2. n. [Formation Evaluation]
A pressure wave in the borehole fluid generated by the passage of either the acoustic compressional wave or the shear wave in the formation. These pressure waves are recorded by logging tools using hydrophones and are the basis for the sonic log. A head wave is generated only when the compressional or the shear speed is faster than the fluid speed. In slow formations, where the shear speed is less than the fluid speed, no shear head wave is created.