Ultrasonic echography is one of the methods which are more and more used for non destructive evaluation of materials, according to the requirements of the users who are often increasingly concerned by quantitative evaluation of defects. This requires a better knowledge of the measurement system, as well as of the ultrasonic field generated by the transducer inside of the material and this has led us to develop methods for the calculation of this field. We present a method for the determination of the ultrasonic field in a solid medium separated by a fluid-solid plane interface, from the knowledge, by modelization or by measures, of the field in a plane of the fluid medium. The method is based upon a decomposition by a plane waves obtained by spatial Fourier Transform, which permits a simple use of the SNELL-DESCARTES refraction laws at the interface. The method has shown quite good agreement with experimental results. Moreover, this method has permit us to predict undesirable effects in the case of oblique incident beams.