What is GPR?

PROMOTING THE USE OF GROUND PENETRATING RADAR

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Introduction to GPR

A simple, quick introduction to... Ground Penetrating Radar.

What is GPR?

HI_MOD Ground penetrating radar (GPR) is an advanced, non-invasive sub-surface imaging technique that typically uses short pulses of electromagnetic energy to ‘see’ into the ground. GPR can image through soil, concrete, tarmac, rock, wood, ice and even water. It is quick, easy to use and inexpensive in comparison to other investigation methods. It is capable of probing down to a few tens of metres (depending on the system type & ground conditions) and provides the user with a ‘cross-sectional’ image of the sub-surface.

GPR is very good at determining the shape, size, extent and depth of a buried object. It can be used to find pipes, foundations, voids, trenches, geological layers, re-inforcing bars, etc. It works best in dry ground environments but can also work well in wet, saturated materials. It does not work very well in saline conditions and through dense clays.

GPR is used by a wide range of people including:

Civil Engineers & NDT Archaeologists
Geologists
Geotechnical Engineers
Glaciologists
Forensic Investigators
Environmental Scientists
Hydrologists

In fact, anyone who needs to investigate buried features. It is particularly popular for utility detection and roadway evaluation.

GPR in action

intro_2 As the GPR system is moved across the surface, pulses of electromagnetic energy are directed into the ground from a transmitting antenna. The energy travels though the ground where buried objects reflect/scatter part of the energy back to the surface, which is picked up by the GPR’s receiving antenna. These returning signals are converted in to a GPR section (or radargram) which is displayed on a computer screen in real-time. The operator then uses knowledge, skill and experience to assess the data and interpret the features seen in the section.

Antenna centre frequency (MHz)	Approximate imaging depth in soil (metres)	Approximate target size in soil (metres)
1500	0.3	0.03
1000	1	0.1
500	2	0.2
250	3	0.3
100	5	0.5
50	10	1
20	20	2

GPR_road Surveys are usually conducted along profile lines (to produce 2D sections). However, any number of closely spaced lines can be combined to create a 3D volume of data. GPR systems are designed to use different antenna frequencies (typically 20-1500MHz).

Low frequency systems (20-250MHz) penetrate the deepest, but only image larger objects. These tend to be used for hydrological, glacial and geological work. Higher frequency systems (>250MHz) see smaller objects but don’t penetrate as deep. These are more commonly used for civil engineering and NDT applications.

GPR is perfectly safe to use but, as with any site technique, appropriate heath and safety risk assessments must be made before carrying out an investigation. To use a GPR system in the EU, operators need to have an appropriate and up-to-date licence, be adequately trained, responsible and follow a code of practice. Operators must ensure that their surveys do not interfere with any vital telecommunications systems (e.g., aircraft, hospitals, defense, etc).

