Now You See It, Now You Don’t: Ground Penetrating Radar Depth Explained

GPR equipment and technician showing ground penetrating radar depth
From wet clay to compacted parking lot gravel to dry sand above Basalt, soil conditions can drastically affect radar suitability.

Almost every day, we get asked, “How deep can your radar see?” Which is why we decided to tackle ground penetrating radar depth head-on in today’s post. We’ll deep-dive into the world of GPR scanning to explain why questions about GPR depth aren’t as easy to answer as you might think. For the purpose of this post, we’ll be focusing on the utility locating aspects of GPR use.

Why Clay and GPR Scanning Aren’t a Match Made in Heaven

Let’s start with a quick history lesson (we’ll keep it short)… Millions of years ago, the Ancestral Rocky Mountains eroded. They were made mostly of granite, so when they broke down, the feldspar in the granite decomposed and turned into clay. This, combined with other geologic sources, resulted in deposits of clay in most of the soils in the U.S. #thanksAncestralRockies 

Concerning GPR suitability, it could be said that most soils fall on a spectrum between wet saline clay and dry sand. 

Okay, but what does this have to do with ground penetrating radar depth?

Well, clay weakens the signal of GPR. It sucks up all the energy of the EMP (a.k.a. Electromagnetic Pulse) coming from the antenna. This means that ground penetrating radar depth is seriously compromised in heavily clayed soils.

Soil maps (like this one for California) can help give us an idea of the clay composition of specific areas, but it’s not always possible to know what was at the site before.

Let’s say your site used to be a geographic basin, and it was filled up and covered years ago to make a strip mall. What was it filled with? What was the soil composition of the fill? If you go far enough back, people didn’t even use soil. They used literal garbage to fill up bodies of water, ditches, and more. The point is, you really have no idea what’s sitting under your feet—that is, until you scan it.

For one project, we went to a site and were only able to see 2 feet down. Then, we drove a mile up the road, in the same city, and we could see 8 feet down! Going by the soil maps (and logic), these areas should have had a similar soil composition, but that wasn’t the case.

It’s truly impossible to predict what the soil conditions are actually like until you’re on site and can assess it with a GPR antenna.

The 5 Things that Affect Ground Penetrating Radar Depth Penetration

Clay isn’t the only thing that affects GPR depth. 

There are five basic factors that can impact ground penetrating radar depth.

  1. Soil Composition: What’s in the soil, and how much clay is in there? (What we talked about above).
  2. Antenna Frequency: Are we using a high or low-frequency antenna? There are different central frequencies of GPR antennas, and the frequency of the tool used can determine the readable depth.
  3. Size of Target: Generally speaking, if a buried target is 2 inches in diameter, it will be much more challenging to locate than a target that is 2 feet in diameter. For example, it would be easier to find a large underground storage tank (UST) at 3 feet down than a 2-inch waterline at the same depth.  
  4. Target Material: Some materials show up better than others. So whether your target is plastic, copper, asbestos, concrete, metal, etc.—it’s going to impact its findability with ground penetrating radar.
  5. What’s Inside the Target? Is it full of water, fuel oil, air, or natural gas? Whether the target is full or empty and what it’s full of will determine the tools and our ability to see the item.  

So, how can we do what we do if there are so many factors outside of our control? It comes down to the tools of the trade and knowing how to wield them.

How We Tackle GPR Scanning and Issues with Depth

A good utility locator has many tools up their sleeve. Here is how we approach some of the main types of scanning:

  • For concrete scanning, we almost always use a high-frequency GPR antenna.
  • For utility scanning, we usually use a low-frequency GPR—though there’s an entire range of frequencies that could all be considered “low,” each with its own pros and cons.
  • Sometimes utility scanning requires a different tool entirely, so we’ll use an EM (electromagnetic) utility wand, an acoustic locator, or something else.

To be frank, sometimes it simply isn’t possible to locate a particular buried target. That’s rarely the case, but it can happen. Occasionally, a utility is truly unfindable, and any utility locator worth their salt will be straightforward about that fact.

Thankfully, locating is getting more effective every day: the technology is progressing quickly, utility owners are making utilities easier to find, and the maps are getting better. Impossible-to-locate scenarios are getting fewer and farther between.

As we like to say, locating today is less like finding a needle in a haystack and more like finding a needle in a handful of hay. (Still, it takes some skill to find that needle.)

The Short and Fast Answer (Average Ground Penetrating Radar Depth)

So, how deep can our radar see?

It depends. I know, I know. It’s not a satisfying answer, but it’s the honest one when it comes to ground penetrating radar depth.

How deep we can see depends on so many factors, as we’ve touched on: soil composition, the tools and antenna used, and the target’s size, material, and type.

For example, a large steel underground storage tank in dry sand will be much easier to find than a 1-inch plastic water line buried beneath wet saline clay. Even if they’re buried at the exact same depth, our ability and means of locating them will vary dramatically.

Still not satisfied? Here’s our quick answer: a GPR antenna with a central frequency selected for utility locating and scanning in average soils can typically see within a range of 3-6 feet in many parts of the U.S. (Vague enough for ya?)

Of course, based on everything we’ve discussed, it’s sometimes more and sometimes less. Ultimately, we won’t know how deep we can see until we get to your site. Fortunately, we always bring along other tools that are great complements to GPR.

Here at Enhanced Scanning, we have an excellent track record of finding utilities. You can rest assured that if we can’t find a buried target, it’s likely that no one can—and you can always trust us to be honest with you about what’s possible. If you’d like help locating pipes, vaults, tanks, and other buried oddities, contact us here.

Give us a call: 951-783-2483