Buried sensors are electronic devices that are designed to detect potential intruders. The sensors are buried along the perimeters of sensitive assets and are able to detect intruder activity both above- and below-ground. Some of these systems are composed of individual, stand-alone sensor units, while other sensors consist of buried cables.

There are four types of buried sensors that rely on different types of triggers. These are: pressure or seismic; magnetic field; ported coaxial cable; and fiber-optic cables. These four sensors are all covert and terrain-following, meaning they are hidden from view and follow the contour of the terrain. The four types of sensors are described in more detail below.

Pressure or seismic sensors are passive detectors that respond to a change or a disturbance in the soil caused by an intruder. Pressure sensors consist of a container filled with liquid, which is connected to a transducer. A seismic sensor consists of geophones that are made up of conducting coils. Pressure sensors are more sensitive to lower frequency pressure waves than are seismic sensors. Pressure or seismic types of sensors would be most useful for detecting intruders by foot.

Magnetic field sensors are also passive detectors that respond to a change in the local magnetic field. This change may be caused by the movement of metallic material nearby, such as movement of an intruder with a metallic weapon. Magnetic field sensors consist of series of wire loops or coils buried in the ground. These sensors can be susceptible to false alarms due to electromagnetic disturbances, such as lightning.

Ported coaxial cable sensors are active sensors that respond to nearby material with a high dielectric constant or high conductivity. Two cables-one acting as a transmitter, the other as a receiver-are run parallel to one another and are spaced approximately two meters apart. The signal leaking from one to the other creates a field between the two cables, and active disturbance of the field triggers the sensor. Materials that trigger these types of sensors can be found in people and metal vehicles.

Optical fibers are long, hair-like stands of transparent glass or plastic that use optical technology to guide light from one end of the fiber to the other. Pressure on the fiber causes a distortion in the light signal, which is detected and analyzed at the far end of the fiber. These sensors are typically woven into a mesh grid to ensure complete coverage of an area to be protected. The fibers require a burial depth of only a few centimeters to be effective. These sensors are ideal for wet environments since the non-metallic, fiber-optic cable is designed as a direct burial cable with a 20-year life expectancy.

Buried sensors are designed to follow the existing terrain and are feasible options if a site is hilly. The sensors are buried at a relatively shallow depth (ranging from a few inches to one foot, depending on the type). These types of sensors are also covert, because they are buried and potential intruders cannot see them.

These systems may be continuous or discrete. A continuous system consists of a continuous cable, such as a fiber optic or ported coaxial cable. These types of systems monitor along a continuos line corresponding to the location of the cable. Discrete sensors consist of individual sensor units that can be buried in non-linear patterns to increase the area monitored. For example, Qual-Tron sensors are designed to monitor all activity within a 30-foot radius in any direction.

A drawback to these type of systems is that they may have different sensitivities when they are buried below different media. For example, if continuous systems are buried below different types of media (such as under a lawn, then under an intersecting concrete driveway, and then back to lawn again), the sensitivities required for different types of media may be different. For example, a good sensitivity adjustment for concrete may be too sensitive for grass. Therefore, it may be best to individually zone those areas.

Another factor that must be considered when using a buried sensor is underground utilities. Underground utilities, such as gas, water, and sewer lines, must be located at a sufficient depth below the detection zone (typically three feet), so as to not cause false and nuisance alarms. Below-ground electrical wires must also be compensated for; however, the potential for nuisance alarms caused by underground power lines is not as great as with other types of utilities.

Several other factors must be considered when using a buried sensor. Rodents have been known to cause maintenance problems by gnawing on the sensor cables; this problem is limited primarily to the Western states. Installations also should not be in areas where running water will either wash away the soil that buries the sensor or cause nuisance alarms during a heavy rain. Table 1 presents the distinctions between the four types of buried sensors.

In order to be effective security measures, sensors must be tied into some type of alarm system or other system that alerts facility personnel when the sensors have been tripped. Many sensor systems include these features; in other cases, these features can be added on and the alert system can be networked to go off when the sensors are tripped.

Stand-alone sensors, such as pressure/seismic and magnetic field sensors, are often sold as individual components, and the cost will depend on the number of sensors required and the sophistication of the transmitting, receiving, and recording equipment. For example, a typical system will require sensor/transmitters and a receiver. In some cases, the system may need a relay if the transmitter and receiver are located at a sufficient distance from one another.

Qual-Tron Inc. provides two types of pressure/seismic or magnetic sensor systems. The first, Mini-Intrusion Detection System (MIDS) includes sensors that transmit on a single channel at a fixed frequency. This system can handle up to 32 sensors. The EMIDS system can transmit on multiple channels and frequencies, and can handle up to 999 individual sensors. Costs for these systems are provided in Table 2 below.

Both of these systems can be installed and maintained by the customer. Qual-Tron indicates that training on installation and maintenance can be accomplished in a day or less. Costs for ported coaxial cable and fiber optic cable systems are provided in Table 3 below.