# Wave equation

The **wave equation** is derived from the *equation of motion*. The equation of motion for a *point mass* is Newton's Second Law:

**(1)**

For an *elastic continuum*, the equation of motion is:

**(2)**

where the force on the left of (1) is replaced by the gradient of the stress tensor , the mass on the right is replaced by the density , and the acceleration on the right is given explicitly by the second time-derivative of the displacement vector . This is three equations for the three components of the unknown displacement , which is *implicit* within the stress, on the left.

To make the displacement *explicit* on the left, we use Hooke's Law:

**(3)**

where the elastic properties of the medium are given by the stiffness tensor , and the (small) strain is given by:

**(4)**

Using (3) and (4), equation (2) becomes:

**(5)**

where the symmetries of simplify the result on the right. This is the *equation of motion*, with the unknown explicit.

If the medium is locally uniform, *i.e.* if is locally constant, then (5) becomes

**(6)**

This is the **wave equation**. When applied to isotropic media, the stiffness tensor simplifies; this form underlies most exploration seismics. It has solutions of P-waves and shear waves.

What is implied by the assumption of "locally uniformity"? Loosely speaking, it means constant on the scale of the seismic wavelength. Of course, the subsurface is in fact highly variable in its elasticity, but we conventionally model this as a set of coarse uniform layers. We then solve the *wave equation* separately in each coarse layer, and match the solutions at the layer boundaries using the boundary conditions of continuity of stress and displacement. However, see the discussion of a more accurate discussion of subsurface heterogeneity on the Equation of Motion page.