  The electrical potential on the surface of $\sim 300$ nm thick SiO$_2$ grown
on single crystalline Si substrates has been characterized at ambient
conditions using electric field microscopy. Our results show an inhomogeneous
potential distribution with fluctuations up to $\sim 0.4 $V within regions of
$1 \mu$m. The potential fluctuations observed at the surface of these usual
dielectric holders of graphene sheets should induce strong variations in the
graphene charge densities and provide a simple explanation for some of the
anomalous behaviors of the transport properties of graphene.
