Buried and Underground Sensors

Buried sensors provide covert perimeter detection — invisible to an intruder, unaffected by weather above ground, and immune to visual obstruction. The sensor is below the surface; the intruder walks, runs, or drives over it without knowing it is there.

This makes buried detection the technology of choice for sites where visible security infrastructure is undesirable (diplomatic facilities, executive residences), where perimeter aesthetics matter, or where above-ground sensors would be damaged or defeated.

How Buried Detection Works

All buried perimeter sensors exploit the same physical principle: a person or vehicle moving on or near the ground surface generates mechanical energy that propagates through the soil. The sensor converts this energy into a signal that is processed to detect and locate the intrusion.

The type of energy detected — seismic waves, electromagnetic field changes, or acoustic vibrations — varies by technology. Each approach has different sensitivity, range, and soil compatibility characteristics.

Sensor Technologies

Ported Coaxial Cable

Two parallel coaxial cables are buried 15–30 cm below the surface, typically 1–2 metres apart. One cable transmits an electromagnetic field; the other receives it. A person walking between or above the cables disturbs the electromagnetic field pattern, and the processor detects the change.

Senstar's OmniTrax is the most widely deployed ported coaxial system, protecting thousands of kilometres of perimeter at government, military, and correctional facilities worldwide. The system divides the cable into zones of 50–200 metres and can localise an intrusion to the triggering zone.

Strengths: Reliable, mature technology with a long operational track record. Works in most soil types. Provides consistent detection regardless of weather.

Limitations: Requires trenching for installation. Performance varies with soil moisture (very wet or frozen ground changes electromagnetic propagation). Two parallel trenches add installation cost.

Seismic/Geophone Sensors

Discrete seismic sensors (geophones) are buried at intervals along the perimeter. Each sensor detects ground vibrations caused by footsteps, vehicle movement, or digging. Signal processing distinguishes human footstep patterns from vehicle vibrations, animal movement, and environmental seismic noise.

SensoGuard produces buried seismic sensors specifically for perimeter security. Each sensor covers a radius of 10–25 metres depending on soil conditions, and the system triangulates target location using arrival time differences between adjacent sensors.

Strengths: Very low false alarm rates from surface events because the sensors respond primarily to ground-coupled energy. Excellent for detecting tunnel-digging activity (the seismic signature of excavation is distinctive and persistent).

Limitations: Coverage gaps between sensors if spacing is too wide. Performance highly dependent on soil type — rocky or very hard ground transmits seismic energy over long distances (increasing false alarm range), while soft or sandy soil attenuates it rapidly (reducing detection range).

Fiber Optic (DAS)

Distributed Acoustic Sensing (DAS) uses a fiber optic cable buried below the surface as a continuous linear sensor. A laser interrogator at one end detects acoustic and seismic disturbances along the entire cable length with localisation accuracy of 1–3 metres.

As a buried technology, DAS offers the longest detection range from a single processor — up to 40+ km of cable from one interrogator. This makes it uniquely suited for very long perimeters: borders, pipelines, railways, and large industrial facilities.

Sintela's Onyx system and OPTEX/Fiber SenSys EchoPoint both support buried fiber deployment. The fiber itself is passive and requires no power along its length.

Strengths: Continuous coverage with fine localisation. No active components in the field. Compatible with standard telecommunications fiber (enabling retrofit on sites with existing buried fiber). Single interrogator covers enormous distances.

Limitations: Fiber is fragile and can be damaged by third-party excavation. Burial depth affects sensitivity — deeper burial reduces surface event sensitivity. Ground conditions affect acoustic coupling.

Soil and Environmental Factors

Soil type is the most critical variable in buried sensor performance. Well-compacted clay and loam provide good energy coupling — seismic waves from footsteps propagate efficiently and sensors respond reliably. Sandy soil absorbs energy, reducing detection range. Rocky ground transmits energy over long distances, potentially causing false alarms from distant legitimate activity.

Soil moisture affects electromagnetic and seismic propagation. Saturated soil can either improve or degrade performance depending on the technology. Ported coaxial systems are sensitive to standing water (which changes the electromagnetic baseline). Seismic sensors may perform better in moist soil due to improved acoustic coupling.

Frost changes soil properties dramatically. Frozen ground transmits seismic energy further than thawed ground (potentially increasing nuisance alarm range). Freeze-thaw cycles can physically displace shallow-buried sensors. Systems installed in regions with freeze-thaw must be buried below the frost line.

Vegetation roots can damage buried cables over time. Tree roots can displace or break fiber optic cables. Sensor planning must account for root zones and future vegetation growth.

Installation

Buried sensor installation is more disruptive and expensive than above-ground alternatives. Trenching, cable routing, backfill, and surface restoration add significant cost and time. For retrofit installations, the surface above the sensor line must be restored to its original condition.

Planning must address underground utilities, drainage systems, and other buried infrastructure. A thorough utility survey is essential before any excavation.

Most buried systems require a commissioning period after installation during which the soil settles and the system baseline stabilises. Immediate performance may not reflect long-term capability.

Key Vendors

Senstar — OmniTrax ported coaxial cable. The market leader in buried electromagnetic detection. Installed at hundreds of sites globally.

Southwest Microwave — INTREPID MicroTrack buried cable system for covert detection. US-based.

SensoGuard — Buried seismic sensors for perimeter and area protection. Israeli.

Sintela — Onyx DAS system for buried fiber optic detection. Australian, recently deployed at Sydney transport infrastructure.

OPTEX / Fiber SenSys — EchoPoint DAS system for buried and fence-mounted fiber optic detection. Japanese parent company.

When to Choose Buried Sensors

Buried detection is the right choice when covert perimeter protection is required, when above-ground sensors would be damaged by the environment or human activity, when the site cannot have visible security infrastructure, or when tunnel detection is a requirement.

Buried detection is not the right choice for temporary installations, for sites where trenching is impractical (solid rock, contaminated ground), for perimeters that change frequently, or when budget does not accommodate the installation cost premium over above-ground alternatives.