Ground Penetrating Radar: Everything You Need To Know About [2024]
Ground-penetrating radar is an electromagnetic technique that provides high-resolution 2D and 3D subsurface images.
In this guide, we dive into how it works and why RTK corrections can significantly improve the accuracy of GPR.
What is Ground Penetrating Radar (GPR)?
Ground Penetrating Radar (GPR) is a geophysical method that emits radar pulses into the ground to measure the reflected signals. It helps us see what’s buried under the surface without digging and provides details about the depth and type of materials or objects based on their differing dielectric properties.
What is GPR Used For?
GPR has many applications across industries because it’s a non-invasive method to image the subsurface.
Here are some common use cases:
- Locating underground objects: Engineers and construction teams use GPR devices to locate underground objects like conducive pipes. Finding these pipes is critical to preventing damage to existing infrastructure and ensuring the safety of workers and the public during excavation or construction.
- Detecting explosives and tunnels: The military may use GPR to detect landlines and unexploded ordnance. GPR also helps them identify and map subsurface tunnels used by the enemy for covert activities.
- Archaeological site mapping: Archaeologists use GPR to locate and map archaeological features like burial sites and artifacts. GPR’s non-invasive nature allows archaeologists to identify and preserve historic structures and objects buried underground.
How GPR Works
GPR involves emitting radar pulses into the ground you’re trying to image or map. The reflected signals are analyzed to image subsurface structures like pipes, conduits, and cables. Here’s how the whole process works:
GPR Equipment
GPR requires various pieces of equipment, such as:
- Transmitter and receiver antennas
- Display and data storage devices
- Mounting and transport systems
- RTF GPR (or a GPS unit) for location tracking and georeferencing
- Coaxial and data cables
- Calibration tools
- Control unit
While not required for ground penetrating radar, real time kinematic (RTK) technology can enhance the effectiveness of GPR. RTK positioning corrects errors in standard satellite data to achieve exceptionally precise positioning data.
This allows for the creation of detailed and accurate maps of subsurface structures, improving the reliability and effectiveness of GPR surveys. The high precision of RTK ensures that the spatial coordinates of the GPR data are accurate, which is essential for applications like utility detection, archaeological surveys, and geological investigations.
For example, you can combine Polaris RTK with GPR to pinpoint exact locations and depths with the highest accuracy. It can be set up in five minutes by connecting to a single NTRIP mount point and it automatically assigns itself to the nearest base station to give you access to local datums.
With Polaris RTK, you get centimeter-accurate positioning, lighting-fast convergence times, and 99.99% network uptime.
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GPR Method
Once you have the necessary equipment, GPR includes:
- Choose an appropriate frequency: Transmitter and receiver antennas operate on a specific frequency. Choose a frequency based on the required resolution and depth.
- Setup and calibration: Place the base station (if you’re using one) and connect all GPR equipment. Calibrate the system based on the site conditions you’re dealing with.
- Pulse and reflection: The GPR device sends pulses into the ground. When these pulses hit materials or objects like pipes beneath the ground, the pulses bounce back and create reflections.
The device records the strength and timing of these reflections. When you send multiple pulses and collect reflections over a single area, you form a scan.
These factors impact the scan:
- Reflection strength: Reflection’s strength differs based on the materials’ properties. For example, moving from a low-dielectric material like dry sand to a high-dielectric material like wet sand creates a strong reflection and vice versa.
- Signal travel time: Some of the energy is reflected to the GPR antenna while the rest continues to travel deeper into the ground until it fades away or the GPR device stops recording. The speed of the signal depends on the material it’s traveling through.
The radar doesn’t emit energy in a straight line of waves. Instead, it spreads the waves out in a cone shape from the antenna. When the antenna moves over a buried object, the reflections come from a hyperbola (an inverted “U”).
The object you’re looking for is located at the peak of this hyperbola in the recorded data.
GPR Data
The GPR device collects data as you move it across the surface. The data is collected in parallel lines and combined using software. The software creates a horizontal view of the ground at a specific depth (called depth slice) to give you a top-down view of the survey area.
The software also helps you remove background noise, adjust hyperbolas, and calculate accurate depths by applying mathematical functions to the data.
Benefits of Using Ground Penetrating Radar
GPR offers various benefits compared to other subsurface imaging technologies. We discuss how GPR is better than other technologies below.
GPR vs LiDar
LiDar is used to map surface topography and vegetation, while GPR detects objects and features below the surface. Unlike LiDar, GPR can identify materials under the ground based on dielectric properties, providing insights into soil composition, buried objects, and voids.
Moreover, GPR can detect changes in soil moisture and identify water-saturated areas, while LiDar doesn’t offer this capability.
GPR vs Sonar
Sonar is primarily used for underwater applications. It requires water to transmit sound waves, which means it doesn’t work on land. GPR is used for terrestrial applications and it can be used in a wide range of environments, including urban areas, deserts, and archaeological sites.
GPR vs Seismic Surveys
GPR is a more cost-effective survey method and is faster to deploy compared to seismic surveys. GPR systems are more portable, easier to carry, and can be stored in confined areas.
On the other hand, seismic surveys require an extensive setup and a large operational area. Seismic surveys are more suitable for deeper investigations but don’t offer great detail in shallow layers. If you want a high-resolution image for shallow subsurface investigations, GPR is a better choice.
Limitations of GPR
GPR is a powerful subsurface imaging tool, but it has its limitations. Here are some limitations of GPR:
- Limited penetration depth: GPR’s penetration is significantly lower in highly conductive soils like clay. Conductive soils absorb and attenuate the radar waves and limit how deep the radar can see. It’s even harder for radar waves to penetrate deeper when there are dense materials like concrete and metal in the way. These materials reflect all of the radar energy, causing signal loss beyond these materials.
