1. n. [Formation Evaluation]
A type of acoustic propagation along the borehole that is visualized as a shaking of the borehole across its diameter. The flexural mode is excited by a dipole source, and measured by dipole receivers oriented in the same direction. Its speed is chiefly a function of the formation shear velocity, the borehole size and fluid velocity, and the frequency. It is used to estimate formation shear velocity, and is the only technique available in slow formations where shear velocity is less than borehole-fluid velocity. In this situation, shear head waves are not generated by a monopole source, so that standard monopole techniques cannot be used. The flexural wave is sensitive to properties of the altered zone, as well as to formation anisotropy, whether intrinsic or stress-induced.