1. n. [Formation Evaluation]

A formation where the velocity of the compressional wave traveling through the borehole fluid is less than the velocity of the shear wave through the surrounding formation. In such conditions a shear head wave is generated, so that standard techniques based on monopole transducers can be used to measure formation shear velocity. In hard formations, several normal modes are excited in addition to the Stoneley and leaky modes.

Antonyms: slow formation

See: acoustic mode,  compressional wave,  head wave,  leaky mode,  shear wave,  sonic log,  sonic measurement