Glossary of common GPR terms

Attenuation	A reduction in electromagnetic signal amplitude caused by energy dissipation in the material where the electromagnetic wave propagates. Attenuation in soil is mainly due to its electrical conductivity and it is the main factor that limits the maximum range (depth) reachable by GPR equipment. In general attenuation increases with frequency so that higher frequencies penetrate less than lower ones.
B-Scan	The typical view of GPR data obtained by showing a collection of radar Scans as they are acquired while the antenna is moved along the soil. Echo delay (or Apparent Depth) is normally represented downwards on the vertical axis while the direction of scan of the antenna is normally towards the right. "GPR Section" is another common name for B-Scan.
Background Removal	A digital signal processing function that filters a B-Scan usually by subtracting an average of a large number of Scans from each individual Scan. Its major purpose is in eliminating the Antenna Cross-Coupling signal which is the main element of clutter in raw GPR data.
C-Scan	A horizontal slice at a given depth (delay) of a GPR data set acquired over a surface. It is normally achieved by using an array of GPR sensors or carefully scanning a single channel GPR in parallel lines. The resulting B-Scans are composed into a 3-dimensional data volume and a slice extracted at a given value of depth (or delay). C-Scan is sometime called a Time Slice.
Clutter	Unwanted reflections that are not of interest. Can also include unwanted electrical signal generated by the system itself or interference from other electromagnetic transmitters (e.g. mobile phones). The single most important source of clutter in GPR is the direct signal propagating from the transmitting antenna to the receiving one on the surface of the soil; this is effectively eliminated by the process of Background Removal.
Conductivity	The property of a material that determines its ability to attenuate an electromagnetic signal. It usually varies with frequency.
Antenna Cross-Coupling	The signal received by the receiving antenna due directly to the energy radiated by the transmitting antenna and propagating at the interface of the antenna and the soil. It carries no information about buried objects and it is the main element of clutter in the raw GPR signal. It is normally removed by Background Subtraction.
Depth	In GPR the depth a target from the surface can only be estimated based upon knowledge of the average propagation velocity of light in the ground.
Dielectric Constant	Another name for Relative Dielectric Permittivity. The average dielectric constant of the soil influences the propagation speed of the electromagnetic wave which is roughly proportional to inverse of the square root of the dielectric constant. The dielectric constant may change with frequency.
Dynamic Range	The ratio between the power of the minimum signal to which the instrument (GPR) is sensible and the maximum input that the system can handle without saturating. In good GPR equipment this can be as high as 100,000,000 (80dB)
Frequency-domain GPR	A GPR system that instead of transmitting short pulses (Impulse GPR) uses continuous or slowly varying frequencies and synthetically reconstructs the short pulse via a Fourier transformation.
Ground Penetrating Radar	A non-destructive radar technology that displays the presence and position of major changes in the electrical properties of the ground which, when interpreted, can reveal the location of buried objects and substances. (typically: Water, Ice, Archaeology, Utility Services etc.). Also known as Ground Probing Radar
Hyperbola	The typical pattern showing on a B-Scan indicating the presence of an isolated target or the crossing of an elongated target (e.g. a pipe) when scanned across with a GPR.
Impulse GPR	The most common electronic scheme for a GPR device whereby a very short electromagnetic pulse (from a few nano-seconds to ~100 pico-seconds) is transmitted and its echo is recorded by the receiving antenna.
Migration	A digital signal processing function that rearranges B-Scan data so that reflections and diffractions are plotted at the locations of the reflectors and diffracting points rather than with respect to observation points on the profile. This transforms the hyperbolic patterns typical of B-Scans into high reflectivity points in the image. To yield good results migration must be fed with an accurate estimate of the electromagnetic propagation speed in the soil.
Nanosecond	A common unit of time used for GPR equal to one billionth of a second (1ns = 10^-9 s). Given a typical propagation speed of the electromagnetic wave in the soil (e.g. 10⁸ m/s), a 1ns delay of the radar echo corresponds 5 cm in depth.
Penetration Depth	The radar wave is attenuated as it progresses due to losses. At radar frequencies in a conductive material (sea water, metallic materials, clay soils, etc.) the rate of attenuation is very great and the wave may penetrate only a short distance (less than 1m) before being reduced to a negligibly small value. In a dielectric earth material (fresh water, granite, quartz sand, etc.), where the losses are low, the depth of penetration can be quite great (>30m).
Propogation Velocity	The speed at which an electromagnetic wave, travels (propagates) through a particular material or substance. In the context of GPR, it is mainly determined by the Dielectric Constant of the soil.
Resolution	The minimum distance of separation between two objects at which their individual responses merge into one. In radar systems this is determined by the radar bandwidth which in impulse systems is the inverse of the pulse duration. A GPR with a 1 nanosecond (ns) pulse can therefore resolve items which echo is separated by 1ns. Given the propagation velocity in the medium, 1ns may correspond to 5 cm (e.g. average soil) to 1.5 cm (e.g. fresh water).
Scan	The recording of the radar echoes by the receiving antenna as a function of time. Knowing the propagation velocity of the electromagnetic wave in the specific soil, allows to transform the time (delay) axis into distance (apparent depth ). In mainstream radar literature a Scan is is often referred to as a Range Profile.
Sensitivity Time Control	A digital signal processing that amplifies the signal received at longer ranges. This is important for human interpretation of GPR data since due to high attenuation in the soil the strength of the signal reflected by identical objects placed ad different depth may vary by a factor of 10,000 or more. Since neither graphic displays nor the human eye can appreciate such wide dynamic ranges, weak signals from deeper targets must be enhanced by STC. When STC is implemented directly in the hardware circuitry of the GPR it is normally referred to as Automatic Gain Control.
Ultra Wide Band	In order to achieve the desired resolution (short pulse width) employing low frequencies (for deeper penetration into the soil) GPR equipment has a high ratio between bandwidth and central frequency, i.e. it is Ultra Wide Band. The development of high dynamics UWB electronics is far more demanding than that of narrow band equipment normally used in telecommunication applications.
Wall Probing Radar	A GPR specifically designed for the non destructive testing of man made structures like walls. Usually it operates at higher frequency than GPR since such structures have less attenuation than soil.