In the field of non-destructive testing there exists the particular problem of examining adhesively-bonded, multi-layered structures for bonding defects and delaminations. An example of such a structure is part of a proposed Rolls-Royce fan blade intended for use in the first stage of a turbo-fan aero engine. The bulk of the blade is made up of a carbon-fibre composite material but the leading edge is reinforced by several adhesively-bonded, stainless steel layers. It is necessary in this case to be able to find—non-destructively—the existence of any non-bonded areas and, of equal importance, their depth locations.