Elastic waves are mechanical disturbances that propagate through a material. Such waves are able to travel over very long distances through the Earth, and thus bring us information about parts of the formations that are otherwise inaccessible. In seismic surveys, elastic waves are generated at the surface, and the echoes from various layers in the formations below are used to map the structure of the underground. Earthquakes are generated by abrupt failure processes and propagate as elastic waves.

Elastic waves in air and water are usually called acoustic waves, or sound waves. These terms are often used about elastic waves in rocks too. Elastic waves in rocks propagate with a velocity that is given by elastic stiffnesses and the density of the rock. These parameters depend on other parameters such as porosity. Thus elastic waves also provide a method by which specific formation parameters can be estimated in the field.

As we proceed, we shall see that this evaluation is not straightforward for example, the stiffness probed by an elastic wave is generally different from the static stiffness. However, the fact that elastic waves are mechanical disturbances means that there has to be a fundamental connection between rock acoustics and rock mechanics.

In this chapter we first present the fundamentals of elastic wave propagation in porous media (Sections 5.1 5.5). Section 5.6 addresses the main focus of this chapter: the connection between rock acoustics and rock mechanics. Finally, we give some background to understand sonic wave propagation in boreholes and seismic waves...