth of a filled rectangular crack

In this paper, an accurate closed form formula is derived to calculate the depth of a dielectric filled rectangular crack. The derived formula is in terms of the crack´s width, echo width measurements, the filling permittivity and the incident angle. The first term of the proposed formula is related to the first null echo width while the second one is a correction term. The calculated depth of the crack is in excellent agreement with the real measurement one up to three decimal points.