What is Deep GPR?
As radar penetration is inversely proportional to the operating frequency, by “deep” GPR, we generally refer to low-frequency GPR. In most cases, this means operating frequencies less than 80 MHz. Such systems were amongst the first to be developed in the 1920s for radio echo-sounding of glaciers.
History of Deep GPR
Advances made during NASA’s Apollo program were commercialized in the 1970s, initially for archeological applications with shielded 100 MHz antennas. Indeed, the majority of initial radar systems though the 1980s and early 1990s were low-frequency systems, partially because such systems are relatively easier to build.
Commercialization and Stagnation
The mid-1990s saw the commercialization of high-frequency antennas used for utility detection, rebar imaging, and pavement studies. These civil infrastructure markets now dominate the vast majority of radar systems sales worldwide.
This lack of market demand, coupled with the restrictions imposed by limitations on transmitted radar power in lucrative jurisdictions (e.g. USA and the EU) brought in during the mid-2000s, have meant that the development of deep radar technologies has stagnated since the 1980s.
Equipment designs have essentially remained the same, including the use of outdated receiver sampling technologies, limited transmitter powers, the reliance on fragile fiber optics, and the need for powerful laptop computers for data capture and storage.
Development of UtraGPR
UltraGPR has addressed each of these shortcomings by employing real-time full waveform capture to enable effective stacking of 32,000 times and powerful transmitters, as well as the replacement of all fiber optics and wires with Bluetooth and Wi-Fi.
The weight of the entire radar system has been reduced to less than 4 Kg through the use of titanium and carbon fiber throughout the system. Data capture and storage is accomplished using any Android tablet or smartphone.
UltraGPR and the Laws of Physics
However, it is important to appreciate that despite UltraGPR’s advancements it does not and cannot defy the laws of physics. UltraGPR does penetrate significantly deeper than consumer-grade GPR systems, but the gains are relative to the environment being imaged.
In situations where the limitation on penetration is the “noise floor”, or the level at which background noise is stronger than the deepest radar reflections, the high stacking rate of UltraGPR will produce two or more times penetration than a similar commercial system. In sands and gravels, this could translate to an increase from 15 metre to over 40 metre penetration. Similarly, in wet clays, whereas a commercial system may produce 50 cm penetration, UltraGPR may be capable of 1 metre. No radar system can defy the immutable limitations of skin depth in conductive media.
Increasing Depth Penetration and Stacking
Depth penetration for a GPR system may be increased in three ways. One could lower the operating frequency of the radar system. One could increase the transmitted power of the radar system, or one could increase the stacking or averaging of the receiver.
Decreasing the radar frequency is the most common approach to increase penetration, although at a loss in profile resolution and an increase in the physical size of the antennas. Although increasing the transmitted power appears to be a simple solution, the governing physics of radar propagation dictates that an increase in penetration requires an exponential increase in transmitted power.
In order to achieve double the original penetration, the transmitter power must be increased 32 times. Current impulse radar systems use transmitters with outputs in the 100s of volts, and a pulse rate of usually 100 kHz. In order to pulse in the 10,000s of volts, the pulse rate must be dramatically reduced to avoid saturating the electronics, thereby offsetting the advantage of the higher power.
The most practical way to increase penetration is through stacking. If one could stack 1000 times, in theory, penetration would double over no stacks. Current radar systems are limited in practice to 32 stacks, whereas UltraGPR stacks 32,000 times.
Extraordinary Claims Demand Extraordinary Evidence
Over the past decade, dubious technologies have emerged in Europe and Russia which make bold claims of astounding penetration through clays, sea water, solid metal, etc. Often, these technologies are vaguely described by their proponents using a variety of unrelated terms, concocted to confuse those not familiar with the principles of physics. The caveat remains: Extraordinary claims demand extraordinary evidence.
One such technology relies on the use of an extremely powerful transmitter of many megawatts, to achieve astounding penetration through clays. There are two obvious issues with this approach, aside from the fact that such spark-gap transmitters have existed since the 1940s. The power of the transmitter and the published radar dynamic range is stated as 120 dB. Numerous references quote clays as having an attenuation of 10 – 200 db/m. Given a two-way travel for the radar energy, even with exceptionally ideal clays of 10 dB/m, a radar system with a dynamic range of 120 dB cannot image greater than 6 m.
A more problematic limitation is that spark-gap 10 MW transmitters cannot stack, they can fire a single impulse only. The analogy often used is that a standard GPR is like a flashlight, whereas a 10 MW transmitter is like a flash. This is entirely true, except that the ground scatters, much like a foggy night. A flash only illuminates the immediate surroundings with a glow for a moment, whereas a strong torch would resolve distant objects.