In the context of Doppler aliasing, which artifact is described as a spectrum mirror image?

• A. Range ambiguity
• B. Propagation speed error
• C. Refraction
• D. Spectrum mirror image

Answer: (D)

Understanding Doppler aliasing hinges on how spectral data are sampled. The velocities measured are confined to a range set by the Nyquist limit, which is half the pulse repetition frequency. When real velocities exceed this limit, the sampled values wrap around and appear as a mirror image of the spectrum on the opposite side of the baseline. This mirrored spectrum is the spectrum mirror image artifact, and it can make high-velocity components seem to move in the opposite direction or show up where they don’t belong. It’s distinct from range ambiguity (depth confusion), propagation speed error (incorrect sound speed causing depth/time errors), and refraction (beam bending at interfaces).