Ground-Penetrating Radar Explained

by James Caviness

Locating underground utilities can be a monumental challenge. Different tools offer solutions specializing in the type of underground utility they are meant to find. Still, for when you need a solution that can locate it all with high precision and accuracy, Ground Penetrating Radar (GPR) may be your best option. While there are some limitations to using this type of technology for utility location, it could be argued there is no better all-encompassing technology for this application.

How Does Ground Penetrating Radar Work?

What Is the Difference Between Seismic Reflection and GPR?

The greatest benefit to using Ground Penetrating Radar, is that GRP can detect targets that other utility locating systems would be blind to. GPR accomplishes this by sending pulses of microwave radiation into the ground and analyzing the signal as it returns. The GRP unit itself consists of a transmitter that sends the signal into the ground and a receiver that will catch the GPR signal as it returns to the GPR unit. Software is a critical part of the GPR unit, as it allows the system to process the signal and gives the operator usable data to work with.

The software within the GPR unit will be able to determine the type of target being detected by processing the data that returns to the unit. GPR uses pulses of microwave radiation and measures the speed of the signal as it travels through the media and back to the receiver. GPR's signal will travel fastest through the air, and slowest through water, with speed values of 1 for air and 81 for water. Different types of utilities will have different speed values to the GPR, which allows the software to map out the locations where the signal met more or less resistance. The capabilities of GPR are similar to that of seismic reflection. However, seismic reflection is more effective for natural horizontal targets underground, whereas GPR provides a much more complete picture of what is underground.

What Can Ground Penetrating Radar Detect?

GPR can detect a wider range of materials underground than any other type of remote detection technology for scanning underground. GPR can detect:

  • Metal
  • Plastic
  • PVC
  • Concrete
  • Natural materials like dense rock and water
  • Underground voids or cavities

Ultimately the applications are limitless, but GPR is most commonly used to find:

  • Underground Utilities
  • Changes in ground conditions and ground composition
  • Locating nearby potential geological hazards or targets
  • Identifying Back-filled areas
  • Finding sources of groundwater
  • Locating voids or cavities underground

With the range of possibilities spanning many different applications, there are some limitations. Since the GPR system uses a 1-81 scale to measure the speed of the signal and ultimately identify targets, targets with similar resistances will produce a similar numerical speed identification so that they may register similarly. Most commonly, this happens with PVC and concrete since they have similar resistances.

Ground Penetrating Radar Purposes

Ground penetrating radar is not limited to archeologists; it can also be used by outdoor enthusiasts who want to know what lies below the surface of their favorite fishing spot. GPR is like a form of sonar that uses electromagnetic radiation to detect hidden materials and objects underground.

Ground penetrating radar is a geophysical method that uses radar pulses to image the subsurface and subsurface structures. This non-destructive method is often used in:

  • Archeology
  • Construction projects
  • Mining operations
  • Search and rescue missions
  • Infrastructure surveys
  • Environmental studies
  • Law enforcement crime scene searches
  • Road building and more

GPR can be used on both stationary objects and moving objects. It works by sending out ultrasonic waves from a transmitter at the ground surface; these signals reflect on interaction with the subsurface strata.

GPR Antenna

GPR antennas send out electromagnetic waves, radar waves that penetrate the ground and bounce off certain materials. Similar to metal detectors, the emitter sends out high-frequency radio waves that transmit information about underground layers such as pipes or stone foundations.

Uses for Ground Penetrating Radar Equipment

Ground penetrating radar work is often used to locate underground utilities like electric cables or sewers, but it can also be used in archeological digs. The radar system works by transmitting an electromagnetic pulse which is then reflected when it hits an object; this bounce-back signal contains information about what lies below the surface of the Earth.

There are many applications for ground penetrating radar. Some of these include:

  • Archeology: Archeologists use GPR to find hidden relics and buried structures that might be difficult or impossible to detect otherwise.
  • Surveying: Surveyors can use a GPR survey system instead of digging exploratory holes when attempting to locate a buried utility line, such as telephone lines, electrical cables, water or gas mains, and sewers. Geophysical survey systems have been used in the oil and gas industry to map subsurface strata, identify oil-bearing layers, and detect the presence of hydrocarbons.
  • Law Enforcement: Law enforcement has been known to use a GPR scan or GPR service. Remember that many crime scene ground searches are over a large area. Besides a GPR solution, it is common for law enforcement to use a metal detector in search of the buried object in question.
  • Construction: Construction companies can utilize ground penetrating radar survey systems for concrete scanning to prepare the best location for construction equipment before beginning work on site.

