Trial of Ground-penetrating Radar, Neutron and Magnetometry Methods in Arid Soil in Egypt

by John F. Crawford

Metal detection and digging are somewhat unsatisfactory approaches to locating landmines. This report presents and examines alternative detection solutions, such as ground-penetrating radar as well as neutron and magnetometry methods. A case study of these techniques in a laboratory setting and in Egyptian soil reveals their effectiveness.

HYDAD-D (left) and PRIS (right) devices, with their operating crews. The string marking the test lane can be seen in both images. A minor problem was reading HYDAD-D’s oscilloscope screen in strong sunlight.
HYDAD-D (above) and PRIS (below) devices, with their operating crews. The string marking the test lane can be seen in both images. A minor problem was reading HYDAD-D’s oscilloscope screen in strong sunlight.
All photos courtesy of John F. Crawford

The Problem

Current methods of finding landmines, based largely on metal detection and careful digging, are not completely satisfactory. Various other techniques are being used or under studynotably ground-penetrating-radar, neutron and magnetometry methods. As discussed below, soil moisture has an adverse effect on the first two techniques.

Ground-penetrating radar. Unlike pulsed radar, continuous-wave radars work by transmitting at a certain frequency (typically for a millisecond) and then stepping to the next frequency. The reflected signal is measured in amplitude and phase at each frequency and stored. After the specified frequencies (typically 256) have been scanned, a fast Fourier transform is carried out on a laptop. This converts from frequency information to time (i.e., space) information, which is displayed to the operator on a laptop screen. GPRs detect discontinuities in the soil’s dielectric constant, such as a landmine. In nonmagnetic soil, radio frequency energy penetrates a distance set by the skin depthwhere ρ is the soil resistivity, λ is the RF wavelength and Ζ is the impedance of free space (about 120Π Ω). With increasing soil moisture, both ρ and δ diminish, and the RF does not penetrate the ground very well.

HYDAD-D (left) and PRIS (right) devices, with their operating crews. The string marking the test lane can be seen in both images. A minor problem was reading HYDAD-D’s oscilloscope screen in strong sunlight.

Neutron methods. Explosives contain significant amounts of hydrogen; for example, TNT contains 2.2% hydrogen by weight, or, more meaningfully, 24% by number. Therefore explosives moderate neutrons effectively. This effect can be exploited for explosive detection as follows: Fast neutrons from a suitable source, such as Californium or Americium-Beryllium, are moderated by the soil, and then detected by slow neutron counters (usually 3He proportional counters). An excess of hydrogen, e.g., in a mine’s explosive, will yield an enhanced count rate. However, this effect depends on the soil not containing too much hydrogen, the presence of which will prevent the neutrons from penetrating and will moderate them, weakening and obscuring the signal. Thus in soil containing the same amount of hydrogen as the explosive, e.g., as water, there will be no signal.

Magnetometry. Magnetometry is an established technique in archaeology. In addition to finding very small amounts of iron, a magnetometer is sensitive to rust. Thus even heavily corroded steel mines, such as those commonly found in North Africa, should be detectable.

Egypt as a Test Area

Both GPR and neutron methods should perform better where the soil is dry, as is the case in North Africa for most of the year. Beyond that, magnetometry should be effective against the steel-cased mines that are common there. Given the successful tests of these techniques in the laboratory,1 a natural next step appeared to be to organize a combined test in Egypt of as many devices as possible, especially GPR and neutron methods. The idea was first discussed at the International Atomic Energy Agency’s Technical Meeting on Combined Devices for Humanitarian Demining and Explosives Detection in Padova, Italy, in November 2006, where many of the laboratory tests were reported.

Table 1: Available anti-personnel and anti-tank mines.All tables courtesy of John F. Crawford/CISR
(Click image to enlarge)
Table 1: Available anti-personnel and anti-tank mines.
All tables courtesy of John F. Crawford/CISR

Three stepped-frequency continuous-wave GHz GPRs were available. A team from Raumfahrt Systemtechnik in Salem, Germany, brought two such devices, both of which they had designed and built: the Hand-held Operational Demining System (HOPE) and Potash Roof Inspection System.2 HOPE works from 2 to 6 GHz and PRIS from 0.55 to 3.8 GHz. Beyond that, the Egyptian National Research Institute of Astronomy and Geophysics provided a Geophysical Survey Systems, Inc. MF20 GPR working at 0.1 to 0.8 GHz and 1 to 2 GHz, and a fluxgate magnetometer. Two neutron detectors were available: the Hydrogen Density Anomaly Detector-D3 from the University of Cape Town, South Africa, and the Egyptian Scanning Landmine Detector (ESCALAD),4 built by a collaboration of Dutch and Egyptian institutes.

