Used to determine the presence of cement behind casing and the quality of cement bond to casing or formation wall. Usually an acoustic log.

The cement bond log is a continuous measurement of the amplitudes of acoustic pulses after they have traveled a length of the casing. The amplitude of a pulse is strong after traveling along unsupported pipe because there is nothing to restrict the vibration of the casing. On the other hand, the vibration of the casing is dampened by the cement sheath in well-cemented pipe, and amplitude is weak. If the formation bond is poor, acoustic energy traveling through the formation is weak; if both the casing and formation are well bonded, only the formation signal is strong. The log may consist of (1) an amplitude log which represents the amplitude of a portion of the acoustic wave train, or (2) a display of the acoustic wave train in the amplitude-time (wave train, x-y) mode or the intensity modulated-time (variable density, x-z) mode.