Active vs Passive acoustic logging. a:3 m constant offset acoustic section, b: constant offset section filtered in the 0.5–7.5 kHz interval, displayed in the 2.5–5 ms interval showing Stoneley waves, c: active acoustic logging: velocity log of the Stoneley wave (V-St), passive acoustic logging: velocity of the LFA wave (V-DT_max), Relative uncertainty DV/V between the 2 velocity logs. The acoustic sections are normalized and displayed with a color scale ranging from 0 (blue) to 1 (red).