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The interfaces reflect and modify the acoustic field. The intensity of the acoustic field received in a point inside the liquid depends on the material, the shape and the number of these interfaces. This intensity is most of the time very difficult to evaluate. This lack of knowledge does not allow a precise determination of the size of the measuring volume.

These interfaces may generate, in certain situations, artifacts and induce modifications in the velocity profiles as presented in the figures.

The ultrasonic beam BC reflected by the far interface transforms this interface in a transmitter. The same particles contained in the liquid will backscatter a second time energy in the direction to the transducer. The depth associated to the path ABC is located outside the flowing liquid. Imaginary velocity components are added to the real velocity profile.

The measurement of velocities near the far interface is affected by this phenomena. The size of the ultrasonic beam determines mainly the level of this artifact.

The interfaces often give strong reflections. Despite of the many reflections which are necessary to reach the transducer, the energy reflected by these interfaces is often stronger than the energy coming from the particles flowing with the liquid.

Most of the algorithms used to compute the Doppler frequency shift do not allowed stationary components. The elimination of these stationary components by high-pass filtering implies an increase in the dynamic range of the analyzed echoes and a reduction in the sensitivity in the measurement of low velocities.

When some interfaces are in movement, the correct estimation of all the velocity field is very difficult. The echoes generated by such interfaces may affect the velocity profile in many places due to the combination of many reflections.

The Doppler frequency shift induced by these movable interfaces can not be removed if their values have the same values as the flowing particles.