Gradient Magnetometer [Passive]:
Magnetometers are passive devices that sense distortions in earth’s natural magnetic field. These detectors exploit the homogeneous property of Earth’s magnetic field. A magnetized ferromagnetic object’s magnetic field creates a distortion in Earth’s magnetic field. The distortion is localized around the object and decreases as distance from the object increases. The level of distortion depends mainly on the object’s size and permeability. The distortion is sensed using a differential approach. Two sensors are arranged a certain distance away from each other and connected in a way to measure a value of zero in a homogenous field. As the magnetometer passes over a ferrous object, each sensor is affected differently producing a non-zero value. The sign (i.e. positive or negative) of the value can indicate the orientation of the object.
Pulse Induction Metal Detectors [Active]:
Metal detectors employ the principle of electromagnetic induction in order to find visually obscured metallic objects (targets). Metal detectors often operate in the Extremely Low Frequency (ELF) and/or the Very Low Frequency (VLF) region of the electromagnetic spectrum. Pulse induction (PI) metal detectors send a current pulse to a search coil quickly creating and collapsing a strong magnetic field. This quickly changing magnetic field induces tiny currents, known as eddy currents, in nearby targets. These eddy currents produce their own magnetic field which is picked up by the search coil.
The United States government fielded a large number of metal detectors to protect warfighters in various theaters. Insurgents responded by reducing or even removing the metallic content from their mines and homemade Improvised Explosive (IED) devices. This produced a capability gap which resulted in an increase of Victim Operated IEDs (VOIEDs) containing low or non-metallic components such as carbon rod/ wire pressure plates, deeply buried homemade explosive (HME) jugs, and other challenging threats.
Ground Penetrating Radar (GPR)
The introduction of non-metallic threats required a different type of technology to be integrated. A technology called Ground Penetrating Radar (GPR) uses electromagnetic waves of a much higher frequency to detect sub-surface objects. Antennas are used to transmit electromagnetic waves into the ground and receive their reflections. Electromagnetic waves exhibit reflections when transitioning between materials having different dielectric constants. These reflections are sent to a digital signal processor (DSP) where intricate algorithms are performed to determine if a target is detected.