Time‐domain electromagnetic detection of a hidden target

Abstract

Numerical modeling of the time‐domain electromagnetic (EM) step response of a vertical tabular target hidden beneath a thin conductive overburden reveals that the target’s presence may be detected only during a well‐defined time window. In a situation where the secondary magnetic field is sensed by an airborne system equipped with horizontal coaxial dipoles, a conductance contrast of about ten between the target and the overburden is needed to ensure target detection. This value will, of course, vary with the size and depth of the target and, to a lesser extent, with the geometry of the system. In general, the time at which the window opens is a function of the geometrical parameters of the target, the height of the system, and the conductance of the overburden. For a given target, its width (defined as the ratio of the time of closure to the time of opening) is only a function of the conductance contrast between the target and the overburden. While the target signal is visible, one observes a maximum value of the target‐to‐overburden response ratio. The time at which this occurs is mainly controlled by the conductance of the target. The presence of the overburden causes the target signal to build up gradually before decaying toward zero. However, once the target signal dominates the overburden response, the signal can be approximated by a simple exponential decay over the time range of interest. The time constant of this decay is determined by the size and conductance of the target. Using this model, it is easy to relate the magnetic field step response calculated here to the response observable with a conventional EM system that transmits a primary field pulse of finite duration and detects the time derivative of the secondary magnetic field.