  We extend earlier work on nonlinear tube wave propagation in permeable
formations to study, analytically and numerically, the generation and
propagation of a difference frequency, Delta omega = omega_1 - omega_2, due to
an initial pulse consisting of carrier frequencies omega_1 and omega_2. Tube
waves in permeable formations have very significant linear
dispersion/attenuation, which is specifically addressed here. We find that the
difference frequency is predicted to be rather easily measurable with existing
techniques and could yield useful information about formation nonlinear
properties.

Condensed Matter