chapter  1
Symmetry of the Main Equation
Pages 30

In this chapter we will consider the symmetry of the acoustic equation for nonhomogeneous locally-isotropic media. Let us write 42(t, r) as the displacement vector of a point of an elastic medium, r = (x, y, z), where t is the time. +Z obeys the linear differential equation (Brehovskih and Godin 1989)

p 5 = (,? + ,u) grad div 4+P + PA % + grad II * div %? + grad p x rot 4P