(57) A system is provided for locating and for characterising the source of attempted
and/or actual penetration of the security perimeters of relatively large geographical
areas. The system comprises an array of structural moment detectors (401, 402, 403)
and electronic circuitry (405) for processing and manipulating signals derived from
the detectors (401, 402, 403) in such a manner as to produce secondary signals indicative
of any seismic forces caused by a penetration of the security penmeter, the effect
of ex traneous seismic forces being excluded. In one embodiment, the electronic circuitry
(405) associated with each structural moment detector (401, 403) is provided in the
same housing as the detector to form a unitary device termed a structural information
detector.
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