Drones have many uses. With the advancements in drones’ payload capacity (ability to carry things), flight time, and stability, drones have been introduced into fields such as surveying, archaeology, and inspections.
But can drones make GPR viable? If we are talking about airborne, maybe. Can drones make GPR viable when on the ground? Absolutely. Can drones make GPR viable when underwater? Without a doubt.
In this article, we will discuss these three forms of how drones can carry GPR, what GPR is, and how it can benefit different fields when using a drone as its carrier.
What is GPR?
GPR stands for Ground Penetrating Radar.
You may be familiar with metal detecting, where you’ve got a handheld device with a cylinder about 4 to 5 feet long protruding from the box you’re holding onto.
A circular ring is attached to the opposite end of the cylinder. This sends electromagnetic pulses down onto the ground and receives those same pulses whenever they bounce back.
This allows a metal detector to identify what kind of metal it is and, to some degree of accuracy, identify how deep the piece of metal is. GPR is extremely similar to a metal detector, so you can think of it as an evolution of the metal detector.
GPR transmitters emit pulses of electromagnetic energy to the subsurface, which are used to detect changes in the subsurface.
Some of the electromagnetic energy reflected back from the surface is caused by a sub-surface change. A receiving antenna mounted on the GPR detects this and records variations in the return signal.
This information is displayed on radargrams. The radargram can be thought of as a topographic map made from aerial photogrammetry or LiDar scan.
However, while the aerial-derived topographic map shows the true elevation of the land in most cases, GPR does have its limitations.
Ground Penetrating Radar is able to detect changes in sub-surface, but it cannot determine their exact nature.
Certain features have particular characteristics in their reflected wave pattern. Reflections from metallic surfaces, for example, have a high amplitude, while those from voids are reverse-polarized and will show up differently on the radargram.
These pieces of data are useful in identifying the features. In some cases, however, absolute data from boreholes or sample cores may be required to augment a Ground Penetrating Radar survey.
What can Ground Penetrating Radar detect in the ground?
Ground Penetrating Radar can be used effectively to identify and differentiate a variety of non-metallic and metallic materials.
GPR is most effective when there is a significant difference in the electromagnetic properties between the materials being surveyed, such as metal objects (e.g. reinforcement in concrete).
Ground Penetrating Radar can detect most materials if there is sufficient electromagnetic difference between the target material and the surrounding material. The most common target materials are:
- Variations in geological features and ground strata
- Valves or air pockets
You can also map excavated areas, backfilled areas, and other ground disturbances.
Ground Penetrating Radar won’t work in some ground conditions, especially if they are clay soils. GPR is not affected by de-ionized water or water with high mineral content (e.g., seawater, which has a high mineral content.
Can drones really use GPR?
GPR, when attached to a UAV, will perform similarly to a LiDAR mapping mission or a photogrammetry mapping mission using a drone.
The drone would most likely fly in an automated pattern, such as a grid or double grid. In some cases, the drone would fly in a linear route.
The drone will collect data as it flies on its route and build a model or radargram of the artifacts underneath the surface. These artifacts could range from a geologic formation to a pipeline to buried treasure.
As discussed previously, the final product will look much different than an orthomosaic created by a photogrammetry mission or even a topographic map created by LiDAR.
However, it’ll be more similar to the contour lines in the topographic map created by LiDAR.
There is a lot to be said for the idea of strapping a GPR unit to a drone, but there are a couple problems with drones using GPR, at least airborne drones.
Namely, most aerial drones have a payload capacity that is beneath the weight of most GPR units. Many GPR units are extremely heavy and hard to move around and are sometimes even motorized.
As technology gets more compact and smaller, GPR units get smaller and get lighter as well. DJI has created a line of payload drones that are able to lift specific GPR units.
Below is a video of a drone providing lift to a GPR unit and performing mapping on an area of buried pipeline.
As shown above, the possibilities for ground penetrating radar on an airborne drone are many. However, as discussed earlier, there are limitations.
GPR sends waves to the ground and receives them as they come up. The issue with using a GPR unit when attached to a drone is the limitation it puts on the depth that it can penetrate the soil.
The benefit of a GPR unit strapped to a drone is that the drone is able to fly clear of any items on the ground, such as mounds of dirt, trees, or rock formations such as cliffs or boulders, or rock outcrops.
However, the drone would have to use some sort of altitude terrain mode where it would sense the objects in front of it and below it so that it could move out of the way. The drone has to stay on course, so it would only have a choice of going up.
If the drone, as shown in the video, is only 4 feet above the ground or so, it would have no trouble collecting the data from the GPR scan.
