연구의 선진화와 생산성 향상에
필요한 최적의 소프트웨어와 컨설팅을
공급하도록 노력하겠습니다.

Detection of IEDs

  • The Improvised Explosive Device (IED) is the principal tool of the Taliban in Afghanistan. Therefore their detection is very important. In principle the IED itself can be detected by the emission of vapors or, if it contains metal, it may be possible to detect it electronically. To reduce the likelihood of personnel losses, the Taliban would prefer to detonate the IED remotely by radio, for example, by cellular telephone. However, these signals can be jammed to neutralize the threat. Therefore detonation is often by command wire. The IED is connected to a battery using a pair of wires running over a sufficient distance from the IED to minimize risk to the triggerman. The detection of command wires themselves is an alternative to the detection of the IED. Furthermore command wires constructed of conducting material, usually copper, offer the possibility of electronic detection. There are two main ways of proceeding. Firstly the wire can be excited using a signal generated locally and secondly signals generated remotely can be employed. In the former case the configuration resembles a metal detector in which the source and the receiver are located close together. In the latter case, the signal is generated some distance away. The exciting signal can be transmitted specifically for the detection process or signals of opportunity can be used. In the latter case, these signals might be broadcast by commercial transmitters in the medium frequency band.
  • While it is possible to design a detection system on a purely empirical basis, reliable estimates of the Electro-Magnetic (EM) fields from excited wires are very desirable. Unfortunately the theory of EM fields from cylindrical conductors located close to or just under conducting ground is quite complicated. Though the basic approach is reasonably straightforward, in that the fields can be expressed as integrals, it is often difficult to evaluate these integrals to obtain useful engineering results in analytic form. The problem is exascerbated by the number of separate cases that must be handled and by the necessity of using saddle point methods, which often leads to functions with complex arguments. These functions may be unfamiliar and in turn require some effort to evaluate.
  • The presence of the ground can be represented as a conducting half space. The wire acts like an antenna and its properties as a receiver are affected by it. This is a classical problem first addressed by Sommerfeld who showed that propagating radio waves induce currents that are associated with surface waves and extend the range of broadcast stations. Broadcast stations typically transmit vertically polarized waves but the surface waves have a longitudinal component (due to the induced ground currents) that permit a horizontal wire to receive them; the wires act like a Beverage antenna. The excited wire scatters the signal in the presence of the ground, which influences the scattering. Practical estimates of the signals from broadcast transmitters can be found in the International Telecommunications Union (ITU) documentation. If the propagation constant and the characteristic impedance of the wire can be estimated, this often permits the current in the wire to be estimated. The scattered fields can then be estimated.
  • Because the ground is conducting and significant currents are induced, the fields in the ground fall off rapidly with distance from the wire. This is the skin effect and at medium frequencies the skin depth is typically some tens of metres. For long horizontal wires this implies that the ground acts as a return conductor.
  • The theory has been developed by a number of authors. Researchers such as Sommerfeld and Banos described the basic concepts in their books and, more recently much progress has been made by the two groups headed by James Wait and Ronold King. These workers have also published books on the subject. However, some of the basic conceptual details can only be found in the early work. This is summarized in J.K.E. Tunaley, "A Summary of EM Theory for Dipole Fields near a Conducting Half-Space", January 2012.