  Bilayer lipid membranes (BLMs) are an essential component of many biological
systems, forming a functional barrier between the cell and the surrounding
environment. When the membrane relaxes from a structural perturbation, the
dynamics of the relaxation depends on the bilayer structure. We present a model
of a BLM in a viscous solvent, including an explicit description of a 'thick'
membrane, where the fluctuations in the thickness of a monolayer leaflet are
coupled to changes in the lipid density within that monolayer. We find
dispersion relations describing three intuitive forms of bilayer motion,
including a mode describing motion of the intermonolayer surface not noted
previously in the literature. Two intrinsic length scales emerge that help
characterise the dynamics; the well known Saffman-Delbruck length and another,
$\ell_r$, resulting from the intermonolayer friction. The framework also allows
for asymmetry in the BLM parameters between the monolayer leaflets, which is
found to couple dynamic modes of bilayer motion.
