GROUND
PENETRATING
RADAR (GPR)
Ground Penetrating Radar (GPR):
Ground Penetrating Radar (GPR) is a geophysical method that uses radar pulses to image the subsurface. GPR is a non-destructive geophysical method that uses electromagnetic radiation in the microwave band (wavelength: 1mm to 1 meter and Frequencies: 300 MHz to 300 GHz) (UHF/VHF Frequencies; where UHF = Ultra High Frequency designates the ITU radio frequency range of electromagnetic waves between 300 MHz and 300 GHz, also known as decimeter band or decimeter wave; VHF = Very High Frequency is the ITU designated of radio frequency electromagnetic waves from 30 MHz to 300 MHz) of radio spectrum and detect the reflected signals from the subsurface structures. GPR can be used in a variety of media including soil, rock, ground water (fresh), ice, structures and pavements. This technology can detect voids and cracks, changes in material and objects.
GPR uses high frequency (usually polarized: "polarization is the property of waves that can move with more than one orientation such that electromagnetic waves and light waves etc, but this is not the case with sound waves that only travel only in the direction they're propagating to) radio waves and transmits into the ground. When the waves hit a buried object, boundary, interface or horizon with different dielectric constants (a dielectric material (dielectric in short) is an electrical insulator that can be polarized by an applied electric field) , the receiving antenna records variations in the reflected return signal. The principles (as shown in Fig.1) involved are similar to reflection seismology except that electromagnetic energy is used instead of acoustic energy and reflections appear at boundaries with different dielectric constants instead of different acoustic impedances.
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Fig.1: A GPR in OperationDepth Range of GPR:
The depth range of GPR is limited by the electrical conductivity of the ground, the transmitted center frequency and the radiated power. As conductivity increases, the penetration depth decreases. This is because the electromagnetic energy is more quickly dissipated into heat, causing a loss in signal strength at depth. Higher frequencies do not penetrate as far as lower frequencies, but give better resolution. Optimal depth penetration is achieved in ice where depth of penetration can achieve several hundred meters. Good penetration is also achieved in dry sandy soils or massive dry materials such as granite, limestone and concrete where the depth of penetration could be up to 15 meter (49 feet). In moist clay laden soils and soils with high electrical conductivity, penetration is sometimes only a few centimeters. Ground Penetrating Radar (GPR) antennas are generally in contact with the ground for the strongest signal strength, however GPR air-launched antennas can be used above the ground.
Applications of Ground Penetrating Radar (GPR)
Like other Geophysical Methods, Ground Penetrating Radar (GPR) has many applications in a number of fields. In the Earth Sciences (Geophysics/Geosciences) it is used to study bedrock (first hard rock under soilar-regolith), soils, ground-water and ice. In alluvial gravel beds (the sediments brought up via rivers especially when floods come and surrounding flooded areas are then covered via soft sediments (alluvium) on the retrieval of the water to its original channel (normalcy), also alluvium gravel beds occur in deltas of rivers) GPR is of some utility in finding gold nuggets and diamonds by searching natural traps in buried stream beds that have the potential for the accumulation of heavier particles (gold and diamond particles). That is why GPRs can also been used on the surfaces of other planets and moons(such as Moon and Mars etc). Similarly other applications of GPR include Non-Destructive Testing (NDT) of structures and pavements. locating buried structures and utility lines, and studying soils and bed rock.
