Understanding Ground Penetrating Radar (GPR) Technology
Ground Penetrating Radar (GPR) technology deploys a geophysical location method using radio waves to capture images of the earth below the surface in a non-intrusive manner. One of the biggest benefits of using GPR technology is the ability to locate subsurface utilities, assets, or features without disturbing the ground.
To capture such subsurface images non-intrusively, GPR technology uses the electromagnetic spectrum (EMS) in the microwave band between 10 MHz and 2.6 GHz. Based on the electrical permittivity of the subsurface structures, these signals are transported through the ground and reflected off of them. The GPR device then records differences in the return signals through a receiving antenna, which in turn are used to produce images that show changes in electrical characteristics.
The Components of a Ground Penetrating Radar
There are three basic components that make up a GPR system:
The GPR transmitter that transmits a pulse of radar energy into the ground is housed in the control unit. It also contains an internal computer, hard drive, and solid-state memory for storing data for analysis after fieldwork. The control unit enables data processing and interpretation without transferring radar files to another computer.
The antenna absorbs the electrical pulse generated by the control unit, amplifies it, and then transmits it at a specific frequency into the ground or another medium. The antenna frequency is an important parameter for depth penetration: The higher the frequency, the more deeply it will pierce the earth’s surface. Smaller objects will also be visible to a higher-frequency antenna. This makes the choice of the antenna one of the most crucial elements in GPR survey design.
A range of power sources, including small rechargeable batteries, car batteries, and standard 110/220-volt, can be used to operate GPR equipment.
How does Ground Penetrating Radar (GPR) Work?
High-frequency (often polarized) radio waves between 10 MHz and 2.6 GHz are used in GPR. Once the GPR transmitter sends electromagnetic pulses into the ground, the differences in permittivity are used to identify subsurface changes and elements. Whenever there is such a change, some of the electromagnetic energy is reflected back to the subsurface. This is picked up by a receiving antenna, and changes in the return signal are noted. A radargram is used to illustrate the data.
It is also important to note that while ground penetrating radar (GPR) can identify changes beneath the surface, it cannot precisely characterize them. Sometimes, the reflected wave patterns may be used to identify certain specific traits, such as metallic surfaces’ high amplitude and reverse polarity.
What can GPR Be Used for?
Ground penetrating radar is used in geological surveys to investigate ice, soils, groundwater, and bedrock. Take for instance the Yutu lunar rover from China that uses a GPR on its underside to study lunar crust and dirt.
GPR technology is also used in archaeology to map features and points of interest, while in environmental remediation it is utilized to define landfills, pollution plumes, and other remediation sites. Law enforcement and emergency responders often use GPR to find hidden and buried objects and evidence. Military applications of GPR include the detection of mines, tunnels, and unexploded ordnance.
Advantages of Using Ground Penetrating Radar (GPR)
Using GPR, a wide range of both metallic and non-metallic materials can be accurately located and distinguished in a benign, non-destructive, non-intrusive manner, which makes it safe for usage in public places. It is an extremely cost-effective, non-intrusive survey method that provides a quick way to learn about the subsurface.
Due to its characteristics, GPR can be used to resolve interfaces at the construction layer. It can be suitably used for the estimation of depth, dimensions of objects, and layer thickness. Faster data collection speeds mean that large areas can be scanned using the technology in lesser time, contributing to overall site productivity. GPR can also pick up on the majority of materials since it requires the electromagnetic properties of the target and the surrounding material to be different enough.
That said, there are also a few disadvantages tied to GPR technology. High-conductivity materials like clay soils and salt-contaminated soils are where GPR’s performance is most severely limited. The scattering of signals in heterogeneous environments, such as rocky soils, also affects GPR performance.
(Featured Image Source: Hexagon Geosystems)