1. n. [Formation Evaluation]

A formation in which the velocity of the compressional wave traveling through the borehole fluid is greater than the velocity of the shear wave through the surrounding formation. In such conditions, there is no critical refraction of the shear wave and no shear head wave generated, so that standard techniques based on monopole transducers cannot be used to measure formation shear velocity. Instead, it is necessary to use dipole sources to excite the flexural mode. The velocity of the latter is closely related to that of the shear wave. In very slow formations, such as in high-porosity gas sands, the formation compressional velocity also may be less than the borehole fluid velocity, causing no compressional head wave. In such cases, it is possible to estimate the formation compressional velocity from the low-frequency end of a leaky mode.

Antonyms: fast formation

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