Although GPR has many applications in environment, archaeology and military as well. But here I'm restricted to discuss its applications only in Geophysics. Geologists and Geophysicists rely on GPR (Ground Penetrating Radar) to gather high resolution subsurface information rapidly. Compared to other geophysical methods nothing comes close in terms of the amount of ground coverage that is obtained with GPR surveying. GPR is the most versatile geophysical technique, used in wide variety of near surface application areas. Even though physical properties of the subsurface will limit resolution with depth, GPR remains as the unmatched champion of high resolution subsurface profiling, object detection and mapping. GPR is generally used for investigations of the subsurface down to roughly 30 meters depth, but in favorable media the technique may penetrate several hundreds of meters. Major business benefits include reduction in the survey's cost are achievable because of GPR's inherent advantages over other geophysical methods; versatility; high speed data acquisition; portability and ease of use; and possibly the most important- the optimization of the resources i.e., one man replacing a large field crew. GPR also offers indirect project benefits by limiting opportunity cost for the system operator and third parties resulting from e.g., limited surveying ability in rough terrain areas. Top applications of GPR are as follows:
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The latest techniques which have been developed in GPR (Ground Penetrating Radar) are discussed as follows:
Helicopter Borne GPR Surveys include GPRTEM- Time Domain EM and GPR (towed with the helicopter) as shown in figure below.
GPR-equipped fixed wing aircraft are used mainly for surveying large areas or inaccessible regions, for example desert areas, permafrost areas and high mountain ranges. A GPR system installed in a helicopter is an effective way to survey large areas with high data density. Large areas even in inaccessible regions can be surveyed within a short time and even limited logistic demands. The high agility of a helicopter allow to increase the data density in areas of special interest. Using Ground Penetrating Radar (GPR) for geological applications a high resolution of near surface structures is necessary. Stepped frequency radar technology offers an attractive alternative to the classical pulse radar systems. For example a newly developed Helicopter Borne GPR system includes following components: transmitter-receiver unit, antenna, data acquisition and a GPS navigation unit. The antenna is a corner reflector with two dipoles in a distance of λ/4. The dipoles are installed in such a way that an optimum bundling is achieved. The antenna is towed to the helicopter via 15 m towing rope and a coaxial cable. The system can be operated mono-statically as well as bi-statically. The normal operating frequency is 150 MHz, but it can vary between 70 MHz and 150 MHz. The above discussed system was modified for near surface application in 1999-2000. These modifications were firstly tested during a survey at the Careser glacier in the Italian Alps in October 2000. The results clearly show the base of this glacier in a depth range of 10 to 80 m. Afterwards a stepped frequency GPR was developed that was also able to be operated from a helicopter. Frequency of the stepped frequency GPR can be varied between 20 MHz and 2 GHz. The system-antenna is mounted on a non-conductive frame connected to the helicopter as a sling load. All electronic parts are installed in the helicopter. This alternative GPR system was flown about along the same flight tracks at the Careser glacier to compare the performance of the SF-Radar and Pulse Radar systems. In the following figures a comparison between PRS (Pulse Radar System) and SFRS (Stepped Frequency Radar System) is given:
  Limitations of GPR:
Similar to other geophysical techniques GPR technology also has some limitations which are as follows:
However, the GPR technology is still developing day by day because of its convenience in getting high density data over large ares especially acquiring data in inaccessible and rough terrains as discussed above. Although the present GPR technology offers a limited scope to delineate the subsurface structures only at shallow depths. Because high resolution electromagnetic waves are used in GPR applications so they limit spontaneously the power of penetration to deeper horizons of the subsurface.
Moreover, from Oil & Gas exploration's point of view; the borehole GPR is an indispensable tool for fracture and ground-water flow analysis. It will provide us high resolution data regarding the rock formations surrounding the borehole.
In writing this material, I'm indebted to the following references:
In Association With:
  
Wilson, M. G. C.; Henry, G.; Marshall, T. R. (2006). South African Journal of Geology (Geological Society of South Africa).
Penguin Dictionary of Civil Engineering (Page # 347; Radar).
Conyers, L.B. (2004).
Walnut Creek, CA., United States: AltaMira Press Limited.
Gaffney, Chris; John Gater (2003).
Stroud, United Kingdom: Tempus.
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geophyx.blogspot.com
Author:
Waqas Haider
M.Sc. Geophysics (2011-2013)
Department of Earth Sciences
Quaid-I-Azam University (45320)
Islamabad (ICT), Pakistan.
Contact Information:
Email: geomindx@gmail.com
Mobile: +923215140154
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