Ground-Penetrating Radar: how do radio and micro-waves reveal what is hidden?
It is -almost- widely known that many archaeological discoveries are supported by radar technology. Without going any further, it was disclosed on May 2018 that a radar-based study revealed the non-existence of secret chambers on Tutankhamun’s tomb [1]. It was not the first time radars helped on this kind of field works: the placement of a Vikings-age manor was recreated in Sweden during 2016 [2] by analyzing Ground-Penetrating Radar (GPR) measurements.
GPR is not only used for archaeological purposes but provides relevant information in different areas like geotechnical and civil engineering, security and forensic, hydrology, geology, and defense. GPR is widely used for terrain profiling, finding corpses or trapped persons, mapping groundwater and caves, and detecting personal landmines [3][4][5].
In general terms, GRP technology can be applied to any buried-object detection or sub-surface study up to certain deepness and resolution. One of the most important characteristics is that GPR is a non-destructive searching method, which does not require to modify the terrain under analysis.
But how do GRP works? What are its limitations and how much information can we extract from GPR measurements? We will read on this article the technological basis for this method and what kind of limitations we can encounter when working with GPR.
Technical background
GPR is based on classical radar technology. Therefore, they transmit electromagnetic energy with controlled frequency and power. An antenna, placed as close as possible to the ground or surface under test, radiates the electromagnetic signal in the desired direction. A second — or even the same — antenna receives the echoes from the objects placed in the way of the signal. It is, detecting the electromagnetic energy reflected back from the surface of any material.
Electromagnetic radiation consists of electromagnetic waves, that propagate at the speed of light. The speed of light depends on the refractive index of the medium, and it is widely known to be 299,792,458 meters per second in the vacuum.
The energy reflected depends strongly on the dielectric constant of the material. When there is a dielectric discontinuity between different materials, part of the energy is reflected. Because of this, the power of the echoes detected variates along time. When the material is more conductive, the electromagnetic wave dissipates quickly. For that reason, GPRs usually achieve better results when working over low-conductive materials [6]. In those cases, the radiation penetrates deeper than in conductive or wet materials.
The dielectric constant K is equal to the refractive index to the square [7]. The dielectric constant of air is K = 1, and the velocity of propagation is 300 mm/ns. However, the constant K = 81 for water and the velocity is 33 mm/ns. An intermediate material like asphalt has K = 4, with a velocity of 150 mm/ns.
When radiation propagates from one medium to another with different velocity, part of the energy gets reflected back to the previous medium. For example, when an electromagnetic wave moves from air to asphalt, there will be a small energy reflection. However, when moving from air to water, there will be a strong reflection.
Received echoes are processed and analyzed over time, generating a profile picture of the different surfaces encountered. Knowing the velocity of the electromagnetic wave on the material, the processor can estimate — up to certain limits— the position of the reflection point. This is possible because the receiver measures the time that takes the signal to reach the object and get back to the antenna.
There are different modulation strategies for GPRs. Most of them are pulsed radars, in which the duration of a short radio-frequency pulse determines the depth resolution of the radar. The carrier frequency is modulated with a square amplitude envelope to work with low duty cycle. However, transmitted pulses are sometimes carrier-free: many GPRs use high-voltage gaussian-based pulses with very high bandwidth.
Despite that pulsed radar are the most common on the market, continuous-wave (CW) radars are also typical architectures for GPRs. Linear-sweep Frequency Modulated CW (FMCW) is one of the preferred modulations for this kind of designs since it is one of the cheaper and simpler options. On the other hand, stepped-frequency CW (SFCW) have some advantages like higher signal-to-noise ratio and wider dynamic range [6][8].
