Introduction

The GPR technology has been used for about 15 years in civil engineering, geology
and archeology for the detection of buried objects and soil study.

GPR works by emitting into the ground, through a wideband antenna, an
electromagnetic wave covering a large frequency band. This can be done by using a
short pulse or a pure sine wave whose frequency is varied continuously or by steps to cover the desired range. Reflections from the soil caused by dielectric variations such as the presence of an object are measured. By moving the antenna it is possible to
reconstruct an image representing a vertical slice of the soil..

A wide frequency band is needed to achieve a good resolution, but since higher
frequencies do not propagate well, the chosen range is always a tradeoff between
resolution and penetration depth. For antipersonnel mines, a center frequency of 1 to 2 GHz seems to be a good choice for most types of soil.

The GPR is able to detect non-metallic materials as long as their dielectric
characteristics are sufficiently different from the surrounding media.

It must be noticed that systems used in civil applications do not have any automatic
recognition algorithms implemented; this is therefore the domain where most of the
work has to be done, to allow the use of this technology for mine detection.
 

GPR basic principles

Basically, GPR systems work by emitting a short electromagnetic pulse in the ground
through a wideband antenna. Reflections from the ground are then measured to form a vector. The displacement of the antenna allows to build an image by displaying
successive vectors side by side.

Any dielectric discontinuity in a propagating media, as for example the presence of an object, will cause a reflection; its intensity will be higher with increasing difference
between the dielectric coefficients. Typical soils permittivity vary from about 4 to 40,
wet soils having a higher permittivity than dry ones (water has a permittivity of 80).
Permittivity of plastic objects is in the range of 2 to 6.

Typical pulses have a width of the order of a nanosecond or less, with rise time of
some hundred of picoseconds, which correspond to a frequency spectrum of some
hundreds of MHz to 1-2 GHz.

High frequencies are needed to achieve a good spatial resolution, but penetration
depth of electric fields being inversely proportional to the frequency, too high
frequencies are useless after some centimeters. Hence the choice of the frequency
range is a tradeoff between resolution and penetration depth. A system working with
an antenna whose center frequency is at 1 GHz is considered by experts as being a good compromise for anti-personnal mines, allowing to work at depths of 50 cm to 1
m in most soils with a resolution of the order of some centimeters.

Penetration depth will also depend on the nature of the soil which have different
attenuation. For example desert sand has an attenuation of about 1 dB/m for 1 GHz
frequency, clay having attenuation of 100 dB/m at the same frequency.

The reflected wave is sampled and digitized by an A/D converter. The region of
interest is quite short, for example in sand, 1 meter depth corresponding to about 20
ns. Typically 512 points are taken through the region of interest, which correspond to
a sampling rate far too high for standard converters. The solution is to repeat the
generation of the pulse (typical repetition rates range from some tens of KHz to 1
MHz) and to acquire only one sample in each reflected wave, the sampling time being shifted by some tens of ps for each pulse. This allows to use standard and relatively
low cost converters.

By moving the antenna along a line and taking regularly spaced acquisitions, it is
possible to construct an image representing a vertical slice of the ground. These
images show hyperbolas whose apex is located at the objects positions. These
hyperbolas result from the fact that antennas have a certain aperture and capture all
reflections coming from a cone-shaped area below them. These hyperbolas can be
focused by software (migration algorithms) to obtain the real image of the objects.

Typical GPR Output

An alternative to the most used (real pulse) GPR is to work with so called synthetic
pulses. The response to pure sine waves of discrete frequencies of the spectrum of a theoretical pulse are measured. Using the appropriate signal processing algorithms
the response to the theoretical pulse can be reconstructed.
 

Research studies:

• The FOA team in Sweden has started a project on this subject, using a 0.3-3 GHz GPR system. Currently published results show the feasibility of the detection of mine-like objects; the next step will be the addition of adaptive and target classification algorithms .

