<p>A conductor (12) has an electrical characteristic which measurably changes in the event of a mechanical disturbance (18). The conductor may have a length of 1 km or more. A signal generator (14) injects a frequency modulated continuous wave signal into the conductor (12, at 19). Reflected signals are received (at 19) by a detector (16) which uses the injected signal and the reflected signal to identify the occurrence of a disturbance, for example by demodulating the reflected signal against the injected signal.</p>
