Ground Penetrating Radar
Ground Penetrating Radar (GPR) Detection in Oregon (Eugene & Portland) and Washington State (Everett, Renton, Seattle, & Tacoma)
Ground Penetrating Radar, commonly abbreviated as GPR, is a geophysical technique that uses radar pulses to create images of the subsurface by detecting the reflected signals from subsurface materials. It is also referred to as Ground Probing Radar, Ground Radar, or Georadar.
RADAR (Radio Detection and Ranging) is a technology primarily used to detect above-ground objects such as aircrafts, ships, vehicles, birds, and rainstorms. In contrast, ground penetrating radar (GPR) antennas transmit an electromagnetic pulse into the ground, and when this pulse encounters a change in material or an object, it echoes back and is captured by the antenna’s receiver. The software used for ground penetrating radar utilizes the frequency and time delay of the transmitted pulses and received echoes to generate information about the target. The depth range and resolution depend on various factors such as radar frequency, transmitted power, the ground material’s dielectric/conductive properties, and the targets’ shape and characteristics.
Objects such as pipes, cables, wires, tanks, reinforcing elements, tree roots, rocks, graves, buried artifacts, etc. produce arch-shaped signals on the GPR image. This is because GPR energy is not confined to a narrow beam but travels in a three-dimensional cone. As a result, reflections can be detected on the GPR image even if the object is not directly beneath the sensor. The GPR sensor can detect the object before and after passing over it, resulting in a hyperbolic response for the image.
GPR is an exceptionally accurate tool for identifying both metallic and non-metallic objects buried beneath the surface. The technology works by emitting a small energy pulse into the ground via an antenna, with an integrated computer recording the time and strength of reflected signals. Any underground variations, whether metallic or non-metallic, will reflect signals back to the GPR equipment. The computer instantly displays all objects that are detected in real-time on the screen as the GPR locates move forward. With experience technicians can even differentiate between metallic and non-metallic features on some reflections. By tracking the location of the top of the hyperbolic responses, the alignment of subsurface utilities can be accurately determined.
When a GPR utility locator is calibrating the dielectrics of the soil conditions in a work area it is recommended that they cross objects at a 90 degree angle. This technique also helps produce more accurate depth estimates. GPR scans taken at an oblique angle (less than 90 degrees) over an object can result in incorrect depth estimation of targets when crossing long, linear targets like pipes or cables. The most commonly used method to locate subsurface utilities is cross and mark, which involves moving the GPR cart perpendicular to the expected utility axis (see the figure above). When the ground penetrating radar antenna crosses a utility, a hyperbolic response is displayed on the monitor, with the top of the arch indicating the utility’s position. The estimated depth to the top of the arch can also be determined. The unique capabilities of ground penetrating radar antennas allow a GPR utility locator to identify a utility in its topographical context which makes GPR an ideal tool for locating utilities before subsurface work is performed. An integrated digital signal processor (DSP) analyzes the reflections on an image map, helping provide the GPR technician with depth information on the reflected object.
The depth that GPR can penetrate into a material depends on the material’s dielectric/conductive properties and the frequency of the antenna being used. Different materials, such as ice, rock, soil, water, metal, asphalt, etc. have unique dielectric and conductive properties that affect GPR penetration. Lower frequency antennas can generally penetrate deeper, but at the cost of a decrease in resolution due to the lower frequency.
To achieve accurate depth estimations of targets in the GPR image, Dielectric Calibration is necessary for GPR locates.
There are three ways to perform a Dielectric Calibration:
There are three ways to perform a Dielectric Calibration:
- Educated Guess (being familiar with dielectric values of materials). This method typically has a 10-20% accuracy.
- Hyperbola Match (matching the shape of the hyperbola of the deepest object on the GPR image). This method typically has a 5-15% accuracy.
- Ground Truth (measuring the depth of a known target). This method typically has a 5-10% accuracy.
Changes in dielectric cannot be accounted for because only one dielectric value can be entered into the system at a time. Concrete tends to stay consistent, and soil can change horizontally and vertically.
Identifying Targets
When GPR antennas intersect with a structure like a pipe, cable, wire, etc. it creates a point target. The radar echoes received by the GPR at the point of intersection reflect the object’s shape, typically as a hyperbola. Additionally, radar echoes from the sides of the target take longer to reach the receiver than those from the top of the target. When the ground penetrating radar antenna intersects a metallic pipe, cable, wire, etc. in its path, the reflected shape of the buried target forms a hyperbola on the GPR image (See Figure A Below). When non-metallic pipes contain water, the hyperbola may appear duplicated (See Figure B Below) due to the radar echoes from the top, the water inside, and the bottom of the pipe. When pipes are buried in trenches with compacted walls, the radar echoes can reflect off the trench walls, resulting in an X shape above the hyperbola (See Figure C Below).
To assess the slope of a drainage pipe, a GPR sweep was profiled along a pipe. The pipe location was determined from Lines 1, 2, and 3 (See the Figure Below).