• The Lawrence Livermore National Laboratory (LLNL) has developed and patented a new technology, the Micropower Impulse Radar (MIR). This radar is characterized by its compact size, light weight and low cost. The small footprint of the antennas (less than 50 cm2) allows to build quite easily an array for a faster and simplified scan of the minefield. First published results on anti-tank mines show interesting pictures produced by a 3D imaging software generating horizontal slices of the soil, although the processing time and needed computer power are not specified.

• The European Microwave Signature Laboratory (EMSL) has conducted experiments on antipersonnel mines using frequencies ranging from 2 to 6 GHz.  They have demonstrated good imaging capabilities for surface-laid mines and mines buried at a shallow depth in sand. But it must be noticed that at such frequencies the penetration depth could be insufficient in other materials and that the move to the field could be difficult, these experiments having been carried out in ideal laboratory conditions (anechoic chamber).

 
• A study conducted in the 1970's at the Ohio State University has already demonstrated the possibility of recognizing targets of about 30cm buried in clay by detecting resonances in the spectrum of the reflected signal. These resonances being specific to each target type. New studies are in progress at that university for the detection of unexploded ordnance using that method .

 
• Another US team, at EG&G, conducts research in the same direction, studying in addition resonances in metal detectors response .

 
• Elta has developed a highly effective, unique solution providing a very high probability of detection for plastic and metal mines.

The low weight radar is mounted on a remotely controlled, low signature         vehicle capable of operating unharmed in adverse terrain.
The high survivability system can advance through mine fields while marking in realtime the detected mines and the vehicle's pathway.
Additional sensors such as a high sensitivity magnetometer, an infra-red camera and a regular T.V. camera can be mounted on the vehicle to achieve enhanced overall system performance.
The EL/M-2190 advanced, innovative high resolution radar is designed for military and commercial applications.

The radar detects, measures, analyzes and classifies objects buried in the ground. The radar has a proven capability in a variety of terrains, soils and environments.
 
• Coleman Research Corp. developed the DIGS (Drop-In GPR Sensor) -

a wide band stepped frequency GPR, with automatic target recognition algos that plugs into a standard Schiebel metal detector.

DIGS - Principles of Operation
 


DIGS was developed for the US Army and meant to be commercialized soon.
 

• GDE Systems' integration of GPR with pulsed induction (EMI) metal detection

provides a compact and lightweight solution that supports mine detection, day or night, in virtually all soil and weather conditions.
By illuminating the ground and target with electromagnetic energy, GDE's detector measures field disturbances, determining target presence and location.
For fully controlled detection operations, where favorable weather conditions and time-of-day can be ensured, Infrared imaging can also be fused with GPR to provide stand-off capability and speedup mine sweeping operations.
Detection data can be used standalone or fed into a GIS database to be merged with terrain data or operational imagery.
 
• Jaycor has developed a proof-of-principle stand-off mine detection system funded by the Department of Defense (DoD) through U.S. Army Communications and Electronics Command (CECOM) at Ft. Belvoir, Virginia. The Jaycor system is the only known system that can detect and identify mine types at a substantial range (up to 30 meters/100 feet) with a low false detection rate. It can detect both surface and buried mines.

A concept vehicle has been built to allow for the demonstration and the improvement of Jaycor's frequency-agile concept of a stand-off mine detection ground penetrating radar. The total concept system weighs about 200 pounds and is constructed on a Hummer. The radar has been developed using only
off-the-shelf commercially available components.
 


 


The system uses a stepped continuous wave (cw) signal, with three horn antennas. The stepped cw approach, going from 0.5 to 4 GHz, allows the system to excite the resonant frequencies of all mine targets. This stepped cw approach creates the ability to obtain detectable signals from all targets, as well as the ability to identify the targets based on resonant frequencies. Of the
three horns, the middle horn transmits, while the outside two receive, which allows for azimuth detection, as well as range determination and identification. The result is a system capable of detecting, locating, and identifying targets at safe stand-off ranges of up to 100 feet. The total radiated power is 1 watt,
although there are plans to increase the power to 10 watts.