An elastic wave, seismic wave. A sound wave in which the disturbance propagated through a medium is an elastic deformation of the medium. The [[wave train]] which results from the emission of an acoustic pulse by a well logging device is detected by one or more [[detector|detectors]] mounted on the [[sonde]] and various aspects of it are measured (see [[acoustic log]]). The wave train is a composite of various modes of energy transfer. The first arrival usually results from [[compression wave|compressional waves]] traveling in the [[formation]]; the inverse of its velocity is measured by sonic tools. A second arrival is sometimes identified as [[shear wave|shear waves]] traveling in the formation. The velocity of the compressional waves traveling through the [[mud]] is frequency dependent and has relatively high frequency content. On the other hand, surface waves of relatively low frequency travel along the borehole wall at lower velocities and are usually called [[tube wave|tube waves]] or pseudo-[[Stoneley wave|Stoneley waves]]. See also [[wave train]].