This work presents a demonstration of the potential of fiber cavity ring-down for remote sensing, by using an OTDR to send impulses down ~20 km of optical fiber at the end of which the fiber ring cavity was placed. The OTDR showed almost no losses in the fiber, so other ring-down cavities could be spliced along the same fiber. To study the sensitivity of the cavity ring an intensity sensor based on a taper was placed in the ring and glued to a translation stage. A displacement of the stage imposes a curvature on the taper and an associated loss. The configuration had a sensitivity of (11.8±0.5) μs/mm.