There are many types of exterior buried detection systems.

• Electromagnetic systems work by creating an electromagnetic field through buried cables. As the transmitting cable signals couple with the receiving cable, an invisible electromagnetic detection field is generated above and below the ground surface and along the cable pair.
• Fiber optic detection uses fiber optic cables which are extremely sensitive to pressure and motion. Fiber optic detection systems can monitor vibration signals along the length of a fiber optic cable and are able to pinpoint the location of an intrusion event. A major advantage of a fiber optic cable is its immunity to electromagnetic frequencies and radio.
• Leaky coaxial sensor cables are used to create an invisible electromagnetic detection field. Apertures within the cables allow energy from the transmitting cable to be detected by the corresponding parallel receiving cable. A coded pulse signal algorithm can be used to determine the exact location of the security breach or intrusion.
• Seismic intrusion detection uses sensors installed underground to create an invisible fence line. The system can be configured with different sensitivity levels to detect and differentiate between footsteps and vehicles. Software is used to show the location of the breach and map the specific location of intrusion to the protected area.

Below is a sample of NFPA 731. For the complete section, see the actual NFPA 731 text at NFPA.ORG. Once there, click on the free access link to the latest edition of NFPA 731.

5.2.4 Exterior Buried Detectors.

5.2.4.1 Exterior buried detectors shall be in compliance with applicable standards, such as ANSI/UL 639, Standard for Safety for Intrusion-Detection Units.

5.2.4.2 Exterior buried detectors shall include, but not be limited to, the following types:

(1) Electromagnetic

(2) Fiber optic

(3) Leaky coaxial

(4) Seismic