Ground penetrating radar

Jouni Lerssi, Geological Survey of Finland, PO Box 1237, 70211 Kuopio, Finland; jouni.lerssi@gtk.fi

Description

Ground penetrating radar (GPR) is sometimes called georadar, ground probing radar, or subsurface radar. GPR uses electromagnetic wave propagation and scattering to image, locate and quantitatively identify contrasts in electrical and magnetic properties in the ground. It may be performed from the surface of the earth, in a borehole or between boreholes, from aircraft or satellites.

GPR uses short impulses of high frequency radio waves directed into the ground to acquire information about the subsurface. The energy radiated into the ground is reflected back to the antenna by features having different electrical properties to that of the surrounding material. The greater the contrast, the stronger the reflection. Typical reflectors include water table, bedrock, bedding, fractures, voids, contaminant plumes and man-made objects such as UST’s and metal and plastic utilities. Materials having little electrical contrast like clay and concrete pipes may not produce strong reflections and may not be seen. Data are digitally recorded or downloaded to a laptop computer for filtering and processing.

The frequency of the radar signal used for a survey is a trade off. Low frequencies (250 MHz – 50 MHz) give better penetration but low resolution so that pipes and utilities may not be seen. Pipes and utilities may be seen using higher frequencies (500 MHz) but the depth of penetration may be limited to only a few feet especially in the wet, clayey soils found in many areas of the NW USA. The GPR frequency is dependent upon the antenna. Once an antenna is selected, nothing the operator can do can increase the depth of penetration.

Radar data is ambiguous. Many buried objects produce echoes that may be similar to the echo expected from the target object. Boulders and debris produce reflections that are similar to pipes and tanks. Subtle changes in the electrical properties along a traverse caused by changes in soil type, mineralogy, grain size, and moisture content all produce “noise” that can make interpretation difficult. Interpreting radargrams is an art as much as a science.

Under some conditions, although a UST itself may not be clearly visible in a GPR record, the excavation or trench in which the UST is buried is evident. Usually GPR data is used to compliment data from other “tools”. For example, a trench-like reflection but no clear UST reflection, combined with a “tank” shaped magnetic anomaly suggests the presence of a UST. Although the UST itself could not be seen using GPR, the radar showed a trench-like reflection. The magnetic data showed a large ferrous object. We would report a possible UST at that location.

GPR is often used in conjunction with magnetic and EM surveys. Magnetic amd EM Surveys are very fast and large areas can be covered cost effectively. Magnetic amd EM anomalies are marked in the field, and then may be further investigated using radar or vice versa.

GPR, like other geophysical tools, is excellent at detecting changes across a site, but it is poor at actually identifying the cause of the change. The only definite way to identify buried objects is through excavation.

GPR has the highest resolution in subsurface imaging of any geophysical method, approaching centimeters under the right conditions. Depth of Investigation varies from less than a meter to over 5,400 meters (over glacial ice sheet), depending upon material properties.

Detectability of a subsurface feature depends upon contrast in electrical and magnetic properties, and the geometric relationship with the antenna. Quantitative interpretation through modeling can derive from ground penetrating radar data such information as depth, orientation, size and shape of buried objects, density and water content of soils, and much more.

Appropriate applications

Ground penetrating radar (GPR) is a relatively new geophysical technique. The last decade has seen major advances as the technology matures. The history of GPR is intertwined with the diverse applications of the technique. GPR has the most extensive set of applications of any geophysical technique leading to a wide range of application spatial scales and concomitant diversity of instrument configurations.

Ground Penetrating Radar (GPR) can be a valuable tool to accurately locate both metallic and non-metallic UST’s and utilities, buried drums and hazardous material at some sites. It may detect objects below reinforced concrete floors and slabs. GPR may delineate trenches and excavations and, under some conditions, it may be used to locate contaminant plumes. It has been used as an archaeological tool to look for buried artifacts. It may accurately profile fresh water lake bottoms either from a boat or from a frozen lake surface. GPR may be used to locate voids below roads and runways. GPR has numerous engineering applications. It can be used in non-destructive testing of engineering material, for example, locating rebar in concrete structures and determining the thickness of concrete and other structural material.