Depending on the purpose of a specific radar, the operating frequency may vary. There is a compromise between the detection depth and the size resolution of the radar [9]. Shorter wavelengths — higher frequencies — will detect smaller objects, but the signal will be strongly attenuated so that the detection range will be shortened. That is the reason for using lower frequencies when there is need to scan several meters depth. Operating frequencies for commercial solutions go from several MHz up to ~4 GHz. [10]
The result of GPR measurements is typically a 2D cross-section image in which the vertical axis (Y) represents the deepness into the surface from the antennas, and the color of every X-Y point represents the intensity of the signal. It is usually drawn using a blue-red color code or simple gray-scales. Furthermore, the transmitter moves along the surface, and the distance along the soil or tested material is represented on the horizontal axis (X).
Interpretation of RPG images is not intuitive, and modern software tools use signal processing and image processing techniques to highlight the most relevant information from the scans, and even auto-detect clear targets [6].
As a conclusion, GPR is a powerful technique which makes use of the electromagnetic theory, classical radio-frequency and radar technology to study sub-surface objects and structures. However, there is still place for improvement and research in the processing and automatic detection field. GPR targets are still not easy to identify, and future systems will rely on specific-purpose software tools to reduce detection times and achieve clear and valuable results.
Innovation is always spinning forward. Just like a Drill.
References
[1] K. Romey, “It’s Official: Tut’s Tomb Has No Hidden Chambers After All”, News.nationalgeographic.com, 2018. [Online]. Available: https://news.nationalgeographic.com/2018/05/king-tut-tutankhamun-tomb-radar-results-science/. [Accessed: 13- May- 2018]
[2] E. De Lazaro, “Ground-Penetrating Radar Helps Archaeologists Find Viking Age Manor”, sci-news.com, 2018. [Online]. Available: http://www.sci-news.com/archaeology/viking-age-manor-04551.html. [Accessed: 13- May- 2018]
[3] ”GPR Applications”, Geotec.co.il, 2018. [Online]. Available: http://www.geotec.co.il/?categoryId=23242. [Accessed: 13- May- 2018]
[4] ”What is GPR: A Brief Description by GSSI”, GSSI Geophysical Survey Systems, Inc., 2018. [Online]. Available: https://www.geophysical.com/whatisgpr. [Accessed: 13- May- 2018]
[5] J. Keller, “Army to upgrade ground-penetrating radar system for detecting hidden IEDs buried in roadways”, Militaryaerospace.com, 2018. [Online]. Available: http://www.militaryaerospace.com/articles/2018/01/ground-penetrating-radar-ieds-upgrade.html. [Accessed: 13- May- 2018]
[6] O. Lopera Tellez and B. Scheers, Ground‐Penetrating Radar for Close‐in Mine Detection, 1st ed. IntechOpen, 2017, p. Chapter 3 [Online]. Available: https://www.intechopen.com/books/mine-action-the-research-experience-of-the-royal-military-academy-of-belgium/ground-penetrating-radar-for-close-in-mine-detection. [Accessed: 18- May- 2018]
[7] “TLP Library Dielectric materials — The dielectric constant and the refractive index”, Doitpoms.ac.uk, 2009. [Online]. Available: https://www.doitpoms.ac.uk/tlplib/dielectrics/dielectric_refractive_index.php. [Accessed: 20- May- 2018]
[8] “Radar Basics — Ground penetrating radar”, Radartutorial.eu. [Online]. Available:http://www.radartutorial.eu/02.basics/Ground%20penetrating%20radar.en.html. [Accessed: 20- May- 2018]
[9] L. Conyers, “Ground-Penetrating Radar”, Pdfs.semanticscholar.org, 2004. [Online]. Available: https://pdfs.semanticscholar.org/9f6b/4df04874ea87bbc46f43d76fe3183d8afe5e.pdf. [Accessed: 20- May- 2018]
[10] “What is GPR: A Brief Description by GSSI”, GSSI Geophysical Survey Systems, Inc.. [Online]. Available: https://www.geophysical.com/whatisgpr. [Accessed: 20- May- 2018]
[11] “Global Penetrating Radar | GPR for Sinkholes | GPR Survey Florida”, Geoviewinc.com. [Online]. Available: http://geoviewinc.com/methods/land/ground-penetrating-radar. [Accessed: 20- May- 2018]
