Subsurface imaging using measured near‐field antenna footprints

Images of the subsurface are made for the detection of land‐mines using a bistatic stepped‐frequency continuous‐wave spiral‐antenna system. While the system moves along the surface, the emitted electromagnetic wavefields are scattered by objects in the subsurface and cause changes in the voltages measured at the receiver. These changes are formulated as a convolution of a sensitivity function and a complex contrast function. Within the Born approximation, this sensitivity function is equal to the inner product of the wavefield emitted by the transmitter and the field from the receiver operating in transmitting mode. For true amplitude imaging purposes, knowledge of the wavefields in the subsurface is needed. Since it is difficult to obtain a model which describes the radiation characteristics accurately, we measure the footprints of the antennae at one level in the near‐field region and propagate the emitted wavefields using Huygens’ principle. We use both synthetic and experimental data to localize objects in a homogeneous space. First, we apply time‐domain synthetic‐aperture‐radar (SAR) imaging in its most basic appearance. Next, we apply a single‐step inversion algorithm to the data, where we use the measured radiation characteristics of the antenna system. This results in an increase in resolution. We refer to this method as ‘minimized back‐propagation’.