An area is established by positioning a string of boundary objects that are reflective to a range of ultrasonic frequencies. Tracking objects may be placed inside the area which also are reflective to the range of ultrasonic frequencies. An array of transducers insonify the area and the reflected signals are received by each transducer, digitized and stored as digital data. The digital data is read out in time reverse order and time reversal signals are generated that are coupled to each corresponding transducer. The time reversed signals focus energy on the reflective sources. The reflected signals resulting from the time reversed signals are analyzed to generate a reference signature indicating relative locations of boundary and tracking objects. The time reversed signals are sent intervals to generate new signatures which are compared to the reference signature to determine if tracking objects have been moved outside the area.