Abstract: A microbend-sensitive optical fiber is embedded in a thin pliable padding and laid under an area to be protected. An amplitude-modulated optical light beam is launched into one end of the fiber. The light beam is recovered from the other end of the optical fiber. The angular phase shift between the launched light beam and the recovered light beam is continuously measured and sampled at desired sample intervals. A change in the measured phase shift between any two sequential sample intervals is indicative of the presence of an intruding entity. The magnitude of the phase shift is a function of the mass of the entity. The pattern of repetitive phase shift differences as a function of time provides an estimate of the dynamic characteristics of the intruding entity.