Conventional impulse ground penetrating radar (GPR) does a reasonable job with the first two criteria above, but is expensive (at around 30,000 USD or more per system) and this has limited its use. In addition, the raw data do not present an easily-interpreted image to any but highly-trained users. Many clever data processing and target recognition algorithms, as well as friendly user interfaces, have been developed, but again at the expense of cost, speed, or compactness.
Microwave holography may be a technology that can achieve all four criteria. It provides similar or better performance than GPR, at a fraction of the cost (only around 5000 USD per system), and provides rapid, real-time, visually interpretable holographic images of buried objects directly from the raw field data.
The physics are simple. Suppose a continuous, oscillating, emitted signal at the source (on the left of the figure below) is described by:
a0(t) ~ cos (2pft + q0)
where f is the frequency, t the time, and q0 is some unknown phase angle. The wave is reflected by some buried object, and will be going in the reverse direction. At the location of the source, it will have a phase difference equal to twice the distance d to the object (because it goes there and back), divided by the wavelength, which is equal to V/2pf, where V is the velocity in the medium. Thus arriving back at the source, the reflected signal is:
ar(t) ~ cos (2pft + 4pdf/V + q0).
The interference signal at the source is the product of the incident (transmitted) and reflected amplitudes averaged over time since it is a continuous signal. The unknown phase angle q0 disappears and the resulting signal has amplitude A which depends on distance d and frequency f, but not time t, and is given by:
A(d) = cos (4pfd/V)
A diagram of this interference function as a function of the obstacle distance d, for several different frequencies f is shown in the figure below. The wavelength of the oscillation with depth decreases as the frequency increases.