Depths shown on the Chirp profiles are relative to the depth of the towfish, not to sea level. Furthermore, the raising or lowering of the towfish during a survey (to avoid obstacles, follow relief, or avoid boat wake) produces a relative shift in the multiple reflections seen in the profile, which may be confused as a geologic feature. Any elevation change of the towfish is typically recorded in the logbooks and is evident on the seismic profiles by steep, abrupt shifts in the seafloor return.