Passive acoustic logging processed by interferometry. a: interference noise section. The section is normalized and displayed with a color scale ranging from 0 (blue) to 1 (red). Arrows indicate the presence of acoustic waves. b: from top to bottom: zoom of the cross-correlation noise section in 0–0.5 ms time interval, picked times DT_max of the maximum of the cross-correlation noise section, flattening of the cross-correlation noise section with DT_max.