  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.