GPR is most often used in areas where the surface of the soil or rock is unstable, uneven, or highly processed, such as city streets with dense traffic. The radar can also detect voids and other subsurface features below a building's foundation.

There are several types of ground penetrating radar solutions available in today's market, which vary significantly in terms of price, performance, portability, and ease of use. Systems consist of a transmitter antenna unit and a receiver antenna unit.

As the radar transmits an electromagnetic impulse into the ground, it measures how long it takes for a reflected signal to come back from different materials and surfaces below the surface. The resulting measurement data is then fed into a computer system which produces a digital rendering of whatever objects lie underneath the earth's surface.

GPR Method Equipment

Geophysical survey systems inc offers the following accessories to purchase their ground penetrating radar equipment: A highly sensitive receiver head, a loop amplifier, coaxial cable, and an adjustable tripod.

Ground penetrating radar systems with ultrasonic flaw detectors for landscapers detect buried objects below the surface on which the turf is growing. This system helps excavators avoid surprising or damaging underground utilities when one digs into an area that has already had some development done.

3d GPR scans create 3d images of the subsurface geology and can be used for a wide range of activities. This is useful for mapping and imaging the underground environment or suspicious-looking surfaces such as land mines or old foundations.

Ground Penetrating Radar Equipment

There are many different types of GPR equipment, but some of the more popular units for utility locations are:

Leica DS2000 4-wheel utility detection radar

This system is a terrific option for underground utility locating. The DS2000 can detect both conductive as well as non-conductive targets to give the operator comprehensive data to work with and ensure all nearby utilities are mapped out. This system uses a tablet to work with the software, which can offer information in real-time, as well as the ability to share the results remotely. Unaffected by dust and moisture, the DS2000 uses a multi-frequency antenna to identify deeper and shallow targets simultaneously.

Leica DS2000 4-wheel utility detection radar starter kit

All the benefits of the DS2000 but with the added benefit of including the CT-1000 field tablet controller with LTE. Another added benefit to the starter kit is a year of support directly from Leica to help get you oriented and up and running quickly.

Leica DSX Utility Detection Starter Kit with DXPLORE perpetual license

The DSX is unique in its capabilities in that it will offer many of the same benefits of GPR along with the added benefit of real-time 2D or the 3D rendering of images captured in the field. This setup will include the CT-1000 field tablet controller with LTE as well as perpetual software licenses for Leica's proprietary DXPLORE software, along with 2-year support for all other software.

What Is GPR Used For?

GPR is clearly a fantastic solution for locating underground utilities. However, the applications do not stop there. Besides utility mapping, contractors search for rebar under concrete structures or concrete slabs and an underground storage tank. Land surveyors could also benefit from using GPR to help them know ground conditions and potential hazards or targets underground to be aware of. GPR can also detect the presence and location of water underground, as well as even be able to detect the amount of groundwater in an underground substrate.

How to Use GPR

Factors that Affect GPR Effectiveness

Some say less is more, but with GPR MORE is more! One of the greatest benefits of using GPR on a project is the amount and accuracy of the information it can provide. Knowing how to use the GPR system and its GPR software effectively will provide the most value from the equipment. Some of the best general tips for using GPR include:

Use a grid pattern

Setting up a grid-like pattern to scan the proposed location will reduce the number of potential errors that you may face. The GPR uses very precise measurements to scan the terrain, so being as methodical as possible above the ground will ensure the greatest accuracy underground.

Another benefit of setting up a grid-link pattern is the ability to more accurately identify targets, as well as get a better idea of the target depth. In fact, the more times you can scan a specific grid in different directions, the more data you will have. More data to work with means more accurate results.

Be familiar with the targets you are looking to locate

As we've mentioned, some targets may register similarly to the GPR system. By knowing ahead of time the potential targets that may cause issues, it is possible to compensate for this, or at the very least be aware of this situation. This is another case where multiple passes would allow for a more accurate representation of what is underground.

Consult plans or use known information in conjunction with the data provided by the GPR unit

Comparing your GPR results to the actual building or survey plans is a great way to ensure accuracy, as well as locate targets more easily. Systems like the Leica DSX would have the capability of comparing their actual produced results with building plans if the plans are in CAD or BIM format. When you want to make absolutely sure you know what you're up against, this method, along with the other best practices, will give you the best information to work with so you can be assured your utility locating is done with as little error as possible.

Copyright 2021 Detector Electronics Corp. - Revised September 2022