Bottom Echo

The bottom echo is the acoustic return reflected from the seabed in echo sounding, the signal whose arrival time fixes the depth. Its strength and shape depend on seabed hardness, roughness, and grazing angle: a rock or sand bottom returns a sharp, strong echo, while soft mud gives a weak, smeared return and sometimes a sub-bottom second echo. The first-bottom detection sets the charted depth; a multiple (a second-bottom echo from a double reflection) can appear at twice the depth and must be rejected. Correct bottom-echo picking, with the sound-velocity profile applied, drives the vertical accuracy graded under IHO S-44.