Ground Penetrating Radar (also known as: GPR, Ground Probing Radar, Ground Radar, and Georadar) is a geophysical method that uses radar pulses to image the subsurface and detects the reflected signals from subsurface structures.
RADAR (Radio Detection And Ranging) is generally used to detect aircraft, ships, vehicles, birds, rainstorms and other above-ground objects. The antenna of the GPR actually transmits a pulse of electromagnetic energy into the ground. When this energy reaches an object, it echoes and is captured by the antenna’s receiver. Computer software uses the frequency of pulses transmitted and the amount of time delay between transmitting pulses and receiving echoes to formulate information about the target. Depth range and resolution are related to the radar frequency, transmitted power, electromagnetic properties of the ground material (soil), as well as to the shape and characteristics of the targets.
Targets like pipes, cables, buried artifacts, tree roots and rocks generate arch-shaped responses on the GPR image. Arches occur because GPR energy does not travel into the ground as a pencil-thin beam but more like a 3D cone. Reflections can appear on the record even though the object is not directly below the GPR sensor. Thus, the GPR sensor “sees” the pipe before and after going over top of it and forms an arch-shaped response on the image.
GPR is extremely accurate when it comes to locating metallic and non-metallic objects. GPR systems work by sending a tiny pulse of energy into the ground from an antenna. An integrated computer records the strength and time required for the return of reflected signals. Any subsurface variations, metallic or non-metallic, will cause signals to bounce back. When this occurs, all detected items are revealed on the computer screen in real-time as the GPR equipment moves along. In some cases users can even tell from the signal returned whether the feature in question is metallic or non-metallic. By moving the GPR back and forth and marking the ground where the top of the arch is observed, the alignment of the subsurface utility can be traced out as the X’s in the figure indicate.
Crossing long, linear targets like pipes or cables at a 90 degree angle can produce a target arch’s suitable for the soil type calibration. The depth estimation of a target will be incorrect if the soil type calibration is done on a target arch produced at an oblique angle (smaller than 90 degrees). The most common method of locating is cross and mark as you go. This method works well in favorable soils and uncluttered settings. Cross and mark is very similar to the use of traditional current tracking utility detectors. The Cart is moved along (sweeps) perpendicular to the anticipated utility axis (see figure above). When the GPR sensor crosses the utility, the image shows an arch. The top of the arch is the position of the utility. The depth to the top of the arch is an estimated depth. The unique ability to see a pipe or cable in its topographical context makes GPR ideal for locating and excavating utilities. The integrated digital signal processor (DSP) analyzes the resulting image map to give the operator information on depth.
Depth of GPR penetration depends on the material being surveyed and also upon the antenna frequency being used. For instance, GPR will penetrate ice, rock, soil and asphalt differently due to each material’s unique electrical properties. Lower frequency antennas will generally penetrate deeper, but there is a loss in resolution with the drop in frequency.
To obtain an accurate depth axis and depth estimations of targets in the GPR image, a Soil Type Calibration must be performed. Soil Type Calibration can be done 3 ways:
- Matching the shape of a target arch
- Using a target at a known depth
- Using the moisture level of the soil
Point targets are identified as the GPR path intersects with a structure (pipe). At the point of intersection, radar echoes received by the GPR reflect the shape of the object as the receiver passes over, generally a hyperbola. The hyperbola is created because the radar echoes from the sides of the pipe take a longer time to reach the receiver than the echoes from the top of the surface of the pipe. If the antenna passes over a pipe or cable in an intersecting path, the reflected shape of buried pipe or cable is a hyperbola (A). Pipes that contain water may show duplicate hyperbolae (B) as the radar echoes from the top of the pipe, the water in the pipe, and the bottom of the pipe. If pipe is buried in a trench with compacted walls, the radar echoes can reflect off the trench walls, forming an X above the hyperbola (C).
To assess a drainage/slope, a transect was profiled along a pipe. The pipe location was determined from Lines 1, 2, and 3.
The depth of the pipe along the traverse is seen in the figure below.
