Ground Penetrating Radar (GPR) uses radar pulses to detect and image underground objects and features. A GPR transmitter emits electromagnetic energy into the ground. When the energy encounters a buried object or a boundary between materials having different dielectric permittivities (a property that defines the speed of electromagnetic waves), it may be reflected to the receiving antenna of GPR. The GPR electronics can then record the variations in the return signal.
Fig.1 - Drone-Mounted GPR Detecting Underground Anomalies
GPR data comes from the sensor in digital form and is not meant for direct human interpretation, unlike photos from cameras. It requires specialized software for processing and interpretation.
These methods of GPR data representation are the most popular, but many additional options exist, including export into formats that can be imported into CAD and GIS systems.
Fig.2 - GPR profile crossing a gas pipeline with interpretation. Data was collected using the Zond Aero 500 GPR system, processed, and interpreted in Radar Systems Prism2 software
The results of GPR surveys can be presented in different forms. The most common and “natural” format for GPR data is a “profile” - a vertical slice of data along the survey line.
Fig.3 - Horizontal slice of the subsurface to visualize the path of utilities. Data was collected using the Zond Aero 500 GPR system and processed in Geolitix
Another popular form is horizontal slices, as they give a better understanding of where the detected objects are under the surface and about the shape of objects.
Fig.4 - 3D representation of the same utilities as on the image with horizontal slices. Screenshot of Geolitix
Many customers prefer to see 3D reconstructions of the underground world - and that is also possible. It will require more processing and preparation steps, but as this method gives maximum understanding in complex situations, it becomes more and more popular, thanks to modern GPR processing software radically simplifying that task.
Fig.5 - Ice thickness grid. Data was collected using the Zond Aero 1000 GPR system and processed in Geolitix
One more popular method is to generate thickness grids, for example, to answer questions like “How thick is the sand layer covering bedrock” or “How thick is the ice”.
Drone-mounted GPR implements the motto “safer, cheaper, faster.” Here are a few situations where the use of GPR on the drone is beneficial
Fig.6 - A drone with Zond Aero LF GPR is taking off to scan the dangerous site with coal burning underground. The picture courtesy of DATUM Ingeniería SAS.
All GPR systems use the same principles but vary in application due to different antenna frequencies:
The antenna design reflects:
The variety of GPR's ensures the right system for every subsurface investigation needs.
The table provides a summary of what we can expect from the GPR systems available for drone use and their recommended applications. Here, we listed GPR systems manufactured by Radar Systems Inc., Latvia, as this line of GPR covers all possible applications for drone-mounted Ground Penetrating Radars. Any other GPR systems with similar center frequency will have more or less the same practical parameters regarding penetration and resolution.
Please note that penetration and resolution in certain places depend on soil composition, humidity, temperature, etc. In the table below, we used the parameters of a typical “average soil”—some substance with a relative dielectric permittivity of 9, low conductivity, and low water content.
By request, Zond Aero LF GPR systems may come with antennas for custom central frequencies.
| Central frequency, MHz | 1000 | 500 | 300 | 150 | 100 |
|---|---|---|---|---|---|
| GPR model | Zond Aero 1000 | Zond Aero 500 | Zond Aero LF | Zond Aero LF | Zond Aero LF |
| Penetration from surface, m | 0.5 .. 1 | 2 .. 4 | 4 .. 8 | 8 .. 15 | 15 .. 20 |
| Penetration from the drone, m | 0.3 .. 0.5 | 1 .. 2 | 2 .. 4 | 4 .. 8 | 7 .. 10 |
| Penetration from the drone in freshwater, m (water conductivity <200 µS/cm) | - | 0.25 | 2 | 4 | 7 |
| Recommended maximum antenna elevation for airborne survey, m | 0.3 (practical limit is 0.6m) | 0.6 | 1 | 2 | 3 |
| Minimum size of detectable objects under the surface from the recommended altitude, cm | 7 | 10 | 20 | 35 | 50 |
| Minimum size of “deep” detectable objects from the recommended altitude, cm | 11 at 0.5m | 26 at 2m | 50 at 4m | 100 at 8m | 180 at 15m |
| Minimum diameter of detectable linear non-conductive objects like an empty plastic pipe, cm | 5 | 10 | 17 | 33 | 50 |
| Minimum diameter of detectable linear conductive objects like a metal pipe or a water-filled plastic pipe, cm | 5 | 8 | 13 | 27 | 40 |
| Applications | |||||
| Small object search |  | | |||
| Glaciology, snow/ice thickness profiling | | | | | |
| Geological stratigraphy • subsurface stratigraphy • structure • bedrock surface | | | | | |
| Geotechnical surveys • cavity search • sinkhole search | | | | | |
| Utility Search • cables • water & sewage pipes • gas pipes • oil pipes | | | | ||
| Underground infrastructure mapping | | | | ||
| Archaeology • artifacts • hidden structures • stratigraphy • foundations | | | | ||
| Archaeology • caves • tombs • tunnels | | | | ||
| Forensics archaeology | | | | ||
| Freshwater Bathymetry | | | | ||
| Mining & Quarrying • rocks • fractures • faults • joints | | | |
These are the tips for effectively utilizing the GPR system. This is not an exhaustive list, and we are always ready to discuss your specific solution in detail.
