Ground-Penetrating Radar (GPR) is an innovative and effective tool for uncovering buried utilities and other items like underground storage tanks. Because of the range of frequencies that GPR has, and the different types of materials that GPR can be used to scan, the answer to the question “ How Deep Does GPR Go? ” is complex.
How Deep Does GPR Go? That Depends on:
Generally speaking, a lower frequency radar signal will travel further through whatever is being scanned. The low frequency signals may detect anomalies up to a hundred feet below the surface! Unfortunately, this lower signal does not provide as high a resolution as the higher frequency signal, and therefore an ultra-low frequency signal may not detect smaller utilities or anomalies, especially at greater depths. A higher frequency signal, on the other hand, has higher resolution and can be used to precisely detect the depth and location of very small utilities or reinforcement.
Another aspect to consider is soil composition. Soils with higher conductivity, like clay and shale, tend to impede the GPR signal, making it more difficult to reliably detect utilities the deeper the signal travels. Conversely, soil with lower conductivity, like sand or gravel, tend to allow the signal to be transmitted much deeper. Utilities are much more easily found in these types of soils. The dielectric setting on the GPR device can be adjusted to counteract some of this signal dampening, but this is only somewhat effective.
Additionally, in some cases the soil conditions can vary from one part of a site to another. Soil may be suitable in one area beneath a parking lot and then if you move over 100 yards, the soil may be unsuitable. Engineered back-fill tends to scan better than original soils that may be filled with rocks and roots from trees. Changes in exposure to regular moisture, such as an area near a drainage culvert or underneath a leaky roof, may also affect the viability of ground-penetrating radar.
Different parts of the country also have different common soil types. Below is a map that is a general guideline for suitability of soil conditions for GPR.
Moisture can also play an important role in how effective the ground-penetrating radar will be. The more moisture soil or concrete has in it, the less effective the GPR scan. So “green” concrete that hasn’t completely cured, or soil – especially soil that holds moisture well, like clay – immediately after a rain are not optimum surfaces to attempt a GPR scan. They can decrease the depth a technician will be able to scan for from 8-10 feet under perfect conditions to only a foot under extremely wet conditions. For concrete, a scan that would usually go 18 inches can drop to only 4 or 5 inches with extremely green concrete.
Luckily, these conditions are usually better for electromagnetic induction locating. As the moisture levels rise, the conductivity of the soil rises, which allows an electromagnetic signal to be carried further at a lower frequency.
The last thing to consider when determining the effectiveness of a GPR scan is the presence of surface structures. Surface structures can be defined as anything above the grade of the concrete or soil to be scanned that could interfere with the smooth gliding of the antenna over the surface. In soil, typically these are things like buildings, signs, fences, trees and roots. The GPR cannot roll over these objects (with the partial exception of roots) and therefore cannot scan directly under them. Roots are often visible to the GPR and indistinguishable from anomalies or utilities.
In concrete, typically the surface structures are portions of the building such as walls, sprinkler systems, wall-mounted utilities, floors, or footings, or utilities coming into the building through cores. Again the GPR cannot roll over these structures, and in many cases, there is no way to scan the areas within 4 inches of them.
With a 400 MHz antenna, the kind most often used to scan the ground for utilities, the safest reliable depth to detect at is about 8 feet or less for most types of soil. Luckily, most utilities are not buried deeper than this, and many common excavations do not go this deep either.
Antennas that run at a frequency of 1600 MHz are much higher resolution, and therefore used in concrete x-rays, since reinforcement can be detected with these antennas much more precisely. The safest depth for this kind of antenna is about 18 inches. While some concrete structures are thicker than than 18 inches, the handheld antenna can often access both sides of a given area. This improves the scan depth to up to 36 inches.
While these are general guidelines for depth, it is important to remember that they are variable dependent upon soil conditions. It is pertinent to point out, too, that once the GPR signal bounces back from a utility, reinforcement, or other anomaly, less of the signal is penetrating directly beneath it. Because of this, there may be hidden utilities or reinforcements in the “shadow” of a detected utility or reinforcement. A layer of tight rebar may render the concrete beneath it effectively impossible to scan accurately. A layer of pan deck or soundproofing will often completely block any GPR signal from travelling below it. Similarly, a large metallic object such as an underground storage tank may block the signal from traveling beneath it, causing any utilities or voids below it to be undetectable.
Encompass Inspections cannot determine before arriving onsite how deep the GPR will be able to go, as conditions can vary from one part of a site to another. But we are always operating under the assumption that we will find what we are looking for, whether it is concrete reinforcements or utilities under soil or concrete. We always use every means at our disposal to ensure safe excavation on your jobsite. Contact us for more information.