- Can’t achieve high resolution and deep penetration: Higher-frequency antennas offer better resolution but shallower penetration. While lower-frequency antennas can penetrate deeper, they offer less detail. There’s always a tradeoff between resolution and penetration depth when using GPR.
- Interference: Electromagnetic interference from nearby power lines, radio transmission, or other electronic devices can introduce noise into the GPR data. So can natural ground variations and clutter. Noise makes it difficult to distinguish meaningful reflections from noise.
- Velocity variability: To accurately determine depth, you need to know the velocity of the radar waves through subsurface materials. If you don’t account for variations in material properties, you could end up with inaccurate depth calculations.
- Similar dielectric properties: You might struggle to differentiate between materials with similar dielectric properties when using GPR. For example, it’s difficult to distinguish between dry sands and certain types of rocks because of similar dielectric properties.
Why You Need RTK for GPR
While GPR is a powerful method, RTK technology can greatly improve the results of a GPR survey. Here’s why:
Improved accuracy
RTK offers centimeter-accurate positioning data, which is crucial for mapping the exact locations of the subsurface features detected by GPR. With RTK, you can create detailed maps of the survey area and ensure the GPR data is correctly georeferenced.
An RTK system includes a fixed base station and a rover unit. Both of them receive GPS signals from satellites: the base station calculates the difference between its known position and the position calculated by the GPS signals and then sends correction data to the rover in real time to improve accuracy.
Efficient data collection & integration
RTK ensures that the spatial coordinates of GPR data points are accurate. This is critical to integrating GPR data with geospatial data, such as maps, aerial images, and other survey data.
Accurate data also eliminates the need for guesswork and manual measurement of survey points. With accurate positioning, your data’s quality remains consistent throughout the survey, which translates to more reliable and interpretable results without a ton of post-processing.
Enhanced safety
Precise location data helps ensure that underground utilities like electrical cables, gas lines, and water pipes and any subsurface hazards like abandoned storage tanks and voids in the survey area are accurately mapped. This helps prevent accidents during construction or drilling activities. You can establish safety zones around detected hazards so personnel and equipment are always at a safe distance.
Since RTK helps you achieve high accuracy, there’s little need for repeated surveys. This means you spend less time in potentially hazardous environments. RTK also ensures systematic and thorough coverage of the survey area and reduces the chances of missing critical subsurface features that pose safety risks.
Cost savings
RTK reduces the risk of errors, which means you’ll spend less time and resources on rework. Moreover, you can collect data much faster with RTK and reduce the time you’d otherwise spend on the field when using GPR.
This also means you need fewer man hours, which reduces labor costs, and you’ll be able to make more efficient use of GPR equipment, extending its life span and lowering maintenance and replacement costs.
Should You Rent, Buy, or Hire Ground Penetrating Radar?
The decision to rent, buy, or hire GPR services depends on factors like cost, frequency of use, and your project’s specific needs. Here’s an overview of these factors that can help you choose between the three options:
Renting GPR Devices
Renting GPR devices can be a good option that offers financial flexibility, access to up-to-date technology, and reduced maintenance concerns. You could consider renting if:
- You have a one-off requirement: When you have a short-term project or a one-time need.
- You want to try various GPR equipment before investing: If you are exploring GPR technology and want to evaluate different models.
- You always want access to latest tech: When you need access to the latest technology without committing to a purchase
Buying GPR Technology
You should buy GPR technology if:
- You want to use GPR for the long run: Buying is more cost-effective in the long run if you plan to use it regularly.
- You need GPR equipment immediately: Purchased equipment is available to use whenever you need it.
- You want to customize equipment: You can choose and configure equipment based on your specific needs.
- You want to claim depreciation: Owning the asset allows you to claim depreciation expense, which reduces your taxable income.
Hiring GPR Professionals
You should hire GPR professionals if:
- You need professional expertise: If you don’t have in-house operators or experts, consider hiring a service that includes experienced operators who can accurately collect and interpret data.
- You don’t want to invest in equipment: Equipment purchase and ownership come with a large cash outflow and expenses towards maintenance and training. If you want to avoid these, hire external GPR professionals.
- You want flexibility and scalability: Equipment logistics can limit your ability to scale GPR surveys based on project size and requirements. If you need more flexibility and scalability, consider hiring GPR professionals.
More About Ground Penetrating Radar
We’ve answered some common questions about GPR.
How much does GPR cost?
The cost of GPR equipment can vary greatly based on factors like the type of system, features, and manufacturer. Entry-level systems may cost between $10,000 and $20,000, while professional-grade systems may cost $100,000 or more.
How deep does GPR go?
The depth of a GPR survey depends on the radar antenna’s frequency, the material properties of the subsurface, and the specific application. For example, low-frequency antennas (50 MHz to 400 MHz) can penetrate up to 30 meters or more in dry or sandy soils because of their low electrical conductivity. However, the high clay content in the soil allows much shallower penetration of up to a few meters because it can significantly attenuate the radar signal.
What can GPR detect?
GPT is a versatile tool that can detect a wide range of subsurface features and objects, including pipes, cables, drainage systems, groundwater, contaminants, sinkholes, and more.
Improve Ground Penetrating Radar with the Best RTK Network
Combining GPRs with RTK correction services is the best way to achieve high accuracy during surveys. RTK’s centimeter-level accuracy helps you find the exact location of subsurface objects like pipes and cables and ensures worker safety.
If you’re looking for the best RTK solution on the market, we’ve got you covered. Point One Navigation offers Polaris RTK, a state-of-the-art RTK solution that offers centimeter-accurate positioning and 99.99% network uptime. The network is easy to use and only requires setting up once–so you can access RTK-enabled GPR quickly and efficiently.
