Ultrasounds, traveling in the medium they image, are attenuated by the very same medium and the further they progress, the faintest the echo comes back.
In order to compensate for this attenuation, the echoes are usually amplified by by a variable gain which renormalize these echoes.
The plots below represent in blue, unsimplified signals, and, in green, the amplified signal, with a gain shifting from 0% to 100% in 200us. It seems that makes the echoes great again =)
More in the log.