The celebrated Rauch–Taylor/Bardos–Lebeau–Rauch geometric control condition
is central in the study of the observability of the wave equation linking this property
to high-frequency propagation along geodesics that are the rays of geometric optics. This
connection is best understood through the propagation properties of microlocal defect
measures that appear as solutions to the wave equation concentrate. For a sufficiently smooth
metric this propagation occurs along the bicharacteristic flow. If one considers a merely
-metric,
this bicharacteristic flow may however not exist. The Hamiltonian vector field
is only continuous; bicharacteristics do exist (as integral curves of this continuous
vector field) but uniqueness is lost. Here, on a compact manifold without boundary,
we consider this low-regularity setting, revisit the geometric control condition,
and address the question of support propagation for a measure solution to an ODE
with continuous coefficients. This leads to a sufficient condition for the observability
and equivalently the exact controllability of the wave equation. Moreover,
we investigate the stability of the observability property and the sensitivity of the
control process under a perturbation of the metric of regularity as low as Lipschitz.