The depth of the drainage pipe while collecting a GPR scan parallel to the top of the pipe is seen in the figure below.
GPR profiles were taken parallel and perpendicular to the drainage in the figures above, revealing a gradual increase in pipe depth from right to left of approximately 3ft (1m). The concrete storm drain’s top and bottom reflections were detected, indicating no metallic structure. Based on the reflection depths and assuming it was an air-filled pipe, the diameter was estimated to be 36 inches. The entire process of locating, marking, tracking depth, and diameter estimation took around 10 minutes.
GPR images can be color-coded using a specific color palette to assist identifying reflections of GPR signals, since some reflections can be more easily identified in different color palettes. Multiple color palettes are available for displaying the image, with some palettes providing better visualization of the target than others. (See Examples Below)
GPR – Locating Pipes & Cables
GPR antennas have the advantage of detecting metallic and non-metallic features, making it a unique tool for locating pipes and cables. Unlike traditional devices that require metal pipes and cables to carry electrical current for detection, GPR can detect plastic, asbestos, concrete, metallic pipes, structures, air voids, etc. It can also estimate target depth using instrument positioning and signal travel time. When equipped with GPS and feature-tagging data loggers, it can provide a locating record as part of the standard locating and marking process.
GPR – Subsurface Utility Mapping
Accurate mapping of buried utilities and support infrastructure is crucial in modern construction projects. SUE (Subsurface Utility Engineering) practices have been found to provide cost benefits up to 4 – 20 times. GPR utility locators utilizing 3d scanning features can provide 3D subsurface images, providing more comprehensive mapping of all buried structures. Further, it enables effective engineering design and construction planning, greatly reducing costly surprises. Service providers now routinely use GPR to detect pipes, cables, and other structures, delivering the complete maps possible. GPR also, helps provide depth estimates that can be displayed on a utility map to reference in the future.
GPR – Applications
- Utility – Identification of Metallic and Non-metallic, Metal Pipe, Clay Pipes, Concrete Pipe, Transite Pipe, Plastic or PVC Conduit, Cable or Wire, Duct Banks/Vaults and Manholes, Water Boxes, Abandoned Lines, Illegal or Unknown Connections, Fiber Optic Lines, Missing Valves, Septic Tanks and Septic Systems, and SUM/SUE Utility Mapping.
- Environmental – Identification of Underground Storage Tanks (UST’s), Landfill Limits, Rubble Limits, and High Saturation Levels.
- Road inspection – Identification of Reinforcing, Cracking, Voids, Water Infiltration, Concrete Sparling, Slab Thickness, and Asphalt Layer Thickness.
- Geophysical – Identification of Strata Layers, Ground Water, Root Mass, Disturbed Soil, Buried, Wood, Bedrock, Boulders and Rocks, Density Changes, and Fill Replacement.
- Archaeology – Identification of Artifact Locating, Structure Mapping, Cemeteries, and Unmarked Graves.
- Military – Identification of Unexploded Ordinance (UXO), Bunker Location, Tunnel Location, and Weapons Cache Location.
- Law Enforcement – Identification of Contraband Location, Objects Hidden in Walls, Buried Caches, and Materials for Forensic Investigations.
GPR – Facts
- Non-Destructive : Despite its name, ground penetrating radar (GPR) is a safe technique that emits only 1% of the power of a cellular phone signal. Its frequency waves do not harm the surrounding area's subsurface, environment, or people.
- Disturbance-Free : GPR generates minimal noise and causes no inconvenience to people in the vicinity while in use.
- Easily Deployed : GPR comes in various user-friendly sizes and is easy to store, transport, and use almost anywhere with few limitations.
- Multiple Applications : GPR is suitable for utility locating, underground locating, structural, archaeological, geophysical, environmental, military, and law enforcement applications.
- Various Strengths : CNI Locates GPR antennas range from 250 MHz to 2,600 MHz to match your specific application and depth requirements, up to 30 feet. Lower frequency antennas (250-800 MHz) are ideal for deeper utility scanning, while higher frequency antennas (1000-2600 MHz) are best for shallow concrete/structural scanning.
- Upgrade To Previous Methods : GPR provides a similar result to having a test trench or boring along the entire project without the need for unnecessary digging or potholing.
- Eliminates Waste : GPR identifies areas for test pits or sample bores instead of using the SWAG method, minimizing useless or misleading data.
- Easy To Read : The high resolution GPR interface allows easy navigation and displays clear image resolution of soil disturbance and other material compositions.
- Computer Friendly : Most ground penetrating radar systems include a USB port for easy transfer of data to printers, computers, memory sticks, and other devices.
- GPR Certification : All equipment complies with FCC, CE, and RSS-220 regulations. For more information, visit Regulatory Information. All CNI Locates GPR utility locators are NUCLA certified to perform ground penetrating radar services.
- GPS Integration : GPR systems can integrate with most GPS systems. The GPS position data files and GPR scans are automatically matched within systems to ensure proper GPS position data is recorded.