Performance

ADVANTAGES – General

When GPR data is properly interpreted subsurface objects can usually be confidently identified. This often requires the GPR data be combined with other geophysical data, surface features and historical information.
GPR provides continuous records along traverses which, depending on the goal of the survey, may be interpreted in the field.
At flat, open sites, for reconnaissance purposes, the antenna can be towed behind a vehicle at several mph.
Many GPR antennas are shielded and are unaffected by surface and overhead objects and power lines.
GPR can be used in conjunction with magnetic or EM surveys to accurately locate buried objects.
ADVANTAGES – Site specific
- With a low frequency antenna, in clean, dry, sandy soil, reflections from targets as deep as 100 feet are possible. Geologic features such as bedrock and cross bedding may be seen at some sites.
- The resolution of data is very high particularly for high frequency antennas.
- Shallow, man-made objects generally can be detected.
- Fiberglass UST’s and plastic pipes can be detected using GPR.

LIMITATIONS – General

To acquire the highest quality data, proper coupling between the antenna and the ground surface is necessary. Poor data may be obtained at sites covered with debris, an uneven surface, tall grass and brush. Objects located at curbs are difficult to see.
Acquiring GPR data is slow. The antenna must be over the target. The signal from the antenna is cone-shaped. Reflections from objects to the side of the antenna may be seen, but their actual location relative to the antenna is not obvious.
Penetration of the GPR signal is “site specific” and its depth of penetration at a particular site cannot be predicted ahead of time. Near surface conductive material, such as salty or contaminated ground water and wet, clay-rich soil, may attenuate the radar signal, limiting the effective depth of the survey to several feet. Reinforced concrete also can attenuate the signal. Rebar may produce reflections that look like pipes.
GPR may not be cost-effective for some projects. For a detailed survey mapping underground storage tanks and utilities, it may be necessary to collect data in orthogonal directions at 1 m line spacing.

LIMITATIONS – Interpretation

Interpretation can be difficult. Radar data are ambiguous. Subsurface objects can be detected but, in general, they cannot be identified. USTs and utilities have a characteristic reflection, however, large rocks and boulders have a similar reflection.
The reflection visible in a GPR record is very complex and may be caused by small changes in the electrical properties of the soil. The target in mind may not produce the reflection. Due to “noise”, the target may be missed. USTs and deep utilities may be missed if they are under debris and/or other pipes.
Other methods may be necessary to aid in the interpretation of the data (use a magnetometer to detect a large metallic mass, then GPR to determine if the object is tank-like, or a utility locator to determine if there are feed lines and fill pipes leading to the object).
Adequate contrast between the ground and the target is required to obtain reflections. UST’s may be missed if they are badly corroded. Utilities made of “earth” materials like clay and concrete may not be detected since their electrical properties are similar to the surrounding soil.
To determine the depth to an object without “ground truth”, assumptions must be made regarding soil properties. Even with ground truth at several locations on the same site, changes in material across a site (therefore changes in signal velocity) can cause errors in depth measurements at other locations.

Prediction of whether GPR will “work” for the problem at hand is not clear cut. In general it is easier to rule out situations where radar is totally unsuitable than to state with confidence that radar will be successful. Again, this is not a unique feature of the GPR method but is a fact of life with all geophysical methods. GPR tends to have more mystery because people have not normally had as much experience with it as with some other methods.

Design requirements

Fig. 1. Descriptive picture of GPR-measurement and radargram (© Dr. Lanbo Liu, Professor, Department of Civil & Environmental Engineering University of Connecticut and Riitta Turunen, GTK).

References

Reynolds, J.M. 2011. An Introduction to Applied and Environmental Geophysics. John Wiley & So n s Ltd, Chichester, 2nd ed.,712 pp.