Since, in the case of airborne use (when the GPR antenna is not in contact with the surface), a significant part of GPR impulse energy can be reflected from the surface, expected penetration from a drone is half of that achieved with a terrestrial survey on the surface. The recommended altitude (or the distance between the antenna and the surface) in the case of an airborne survey should be less than the length of the EM wave in the air corresponding to the central frequency of the antenna.
Penetration in good conditions, like very dry sand in the desert after the dry season, can be up to 2 times better. In ideal conditions (snow and ice), penetration can be 3-4 times better. Conditions like dry sand or snow/ice are also very good for airborne use. If the recommended altitude is maintained, we don’t see any significant degradation of maximum penetration into ice or snow compared with terrestrial use.
The minimum size of a detectable object is the diameter of the top flat surface of an underground object oriented horizontally. Sometimes (depending on the GPR travel direction), it is impossible to detect a sheet of metal even twice the minimum required size if, for example, it is positioned at a 45-degree angle.
“Minimum size” or “Minimum diameter” means that you are extremely unlikely to detect smaller objects. But it is not guaranteed that it will be possible to detect bigger objects – that will depend on dozens of other factors.
The diameter of the flat detectable reflector is estimated using a "rule of thumb" as 10% of the distance between antenna and object (antenna elevation + depth) OR half of the wavelength in host material—whichever is bigger.
The minimum diameter of empty detectable plastic pipes is estimated as GPR central frequency wavelength in air divided by 2.
The minimum diameter of detectable conductive objects (metallic pipes, water-filled plastic pipes) is estimated as 40% of the GPR central frequency wavelength in a host material (source: Ground‐Penetrating Radar for Geoarchaeology, Lawrence B. Conyers).
NEVER plan surveys using estimations close to the penetration limits, size of detectable objects, etc. Always use more conservative values.
A typical mistake of new GPR users is ordering a GPR system with maximum penetration and trying to detect smaller subsurface objects with it. Remember – good maximum penetration means poor resolution/capability to detect small objects.
When ordering a new GPR system for a particular application, please consider what penetration is necessary, i.e., do not exceed it too much. Potential customers often ask for a system for utility search with a maximum penetration of up to 20m. However, the usual depth of pipes/cables is 1-2m. It’s much better to order a 500 Mhz system, which will allow the detection of smaller/thinner objects.
A clay layer with even a small amount of water will ruin the acquired image. If there is clay or clayey soil in the survey area, the survey must be planned after a dry season or a long period of dry weather.
Electromagnetic waves don’t penetrate through salt water. So, GPR can’t be used for sea/saltwater bathymetry.
The GPR calculator can be used to estimate the detectability of targets at a particular depth and flight altitude (antenna elevation).
Enter information about Antenna Elevation, type of GPR system, Estimated Target Depth, and Material/Soil type to get the results.
Fig.7 - GPR calculator by SPH Engineering
And we will tell you how to implement a GPR solution for your application
The GPR system for UAVs consists of multiple components: GPR, SkyHub, laser/radar altimeter, UgCS and data processing software. Discover the components of airborne GPR system, training and certification on SPH Engineering's shop »»»
Compatible drones: DJI M300/M350/M600, Inspired Flight IF1200A, Harris Aerial H6, and Wispr Ranger Pro and similar UAV
And we will help you choose the best option
