A type of acoustic energy that propagates in one direction while being confined in the other two directions, in this case by the borehole wall. Leaky modes can be considered as multiply reflected and constructively interfering waves propagating in the borehole. Each time a compressional wave hits the borehole wall, part of the energy is reflected into the borehole, while the rest is converted to compressional or shear energy that radiates into the formation, hence the term 'leaky'. Leaky modes are dispersive, starting at a certain cutoff frequency with the formation compressional velocity and increasing towards the borehole fluid velocity at high frequency. In slow formations, where no head wave is generated because the borehole fluid is faster than the formation compressional wave, the low-frequency end of the leaky mode can be used to determine formation compressional velocity.

The term 'hybrid mode' is used to describe a form of leaky mode that is associated with an altered zone.