Overview

The concept of radar is familiar to most people. With ground penetrating radar, the radar signal – an electromagnetic pulse – is directed into the ground. Subsurface objects and stratigraphy (layering) will cause reflections that are picked up by a receiver. The travel time of the reflected signal indicates the depth. Data may be plotted as profiles, as planview maps isolating specific depths, or as three-dimensional models.

GPR can be a powerful tool in favorable conditions (uniform sandy soils are ideal). Like other geophyscal methods used in archaeology (and unlike excavation) it can locate artifacts and map features without any risk of damaging them. Among methods used in archaeological geophysics it is unique both in its ability to detect some spatially small objects at relatively great depths and in its ability to distinguish the depth of anomaly sources. The principal disadvantage of GPR is that it is severely limited by less-than-ideal environmental conditions. Fine-grained sediments (clays and silts) are often problematic because their high electrical conductivity causes loss of signal strength; rocky or heterogeneous sediments scatter the GPR signal, weakening the useful signal while increasing extraneous noise.

Individual lines of GPR data represent a sectional (profile) view of the subsurface. Multiple lines of data systematically collected over an area may be used to construct three-dimensional or tomographic images. Data may be presented as three-dimensional blocks, or as horizontal or vertical slices. Horizontal slices (known as “depth slices” or “time slices”) are essentially planview maps isolating specific depths. Time-slicing has become standard practice in archaeological applications, because horizontal patterning is often the most important indicator of cultural activities.

Here is a basic introduction to some of the key concepts of Ground-Penetrating Radar (GPR).

___

GPR Equipment

A GPR system is made up of three main components:

1. Control Unit
2. Antenna
3. Power Source

GPR equipment can be run with a variety of power supplies ranging from small rechargeable batteries to vehicle batteries and normal 110-volt current.  The unit in the photo above is running from a small internal rechargeable battery.

The control unit contains the electronics that produce and regulate the pulse of radar energy that the antenna sends into the ground. It also has a built in computer and hard disk to record and store data for examination after fieldwork. Some systems are controlled by an attached Windows laptop computer with pre-loaded control software. This system allows data processing and interpretation without having to download radar files into another computer.

The antenna receives the electrical pulse produced by the control unit, amplifies it and transmits it into the ground or other medium at a particular frequency. Antenna frequency is a major factor in depth penetration. The higher the frequency of the antenna, the shallower into the ground it will penetrate. A higher frequency antenna will also ‘see’ smaller targets. Antenna choice is one of the most important factors in survey design. The following table shows antenna frequency, approximate depth penetration and appropriate application.

Depth Range (approximate)	Primary Antenna Choice	Secondary Antenna Choice	Appropriate Application
0-1.5 ft 0-0.5 m	1600 MHz	900 MHz	Structural Concrete, Roadways, Bridge Decks,
0-3 ft 0-1 m	900 MHz	400 MHz	Concrete, Shallow Soils, Archaeology
0-12 ft 0-9 M	400 MHz	200 MHz	Shallow Geology, Utilities, UST’s, Archaeology
0-25 ft 0-9 m	200 MHz	100 MHz	Geology, Environmental, Utility, Archaeology
0-90 ft 0-30 m	100 MHz	Sub-Echo 40	Geologic Profiling
Greater than 90 ft or 30 m	MLF (80, 40, 32, 20, 16 MHz)	20 m	Geologic Profiling

___

GPR Method

GPR works by sending a tiny pulse of energy into a material and recording the strength and the time required for the return of any reflected signal. A series of pulses over a single area make up what is called a scan. Reflections are produced whenever the energy pulse enters into a material with different electrical conduction properties (dielectric permittivity) from the material it left. The strength, or amplitude, of the reflection is determined by the contrast in the dielectric constants of the two materials. This means that a pulse which moves from dry sand (diel of 5) to wet sand (diel of 30) will produce a very strong, brilliantly visible reflection, while one moving from dry sand (5) to limestone (7) will produce a very weak reflections. Materials with a high dielectric are very conductive.

While some of the GPR energy pulse is reflected back to the antenna, energy also keeps traveling through the material until it either dissipates (attenuates) or the GPR control unit has closed its time window (Figure 2). The rate of signal attenuation varies widely and is dependant on the dielectric properties of the material through which the pulse is passing.

Materials with a high dielectric are very conductive and thus attenuate the signal rapidly. Water saturation dramatically raises the dielectric of a material, so a survey area should be carefully inspected for signs of water penetration. Radar surveys should never be conducted through standing water, no matter how shallow. Depth penetration through a material with a high dielectric will not be very good.

Metals are considered to be a complete reflector and do not allow any amount of signal to pass through. Materials beneath a metal sheet, fine metal mesh, or pan decking will not be visible.

Radar energy is not emitted from the antenna in a straight line. It is emitted in a cone shape (Figure 3). The two-way travel time for energy at the leading edge of the cone is longer than for energy directly beneath the antenna. This is because that leading edge of the cone represents the hypotenuse of a right triangle.

