Buried improvised explosive devices (IEDs), due to their relative ease of construction, availability, and destructive capacity, remains the main and current (and likely future) asymmetric threat directed at US and coalition forces. The IEDs can be hidden anywhere: in vehicles, on animals, planted in roads or strapped to a person and can be deployed everywhere: in a combat environment or in the middle of a busy city. The adaptability of IEDs to almost any situation makes them difficult to detect, identify and neutralize using standard subsurface sensing technologies, such as low frequency electromagnetic induction (EMI, DC to 100kHz) and ground penetrating radar (GPR, operating above 50MHz). Much research over the past few years has been focused on exploring, developing, and building new systems for buried IED detection. One such technology is the high frequency EMI (HFEMI) sensor developed under an Office Naval Research project. As a part of the project, IEDs detection studies were conducted at a DoD test site using the HFEMI system. The objective of this paper is to illustrate subsurface IED targets detection and identification capabilities using the HFEMI data and models. Namely, first the paper demonstrates the HFEMI data sets collected over IED targets, including intermediate conducting, low-metal content targets, and explosive filled voids; Then, the advanced EMI models and signal processing approaches are adapted to the HFEMI data sets; and finally, applicability of the advanced models are illustrated by postprocessing and inverting HFEMI data sets.
High-frequency electromagnetic induction (HFEMI) extends the established EMI frequency range above 100 kHz to perhaps 20 MHz. In this higher frequency range, less-conductive targets display heretofore unseen responses in their inphase and quadrature components. Improvised explosive device constituent parts, such as carbon rods, small pressure plates, conductivity voids, low metal content mines, and short wires respond to HFEMI but not to traditional EMI. Results from recent testing over mock-ups of less conductive IEDs or their components show distinctive HFEMI responses, suggesting that this new sensing realm could augment the detection and discrimination capability of established EMI technology. The electrical conductivity of soil may contribute, in effect, to the imaginary part of the permittivity of soil and may then, in turn, generate perceptible responses in traditional EMI. In HFEMI, both the real and complete imaginary parts of soil permittivity produce notable effects. Pursuing this, lab tests with tap water and variously saturated Ottawa sand were compared with results from time domain reflectometry.
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