Abstract: A method for evaluating a composite structure includes providing a model of said structure and locally varying a material property to determine complex reflection and transmission coefficients at said locality. From these coefficients at least one ultrasonic response characteristic for said material property can be found and compared to a measured ultrasonic response of a sample to determine a local measure of the material property. This method exploits the fact that certain material properties contribute to the ultrasonic frequency response substantially independently of one another. The frequency response of a region of porosity and of a thick resin layer in particular are evaluated. In one embodiment, the modelled responses are used to provide frequency domain basis functions for material properties, which can in turn be used in a decomposition method.