GPR: A Brief History and Its Applications
GPR stands for Ground Penetrating Radar. While GPR was used in the late 1950s, it was not commercially available until the early 1970s. However, the mainstream use of digital signal processing led to the GPR instruments used today. Today Ground Penetrating Radar is used because it is a highly effective and non-invasive method of detecting and investigating the existence of subsurface objects. Its variety of applications include the following:
- Concrete Imaging. It is common to use GPR to locate rebar and reinforcing steel in concrete, as well as identify conduits and post-tension cables. Additionally, it can be used to detect the presence of voids in the subgrade below a concrete floor, as well as the structural integrity of the concrete. This is a non-invasive procedure which does not cause any damage to the structures.
- Utility Locating. GPR is also useful in identifying utilities such as fiber optic lines, conduit, cables and wires, water boxes, metallic and non-metallic objects, concrete pipes, valves, septic tanks, transit pipe, and more.
- Road Inspection. GPR can be safely and effectively used to evaluate the pavement of roads, both the base and sub-base layers. Problems below the surface can be easily detected; this is useful for pavement preservation, planning, or restoration.
- Environmental. GPR can also be used for environmental reasons, such as determining landfill limits, soil contamination or soil saturation level, as well as locating underground storage tanks.
- Archaeological. For non-invasive investigation or mapping, GPR is ideal. It can be used for location of sensitive cultural resources for either preservation or avoidance or for locating sites such as unmarked graves.
Technical Training - Reporting
How GPR Works
The way that Ground Penetrating Radar works is that electromagnetic energy (in the form of a radio wave) is pulsed into the surface via a frequency of usually between 10 MHz and 3000 MHz. Some of this energy reflects off of an object (e.g., pipe, conduit) and is captured by a receiver antenna. The GPR equipment records the amplitude and frequency of this reflected signal, as well as the time delay between transmission and reception, in order to calculate the depth of the object. Detected objects are revealed, over time and distance, on GPR’s computer screen in real-time and, in fact, the GPR sensor is able to “see” the object before it actually goes over it.
The electrical and magnetic properties of the place being investigated determine the speed of transmission and frequency of the radar waves. The radar waves reflect off different objects differently. When more energy is reflected, the image is brighter on the radar profile. The maximum effective depth of penetration of GPR waves is a function of two primary factors: the frequency of the waves that are pulsed into the ground and the physical characteristics of the material through which the waves must pass through.
GPR is especially useful in detecting metal objects like pipes or rebar because the radar energy will not penetrate metal; thus, metal objects can usually be easily seen in the GPR profile.
Benefits of GPR
GPR has two main advantages over other non-invasive scanning techniques. First, GPR provides a three-dimensional image that can be easily converted to depths that are extremely accurate. Second, GPR responds to both metallic and non-metallic objects. These benefits make GPR a highly effective tool for scanning archaeological sites and for use in military, security, law enforcement, concrete construction, utility detection, and urban planning. GPR clearly and accurately indicates where it is safe to drill, cut, or dig, as well as where extra precaution should be taken.
GPR vs. X-ray
Both GPR and X-ray can provide valuable information about subsurface objects. Some key differences, however, are that GPR does not expose the user or others to radiation, whereas X-ray does. Additionally, with GPR, only one side of the surface is needed. It is also a much quicker procedure, taking place in real-time. Also, because it’s non-invasive, it can be done during normal work hours without disruption.
GPR equipment generally consists of a transmitter and receiver antenna, a radar control unit, and suitable data storage and display. Differing GPR equipment options and antennas make it possible to explore the subsurface of the earth and inspect infrastructure systems for diverse applications such as:
- Concrete inspection
- Utility locating
- Road inspection
- Bridge inspection
There are different approaches to GPR scanning. It depends on the project’s end-goal and whether real-time data or post-processing data is needed. We generally use closely spaced transect grids (rectangular or rectilinear) over the area; these allow for easy export to the GPR computer. A scan can be conducted by either a line scan or a grid scan. The line scan allows for a sectional view of data in real-time; a grid scan provides quick data collection that can be then processed to create maps. Three-dimensional data can be generated with GPR, too. This means that the number of scan lines would likely have to be doubled or quadrupled. This is not usually used for large-scale surveys. Additionally, these 3D presentations will require some post-processing.
Regardless of the approach, there is typically a three-step process to a GPR scan:
- Visual examination of the area
- The GPR scan
- Use of other utility locating technology, such as electromagnetic equipment, for confirmation
Data presentation is meant to provide a display of processed data that shows a close approximation of what is in the subsurface. Data can be displayed as 1.) a one-dimensional trace, 2.) a two-dimensional cross-section, or 3.) a three-dimensional display.
A one-dimensional trace by itself is generally not useful and are not used by themselves. Several traces would be needed to produce a two-dimensional cross-section.
Three-dimensional GPR data can be represented in any of the following ways:
- 3D alignment of 2D traces, which requires almost no post-processing
- Depth slices, which are effective for modeling linear features (e.g., rebar, conduit)
- Isosurfaces, which can be effective for modeling more complex features, but this requires the greatest amount of post-processing.
GPR Reporting Methods
Types of GPR Reports:
- A BASIC report provides a written description of what the GPR scan found and includes pictures and data screen shots.
- A COMPLEX report includes the same information as a BASIC report but also includes a sketch of the area where the scan was conducted which includes the location of utilities as well as their depth.
- A GPS (Global Positioning System) report is useful in recording positions of utilities and addressing questions about markings. This data can be exported into CAD formats and formats compatible with Google Earth.
- A CAD report is most useful for interior projects where the GPS report by itself is ineffective. A CAD report includes the BASIC report and sometimes the GPS report. The findings are layered, as needed, into an existing CAD file.
- A LASER SCANNING report requires the use of a laser to scan the GPR scan markings and export them into a 3D image. That image can be used to pull measurements and determine the precise location of objects. This report can be very useful for mapping post-tension cables within large areas of concrete, finding buried electrical conduit, or plotting the precise location of outdoor underground utilities.
Variables Affecting an Accurate GPR Scan
Many factors can affect the accuracy of a GPR scan; some of these include the following:
- Composition of the material being penetrated
- Depth of the target
- Moisture content
- Varying velocity of the natural layers within a geological composition
- Areas with strong electromagnetic signals
- Areas containing a complex assortment of objects