However, if it ran into an object such as a tree and elevated itself to 20 feet, the data collected by the GPR scan would be near useless as the technology stands today.
That isn’t to say that the technology is completely useless on an airborne drone. The example before supposed that the area that was being surveyed with the drone GPR had structures or objects that would obstruct the drone’s flight path.
Shown below is an example of how drone GPR is being used in areas where there would be no obstructions to a drone flying close to the ground.
This usage of drone GPR is an excellent example of one way drones are being used to carry GPR right now in industries such as archaeology.
With no obstructions in the flight path of the drone, the drone can remove any human error that would normally occur when GPR is being used to walk the path and scan the Earth.
By programming the drone to have a flight path that is 70% overlap, you make sure that you’re not missing any piece of the area that you’re surveying. You’ll be able to build an entire model of the structure beneath the ground without any missing spots.
This is extremely useful in areas that have large spans of wide-open areas that would be difficult for a human to walk with any accuracy, such as a desert.
Not only is this form of GPR surveying easier on the surveyor, but it is also quicker.
A drone can cover an area of land while scanning with GPR much faster than a human can, so while there are limitations on what a drone can do with GPR, it is still a useful tool.
These limitations, as discussed above, are really only on aerial GPRs, however. If the drone in question is not an airborne drone, it is possible that these obstructions that largely limit a drone’s GPR unit could be avoided.
Ground-based drones using GPR
If a drone is completely ground-locked and is made to traverse over tough terrain, it is possible that this could be the perfect combination between airborne GPR and standard human-powered GPR.
You may be familiar with the automated AI-driven vacuum cleaner otherwise known as Roomba.
The technology applied in Roomba could be applied to a drone GPR unit that is ground-based. In fact, there are already units that are designed to cut grass that use the same technology as a Roomba, and a GPS drone mapping mission that traverses over tough terrain.
The only technology that will need to be applied is the GPS unit, possibly an RTK/PPK unit, sensors to avoid any objects, such as a tree or large boulders, and the GPR unit itself.
Navigating around the tree and mapping shouldn’t be an issue for the all-terrain ground drone. However, the large boulder would be an issue, but these are issues that human GPR-powered units already run into.
While the drawbacks are much less than the airborne GPR unit, the benefits are very much the same.
While a GPR-driven ground-based drone may survey the site slightly slower than an airborne drone, it could navigate around tough terrain, move faster than a human pushing a GPR unit, and run continuously as opposed to a human, who may get tired.
All that the unit would need is a large supply of batteries, and it could survey a site mile by mile and beyond. We will likely see an evolution geared towards using GPR on drones in the field, with the unit of transportation being a ground-based drone.
Underwater drones using GPR
Another interesting usage of ground penetrating radar could be on underwater drones.
Underwater drones have come a long way and have had a more difficult path than airborne drones as far as mapping and serving capability is concerned.
One company out of Australia has designed a drone to get around problems in mapping that occur.
These problems that you run into when using underwater drones for mapping are that you are limited by the length of the power cable that attaches from the drone to the remote controller or base station.
You might be asking, “Why even use a cable and not use a battery such as on an airborne drone or ground-based drone and transmit the signal between the RC and the drone via Wi-Fi, radio, or 5G?”
The answer is that with underwater operations, there isn’t a signal out of these three that can transmit dependably through water.
This means it’s very likely that at a depth where using an underwater drone GPR would be necessary. You would have little to no control over it.
However, the Australian company mentioned above has developed a signal that is able to transmit data to and from the drone to control the movement and track its data.
Another solution would be to simply have an automated dive already programmed on the underwater drone and have artificial intelligence solve problems underwater, such as any obstructions in its path.
Using underwater drones with GPR would very likely result in the archaeological community finding civilizations that were unheard of.
Sea levels throughout history have risen and fallen as ice caps have melted and frozen, and we have found entire tribes and kingdoms underneath the ocean.
However, the remains of these civilizations are limited because we only find them due to their exposure on the surface.
If an underwater drone with a GPR unit could map the seabed or lakebed, the likelihood of archaeologists finding remains of civilizations lost to the past increases drastically.
Other uses for underwater drones with GPR could be inspections of underwater infrastructures, such as power cables or buried pipelines.
Final thoughts on drones using GPR
Without a doubt, using drones as a unit of transportation for surveys with GPR is an all-around benefit, and it’s definitely possible.
The issues that we run into today when using airborne drone GPR will likely be solved as GPR technology increases and drones become better at carrying payloads while becoming smaller themselves.
Not only this, but ground-based GPR is already a reliable way to map land masses.
Underwater GPR poses new adventures and possibilities for archaeologists. Ground penetrating radar would likely see a leap in its usage in many industries.