GPR cross-sections were parallel and perpendicular to the sewer. From the transect it was apparent that the depth of the pipe increased from right to left by about 3ft (1m). In addition, the reflections from the top and the bottom of the pipe were detected. The storm drain appeared to be concrete with no metallic structure. Using the reflection depth of the top and bottom of the pipe and assuming the pipe was air filled, the pipe diameter was estimated to be 36 inches (90mm). The entire exercise of locating the pipe, marking the alignment, tracking the depth, and estimating its diameter took about 10 minutes.
GPR images are displayed in colors corresponding to a color palette. In general, stronger GPR signals appear in stronger colors. A number of different color palettes are available to display the image. Some color palettes may show the target better than others.
GPR – Locating Pipes & Cables
GPR’s ability to respond to both metallic and non-metallic features gives it a unique capability for pipe and cable locating. Unlike conventional cable locating devices that need metal pipes and cables to carry electrical current to be detected, GPR detects plastic, asbestos and concrete pipes and structures as well as metallic ones. Further, exploiting the instrument positioning and signal travel time enables estimating target depth. When outfitted with GPS and feature-tagging data loggers, a locating record can be provided as part of the standard locating and marking activity.
GPR – Subsurface Utility Mapping
Everyday construction now needs a detailed mapping of buried utilities and support infrastructure. Depth and location are critical in new structure designs. Extensive studies have shown that using SUE (Subsurface Utility Engineering) practices leads to cost benefits of 4 to 20 times. Successful SUE requires comprehensive mapping of all buried structures. Engineering design and construction can be planned effectively thereby greatly reducing costly surprises. GPR operating in survey and map mode delivers 3D images of the subsurface. SUE service providers now routinely exploit GPR’s ability to detect pipes and cables as well as other structures to deliver the most complete maps possible.
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, SUM/SUE Utility Mapping
- Environmental – Underground Storage Tanks (UST’s), Landfill Limits, Rubble Limits, High Saturation Levels.
- Road inspection – Reinforcing, Cracking, Voids, Water Infiltration, Concrete Sparling, Slab Thickness, Asphalt Layer Thickness.
- Geophysical – Strata Layers, Ground Water, Root Mass, Disturbed Soil, Buried, Wood, Bedrock, Boulders and Rocks, Density Changes, Fill Replacement.
- Archaeology – Artifact Locating, Structure Mapping, Cemeteries and Unmarked Graves.
- Military – Unexploded Ordinance (UXO), Bunker Location, Tunnel Location, Weapons Cache Location.
- Law Enforcement – Contraband Location, Objects Hidden in Walls, Buried Caches, Forensic Investigation.
GPR – Facts
- Non-Destructive : Although “ground penetrating radar” may sound like a hazardous technique, it is extremely safe and emits roughly 1% of the power of a cellular phone signal. Frequency waves inflict absolutely no damage on the subsurface, environment or surrounding people.
- Disturbance-Free : Makes little noise and doesn’t bother surrounding people during use.
- Easily Deployed : Multiple size options that are user friendly, so it can be stored, moved and utilized almost anywhere with few limitations.
- Multiple Applications : Ideal in most utility locating, underground locating, structural, archaeological, geophysical, environmental, military and law enforcement applications.
- Various Strengths : Antennas range from 100 up to 2,500 MHz to appropriately match your application and depth requirements of up to 100 ft. (30.5 m). Lower antenna frequencies 250-600 MHz are used for Deeper Utility scanning. Higher antenna frequencies 1000-2500 MHz are used for shallow Concrete/Structural scanning.
- Upgrade To Previous Methods : Similar to having a test trench or boring along the whole length of the project without the unnecessary digging.
- Eliminates Waste : Points out areas to test pit or sample bore rather than using the SWAG method, minimizing useless or misleading data.
- Easy To Read : Touch-screen operator interface provides easy navigation and has a bright screen that shows clear image resolution of soil disturbance and other material compositions.
- Computer Friendly : A USB port allows for easy transfer of data to printers, computers, memory sticks and more.
- GPR Certification : Yes, all equipment is in full compliance with FCC, CE and RSS-220 regulations. For more information, please visit Regulatory Information.
- GPS Integration : GPR systems can integrate with most GPS systems. The GPS position data files and GPR scans are automatically matched within systems so that the resulting data shows proper GPS position.