How It's Done

Ground Penetrating Radar

What it Ground Penetrating Radar (GPR)?

This technology uses low intensity electromagnetic radiation to detect objects beneath the surface being scanned. This radiation leaves the scanner, bounces off an object below the surface, and returns to the scanner. The data generated from this may be used to interpret object material, depth, direction, and end points.

Here we will explain some of the common concepts people wonder about when considering requesting a radar scan. Let’s look at depth interpretation.

The scanner calculates how far down the object must be, given how long it took the signal to arrive back at the scanner. Let’s take a look!

As we scan, we are looking for objects that run perpendicular to the motion of the scanner, as pictured below.

Viewing this from the side, we can see that as the scanner approaches the object, it gets closer and closer until it is directly above. Then, as it passes, the distance will increase again.

Now let’s look at how this is represented on our scan data. 

Each of the hyperbolas indicated here represents an object reflecting the GPR signal back at the scanner. The Y-axis shows us the depth of the object, measured by how long it took the signal to come back to the scanner. The center of the hyperbola is where the scanner was when it was closest to the object in the slab (meaning it was directly over top of it). The ‘tails’ on either side of the hyperbola are the scanner detecting that object when the scanner is on either side of it, and the distance is thus getting larger and larger.

Once the scan technicians knows where the object is below the slab, they will mark that spot on the surface. Here, we’ve done this with an ‘X’. Now they move several inches away and perform another pass with the scanner as shown below. Then, they mark it again.

Now that they know two points where the object is, they simply connect the dots and draw a line between them to represent that an object is there.

What are some reasons why a radar scan cannot always be done with 100% accuracy?

There are many factors that come in to play when gathering and interpreting the data. Below we outline a few of them. There are other limitations which are not listed here. This overview is meant to give a general understanding of how the technology works so you can be better equipped to use the information your scan technician gives you.

As with all radar technology, the data cannot be used to determine the diameter of objects in the slab. Your scan technician cannot tell you if he is detecting ¾” rebar, or 1- ¼” rebar.

Because objects in the slab deflect the signal (especially metal objects), other objects directly below an object on top will become very hard for the scanner to detect. This problem grows as the signal reaches deeper and deeper into the slab. This is considered to be the main limitation of radar technology.

One can’t know with total certainty how fast the signal is able to travel through each given slab of concrete, as each slab has a slightly different dielectric constant (how much moisture is in the slab). That moisture resists the signal, slowing it down. So the more moisture you have, the more you slow it down. Since the scan technology calculates object depth based on the time it takes for a signal to get to the object and back again, it becomes less precise because of this.

If there are many objects in a slab, it may be hard to determine which object is running which direction. Since the scan is a variation of “connecting the dots”, it may be difficult to see which dot connects to which when there are 15 of them marked on the slab.

Hire only trusted experts with a breadth of experience in the industry to perform and interpret the scans on your projects. Our technicians will take thorough care to ensure your objects are marked out clearly and accurately, and to communicate openly any barriers to the scan performing optimally, so your project remains on time and on budget.