An electromagnetic model for detecting explosives under obscuring layers

Recently, there has been increasing interest in object imaging and detection behind obscuring layers because of applications in detection of improvised explosive devices (IEDs) and suspicious materials. Millimeter (MMW) and terahertz waves show promise in detecting and imaging objects through clothing or packaging materials. In this paper, we discuss an electromagnetic model based on a multi-layer medium which may represent hidden explosives underneath clothing on a person. The dielectric model of the material of interest has been studied. Simulations of the wave propagation through this multi-layer medium model are performed, and the results show indications that MMW and terahertz waves can be used in detection and imaging of suspicious objects underneath normal clothing, which leads to possible detection of IEDs. Introduction Millimeter (MMW) and terahertz wave propagation and imaging through obscuring layers are topics of recent interest with practical applications in security, inspection, and surveillance. Both millimeter and terahertz waves show promise in detecting and imaging objects under obscuring material. One important and practical scenario consists of hidden explosives underneath normal clothing in a standoff position. An electromagnetic model for this scenario is necessary, including the dielectric modeling of materials. We present an electromagnetic model based on multi-layer media, where multiple reflections within each layer are included. It has the flexibility for more complicated structures to be added, including rough surface interfaces and small inclusions in layers, which were used in our previous study on sea-ice remote sensing [1]. The model can also be used to calculate the angular correlation of two waves at two observed angles. Simulations of wave pulses propagated through this multi-layer air cloth air Human skin Incident wave Configuration B, suspicious material not present c ε h ε air cloth plastic Human skin Incident wave Configuration A, suspicious material present p ε c ε h ε structure are presented. This model provides a tool for estimating the response at different frequencies and configurations which is instrumental for modeling the imaging and detection of objects. An electromagnetic multi-layer model The multi-layer model is shown schematically in Fig. 1. This model represents electromagnetic wave propagation through clothing and human skin in a standoff position. Configuration A is where there is a plastic explosive composite present, while configuration B is where there is no explosive present. The analyses in this paper are based on normal incident waves. Fig. 1: Multi-layer media model for hidden objects underneath clothing. First, we find the properties of the materials of interest. The dielectric models are based on the Debye model with parameters fitted experimentally. In the MMW range, the dielectric model for plastic explosive composite (C-4) is given by Federici [2] ( ) 2 2 3 3 _ 1 2 3 , 1 1 1 s p MMW j j j ε ε ε ε ε ε ε ω ε ωτ ωτ ωτ ∞ ∞ − − − = + + + + + + (1) where 3 s ε = , 2 2.97 ε = , 3 2.94 ε = , 2.9 ε∞ = , ( ) 1 1/ 0.72 THz τ = , ( ) 2 1/ 1.26 THz τ = , and ( ) 3 1/ 1.73 THz τ = . This is an extension to lower frequencies of the model used for the sub-terahertz range. The range of validity is approximately 100 GHz to 3 THz. For the terahertz range, we employ the dielectric constant model for C-4 from Yamamoto [3]