A seismic and pressure wave sensor adapted to be buried in the earth to detect disturbances comprises a deformable tube containing an incompressible liquid in which a force-responsive transducer is immersed. The output of the transducer is an alternating signal having spectral components corresponding to both seismic (10-100 Hz) and pressure (< 1 Hz) type earth vibrations produced by human footsteps remote from and near to the buried sensor. In order to provide perimeter protection, an even-numbered plurality of such sensors are arranged in a row along a single curved or straight line and are electrically connected in parallel with the polarities of successive transducers reversed. A single processor connected to the outputs of such plurality of sensors has frequency-separated band-limited channels for isolating and detecting seismic and pressure signals, respectively, and for audibly, visually or otherwise indicating the pressure of either or both signals. An array of several multisensor units, each unit having a frequency coded processor, is connected to a remote alarm station which decodes the alarm signal to identify the sensor unit at which the intrusion occurred.