Ground Penetrating Radar
Ground Penetrating Radar (GPR) Ground Penetrating Radar (GPR) Ground Penetrating Radar (GPR)

 

Ground Penetrating Radar

Detailed subsurface data with specialized drone-mounted GPR systems

How it works

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.

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Deliverables

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”.

Benefits of drone-mounted GPR

    Drone-mounted GPR implements the motto “safer, cheaper, faster.” Here are a few situations where the use of GPR on the drone is beneficial

  • Rough terrain where surface surveys may be impossible (ice and snow-covered ground, rocky and uneven terrain, across rivers and in avalanche-prone areas)
  • Areas with safety or health risks for the operator (glaciers with crevices, contaminated soils, etc.)
  • Large unobstructed areas where the productivity of terrestrial surveys will not be economically reasonable (for example, scanning huge fields for solar panel farms for the depth of bedrock and presence of rocks)

    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.

Why do we need different GPR systems?

All GPR systems use the same principles but vary in application due to different antenna frequencies:
  • Low-frequency GPR systems penetrate deeper with lower resolution and are suitable for larger objects at greater depths.
  • High-frequency GPR systems offer higher resolution for smaller objects but have limited penetration.

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    The antenna design reflects:
  • Low-frequency antennas are larger and unshielded
  • High-frequency antennas are compact and shielded to reduce noise.

    • The variety of GPR's ensures the right system for every subsurface investigation needs.

      Zond Aero LF with 50 Mhz 3m long antenna

      Deepest possible penetration - up to few dozens of meters in very-low conductive soils (dry sand or rocks), or hundreds of meters in ice - making this system an excellent tool for glaciology and deep scanning.

      Zond Aero LF with 75…400Mhz antennas

      Zond Aero LF on drones like DJI M350/M300 RTK can be used with antennas in frequency range 75…400Mhz, allowing to select appropriate frequency for a particular application. Changing the antenna takes a couple of minutes.

      Zond Aero 500 Mhz

      A pretty universal system with a shielded antenna is capable of penetrating deep enough for many applications of engineering geophysics and detecting relatively small objects or thin utilities.

      Zond Aero 1000 Mhz

      Best possible resolution and possibility to detect small targets, but penetration under the surface in normal conditions will be less than 0.5m.

    Typical applications

    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.

    Best Practices for Using the GPR System

    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.

    • 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.

    • “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.

    • 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.

    GPR calculator by SPH Engineering

      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

     

     

      System components

      The GPR system for UAVs consists of multiple components. For your convenience, all necessary components, software, and services are combined into GPR bundles.

      Compatible drones: DJI M300/M350/M600, Inspired Flight IF1200A, Harris Aerial H6, and Wispr Ranger Pro and similar UAV

    Data Sets

    Discover the large collection of data examples for different GPR systems and applications by SPH Engineering »»»

    Case Studies

    • Drone-based GPR system for alpine glaciological applications

    • Drone-based GPR for Retaining Wall Quality Control

    • Drone-based GPR and Echo Sounder for Mining Operations Enhancement

    • Airborne GPR System for safer and more efficient Snow and Ice Studies

    • High-frequency airborne GPR system for bathymetric surveys

    Frequently Asked Questions

      Can we use GPR to detect landmines?

      In theory yes, in practice no. The main issue here is the extremely high rate of false positive detections in the real environment. Please check our report from the UXO/landmines test range.

      The only feasible application of GPR here is to use it as an auxiliary sensor to collect more information (depth, size) about targets detected using sensors utilizing different physical principles (magnetometers, metal detectors).

      Can GPR measure the depth?

      Physically, GPR measures the time (in nanoseconds - ns) when a reflected signal was received, using the moment when the signal was sent as a zero point. That time period is called Two-Way Time (TWT). GPR data processing software can recalculate time in depth if you inform the software what kind of soil/medium was in your survey area.

      What is the recommended flight altitude for GPR?

      As low as possible, please account for the dead zone of single-antenna systems like Zond Aero LF.

      What is the dead zone regarding GPR?

      Some GPR systems (for example, Zond Aero LF) use a single antenna to transmit and receive signals. During the transmission cycle and some time after it, GPR can’t receive reflected signals. The dead zone for low-frequency antennas can be pretty large—up to a few meters under the surface. The GPR online calculator by SPH Engineering can be used to estimate the dead zone under the surface for a given type of GPR and antenna elevation.

      Can I fly over the forest and collect subsurface data?

      No. In theory, it is possible to use GPR systems with very powerful transmitters and narrow-beam antennas. Still, they don’t exist and, anyway, will not be suitable for small and medium UAVs because of the large size of directional antennas for low frequency of GPR. Also, the power of GPR transmitters is very limited in almost all countries.

      Why do we see hyperbolas in GPR data?

      Hyperbolas in the GPR profile correspond to small objects or linear objects crossed by survey lines. GPR antennas have a pretty wide beam, and they start to “sense” reflections from the target before the GPR antenna passes over the object and sometime after that. Recorded reflections will form hyperbolas if the antenna moves more or less constantly. The top of the hyperbola will be at the point when the GPR antenna is right above the object.

      Does it make sense to use GPR on drones in cities, to scan utilities under streets, etc.?

      No. GPR on UAVs is not suitable for confined spaces. In all the situations mentioned, it is better to use GPR with a cart in the traditional way. We supply terrestrial GPR carts for Zond Aero 500 and Zond Aero 1000 GPR systems.

      What mediums are the best for GPR?

      Snow and ice. In terms of EM wave propagation, ice and snow are almost the same as air. That makes snow and ice studies a very popular application of GPR (both drone-mounted and terrestrial).

      Dry sand and sandy soils are also very favorable for GPR.

      What mediums or soils are worst for GPR?

      Any mediums and soils with high conductivity, such as clay or clayey soil, farm fields with a lot of fertilizers, seawater, or contaminated water.

      What are additional “show stoppers” for drone-mounted GPR?

      In most cases, you may not use drone-mounted GPR after rain or when the top layer of soil is saturated with water and if there is tall vegetation or trees in the survey area.

      Are drone-mounted GPR systems legal?

      In many countries, there is a limit for the elevation of the GPR antenna over the ground, usually 1m. If the elevation of the antenna when used on a UAV is inside this limit, you are OK.

      But from another point of view - in the majority of cases, except snow/ice/very dry soils, drone-mounted GPR is useless when the antenna is elevated for more than 1m. So here, we have strict regulations, but they don’t limit the practical use of drone-mounted GPR.

      How to estimate what size of targets or diameters of the pipes can be detected using GPR mounted on the drone?

      The GPR online calculator by SPH Engineering can be used for that purpose.

      Select the GPR model, antenna elevation, estimated target depth, and type of soil/medium. The GPR calculator will estimate and display a lot of useful information.

      Please note that all these numbers are for favorable conditions and are not guaranteed for any particular conditions.

      How complex is GPR data processing?

      In most situations, data processing is simple and straightforward and requires just a few operations:

      1. Load RAW data from the sensor into processing software

      2. Remove background signal (constant noises) that hides reflections from objects or features of interest

      3. Increase the gain (amplification) of the signal to make subsurface reflections visible better and unveil weak anomalies

      If you don’t see your targets of interest after just these simple steps, most probably the thing you are looking for is not there, or data quality is bad, or subsurface conditions don’t allow the electromagnetic energy to penetrate deep enough and return to the receiver antenna of GPR. More complex processing steps can increase the contrast of anomalies and may allow extracting additional information about targets, but can’t help to find something in “scrap” data.

    Available GPR systems for drones - use and recommended applications

    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 N15 .. 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

    DATASHEETS