This invention uses an interferometric fiber optic sensor, particularly a Sagnac or Michelson interferometer, in a first fiber to monitor a sensing length of the first fiber and to detect disturbances. Signals indicating disturbances are classified as being of interest or not of interest, depending on predetermined criteria. Disturbances of interest can be, for example, the breaking of reinforcement wires in concrete pipe, the breaking of wires in suspension cables, a fire, a pipeline leak, or an intrusion. A location sensor system is used to determine the location of disturbances of interest, and to confirm the interferometer signal to reduce noise. The location sensor system is a fiber optic sensor, such as a phase OTDR sensor or a Brillouin effect sensor, which can detect the location of events it senses. It is present either in the first fiber or in a separate fiber laid adjacent the first fiber along its sensing length, as for example in the same optical cable.