How Does GPR Scanning Work?
From Earth science to engineering, there are many industries that, at one time or another, may require detection of what lies beneath the ground's surface.
When this is the case, one of the best ways to get a clear and undisrupted view of the subsurface is through Ground Penetrating Radar scanning, also known as GPR scanning.
In this method, a transmitter sends a radio signal to the surface, then a receiver inputs and records the signals that are reflected back. Comparing these signals can help users determine the location and makeup of the unseen material.
Today, we're taking a closer look at exactly how GPR scanning works, and some of its most common applications.
Ready to learn more? Let's get started!
GPR Scanning: Top Applications
There are many reasons one may need to "see" below the ground's surface.
Earth scientists study the composition of the ground's makeup. To do so, they must analyze substances such as groundwater and bedrock.
Engineers must detect buried electrical lines and perform other nondestructive testing routines. Law enforcement officers seek to find hidden evidence. Concrete workers need to locate rebar, conduits, and other obstructions.
It's also of great value to the military, archeologists, and even astrophysicists. In fact, China's "Yutu" rover used GPR scanning to analyze the surface of the moon!
In these applications, GPR scanning provides the evaluation approach that these professionals need. It does so by keeping the underground substances intact while giving users a detailed view of what lies beneath.
How Does It Work?
So just how does GPR scanning work?
When the GPR transmitter emits the radio signal into the ground, it does so via a transducer or antenna.
The user will penetrate the ground with the antenna, then move it along the desired path. This can be done manually (where the user walks in front of the antenna and pulls it along). Alternatively, a vehicle can be used to do the job.
As it moves along the subsurface, the antenna is looking for one of two things. It could be looking for a physical object (such as a pipeline). Or, it might be searching for a change in the earth's natural composition (such as at what level bedrock begins).
If the antenna comes into contact with a buried substance or encounters a contrast between the earth's underground layers, it will reflect that target's signal.
Then, the antenna captures those returned waves and stores their image on digital media. This is usually in the form of an LCD control unit on the GPR scanning device.
What Does It Reveal?
As it passes along the subsurface, the antenna comes into contact with myriad types of material.
From sandy clay soil to concrete, debris, and more, it interacts with elements that have differing dielectric properties. These materials also have varying levels of conductivity.
Adding to the complexity is the fact that the antenna is sending and receiving signals up to one thousand times per second. That's a lot of data to track!
As such, it takes a professional to analyze the signals for meaningful changes. The operator can check the signals in real-time as the antenna tracks them. Or, he or she can save them on the system for later viewing.
Analysts can use this process to determine where a target is located. They can also use it to determine how far down the target lies.
To do so, they look at how long it took for the emitted signal to locate the target, then return to the surface. This can help them determine how deep the target is buried.
Analyzing Targets in Concrete
When GPR scanning is used in concrete surfaces, the review process is slightly different, though the overall aim is the same.
Concrete is one of the most-scanned surfaces -- and for good reason. A construction worker shouldn't cut into or core a concrete slab without knowing what's beneath it. If this happens, serious and costly implications could occur.
Sometimes an operator simply needs to know how deep the concrete extends. In this case, they can use GPR scanning to create a line scan that reveals this data.
Yet, they often need to identify the precise location of buried objects. These could include rebar, conduits (electrical or communications), post tension cables, and more.
In this case, changing the data into a grid view will help them map any targets buried within the material. They can also virtually slice the image to create a 3D map of the target. This will help them learn more details about it, such as how deep it lies.
When scanning concrete, a high-frequency antenna is typically required. This is because the targets are often shallow and a high-resolution result is preferable.
What are the Depth Limits?
How deep a GPR scanning system can travel is dependent on the strength of the antenna's frequency and the type of subsurface it's descending into.
A low-frequency antenna is typically defined as one that goes up to around 200 MHz. The signals they send back are typically low-resolution. Yet, they are often able to travel to significant depths (of around 100 feet or more). This makes them ideal for investigating deeper sites, such as sinkholes.
On the other hand, a high-frequency antenna (up to 1,500 MHz) is usually used to detect more shallow surfaces, around 18 feet deep or less. The reflections they capture are in high-definition. As such, these are often used to analyze targets relatively close to the surface, such as buried utilities.
Some materials, such as dry sand, carry low levels of conductivity. Here, antennas can often reach the deepest levels, ascending to around 100 feet.
Other materials, such as wet clay or shale, have higher conductivity levels. These can actually serve to reduce the strength of the signals as they're emitted. As such, the antenna can only penetrate about three feet into their surface to achieve an accurate measurement.
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