Methodology

Table 2: APM lane results. Column 3 shows the cover depth; Column 6 the depth estimated by PRIS.
(Click image to enlarge)
Table 2: APM lane results. Column 3 shows the cover depth; Column 6 the depth estimated by PRIS.

In a convenient part of the Inshas Centre of the Egyptian Atomic Energy Authority, two test lanes were marked out in flat, dry, sandy soil by means of strings stretched between pegs. Every 2 m, a position was marked with a knot. At each position an object could be buried (either a mine or scrap metal) or else nothing was buried there; selection among these three options was done at random. Objects were covered to depths of 0, 10 or 20 cm, again at random. Surface objects were covered by just enough soil to conceal them. The soil at empty positions was disturbed enough to avoid providing a visual clue. The trials were “single blind” in the sense that the testees were not told which objects were in which positions. Five anti-personnel mines and five anti-tank mines (see Table 1) were buried in the test lanes; two additional mines were required for simultaneous tests with other equipment. The two photos on the previous page show HYDAD-D and PRIS with their operators.

Test Lane Results

Tables 2 and 3 show the results of the above tests. Neither HYDAD-D nor PRIS detects landmines as such; rather they detect proxies in the form of hydrogen anomalies and radar reflections, respectively. Accordingly, the statements “Positive” and “Negative” in the Tables refer to the apparent presence or absence of these proxies. “True” and “False,” on the other hand, refer to the presence or absence of a mine.

The following are remarks on the APM test lane results:

The following are remarks on the ATM test lane results:

Extra Results

Although the trial concentrated on the test lanes, some further useful results were obtained:

Table 3: ATM lane results. Column 3 shows the cover depth; Column 5 the depth estimated by PRIS. Because of the size of these mines, there were no false results and PRIS was able to estimate their depths fairly well. Time did not permit HYDAD-D to be tested on this lane.
(Click image to enlarge)
Table 3: ATM lane results. Column 3 shows the cover depth; Column 5 the depth estimated by PRIS. Because of the size of these mines, there were no false results and PRIS was able to estimate their depths fairly well. Time did not permit HYDAD-D to be tested on this lane.

 

Comments and Conclusions

Acknowledgments

This work relies heavily on the contributions of many people, in particular the following:

Visiting teams:

Local participants:

 

Biography

John CrawfordOriginally from the United Kingdom, John Crawford is a retired Swiss scientific civil servant who started out as a particle physicist in the 1960s, went on to work in cancer therapy and accelerator physics, and then on landmine problems. These problems are important and interesting enough to keep him involved during his retirement.


Endnotes

  1. “Combined Devices for Humanitarian Demining and Explosives Detection.” Proceedings of an International Atomic Energy Agency Technical Meeting, 13–17 November 2006, INFN, Padova, Italy (2007). http://www-pub.iaea.org/MTCD/publications/PDF/Pub1300_label.pdf. Accessed 30 April 2010.
  2. RST-Group. 2009. http://rst-group.net/index2.htm.
    Accessed 1 July 2010.
  3. F.D. Brooks and M. Drosg, Appl. Rad. and Isotopes 63 (2005) 565–74.
  4. V.R Bom, A.M. Osman and A.M.A. Monem, IEEE TNS 55 (2008) 741–747.
  5. F.D. Brooks, private communication. November 2007.
  6. RST Report GPR-RST-RP-0022, available from RST, Bahnhofstrasse 108, D-88682 Salem (Germany); E-mail: rst@w-4.de
  7. This mine can be supplied with an 18-g metal plate to make it easier to find.
  8. V.R. Bom, e-mail message to author. 27 April 2010.

Contact Information

John F. Crawford, B.Sc., Ph.D.
Hardstrasse 68c
CH-5430 Wettingen / Switzerland
Tel: +41 56 281 2024
Mobile: +41 79 782 8007
E-mail: john.crawford@psi.ch