We propose a comparative study of X-ray tomography and ultrasonic reflection methods, for determining the porosity of a composite plate realized in LOMC with an industrial process. We measure the attenuation of ultrasound propagating in the thickness by using 10MHz plane transducer in pulse-echo mode. Comparing these results to the 2D porosity tomographic map allows establishing a relation between attenuation and porosity. A C-scan picture of the plate given by the echoes reflected by the rear surface also provides a local information on the attenuation. Furthermore, we propose a method for the mapping of the reflecting sources as the included bubbles and the interfaces resin/fibers.