Because it takes longer for that energy to be received, it is recorded farther down in the profile. As the antenna is moved over a target, the distance between them decreases until the antenna is over the target and increases as the antenna is moved away. It is for this reason that a single target will appear in a data as a hyperbola, or inverted “U.”  The target is actually at the peak amplitude of the positive wavelet (Bottom Figure 3).

Data are collected in parallel transects and then placed together in their appropriate locations for computer processing in a specialized software program. The computer then produces a horizontal surface at a particular depth in the record. This is referred to as a depth slice, which allows operators to interpret a planview of the survey area.

___

Data Processing

In many situations, a GPR operator will simply note the location of a target so that it can be avoided. For these clients, it may only be necessary to use a simple linescan format in order to mark the approximate area of the target on the survey surface. Other clients may require detailed subsurface maps and depth to features. These situations will require the operator to use GPR processing software, which applies mathematical functions to the data in order to remove background interference, migrate hyperbolas, calculate accurate depth and much more.

Ground-penetrating radar (GPR) is a geophysical method that uses radar pulses to image the subsurface. This non-destructive method uses electromagnetic radiation in the microwave band (UHF/VHF frequencies) of the radio spectrum, and detects the reflected signals from subsurface structures. GPR can be used in a variety of media, including rock, soil, ice, fresh water, pavements and structures. It can detect objects, changes in material, and voids and cracks.

GPR uses transmitting and receiving antennas or only one containing both functions. The transmitting antenna radiates short pulses of the high-frequency (usually polarized) radio waves into the ground. When the wave hits a buried object or a boundary with different dielectric constants, the receiving antenna records variations in the reflected return signal. The principles 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 acoustic impedances.

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 also 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 the 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 m. In moist and/or clay-laden soils and soils with high electrical conductivity, penetration is sometimes only a few centimeters.

Ground-penetrating radar antennas are generally in contact with the ground for the strongest signal strength; however, GPR air launched antennas can be used above the ground.
Cross borehole GPR has developed within the field of hydrogeophysics to be a valuable means of assessing the presence and amount of soil water.

___

Applications

GPR has many applications in a number of fields. In the Earth sciences it is used to study bedrock, soils, groundwater, and ice. Engineering applications include nondestructive testing (NDT) of structures and pavements, locating buried structures and utility lines, and studying soils and bedrock. In environmental remediation, GPR is used to define landfills, contaminant plumes, and other remediation sites, while in archaeology it is used for mapping archaeological features and cemeteries. GPR is used in law enforcement for locating clandestine graves and buried evidence. Military uses include detection of mines, unexploded ordnance, and tunnels.

Before 1987 the Frankley Reservoir in Birmingham, England UK was leaking 540 litres of drinking water per second. In that year GPR was used successfully to isolate the leaks.

Borehole radars utilizing GPR are used to map the structures from a borehole in underground mining applications. Modern directional borehole radar systems are able to produce three-dimensional images from measurements in a single borehole.

One of the other main applications for ground penetration radars to locate underground utilities. Being able to generate 3D underground Images of Pipes, Power, Sewage and Water mains.

___

Three-Dimensional Imaging

Individual lines of GPR data represent a sectional (profile) view of the subsurface. Multiple lines of data systematically collected over an area may be used to construct three-dimensional or tomographic images. Data may be presented as three-dimensional blocks, or as horizontal or vertical slices. Horizontal slices (known as “depth slices” or “time slices”) are essentially planview maps isolating specific depths. Time-slicing has become standard practice in archaeological applications, because horizontal patterning is often the most important indicator of cultural activities.

___

Limitations

The most significant performance limitation of GPR is in high-conductivity materials such as clay soils and soils that are salt contaminated. Performance is also limited by signal scattering in heterogeneous conditions (e.g. rocky soils).

Other disadvantages of currently available GPR systems include:

• Interpretation of radargrams is generally non-intuitive to the novice.
• Considerable expertise is necessary to effectively design, conduct, and interpret GPR surveys.
• Relatively high energy consumption can be problematic for extensive field surveys.

Recent advances in GPR hardware and software have done much to ameliorate these disadvantages, and further improvement can be expected with ongoing development.

___

Similar Technologies

Ground penetrating radar uses a variety of technologies to generate the radar signal, these are impulse, stepped frequency, FMCW and noise. Systems on the market in 2009 also use DSP to process the data, while survey work is being carried out rather than off line.

GPR is used on vehicles for close-in high speed road survey and landmine detection as well as in stand-off mode.

Pipe Penetrating Radar (PPR) is an application of GPR technologies applied in-pipe where the signals are directed through pipe and conduit walls to detect pipe wall thickness and voids behind the pipe walls.

Wall-penetrating radar can read through walls and even act as a motion sensor for police.

GPR is used for hand held landmine detection to reduce the false alarms experienced by the standard metal detector and systems are available off the shelf.

The “M-ineseeker Project” seeks to design a system to determine whether landmines are present in areas using ultra wideband synthetic aperture radar units mounted on blimps.