WHAT IS GROUND PENETRATING RADAR (GPR) & HOW DOES IT WORK?

Ground penetrating radar (GPR) is a geophysical locating method that uses radio waves to capture images below the surface of the ground in a minimally invasive way. The huge advantage is that it allows crews to pinpoint the location of underground utilities without disturbing the ground.

Ground-penetrating radar provides images of the subsurface by using the electromagnetic spectrum (EMS) in the microwave range between 10 MHz and 2.6 GHz. These signals are transmitted through the ground and reflect off of subsurface structures based on their electrical permittivity. A receiving antenna records variations in the return signals, which the GPR device uses to generate images that generally indicate changes in electrical properties.

How Does GPR Work?

It uses energy waves in the microwave band, ranging in frequency from 1 to 1000 MHz. GPR requires two main pieces of equipment – a transmitter and a receiving antenna. The transmitter sends electromagnetic energy into the soil and other material. Ground Penetrating Radar works by emitting a pulse into the ground and recording the echoes that result from subsurface objects. GPR imaging devices also detect variation in the composition of the ground material.

If the electromagnetic impulse hits an object, the density of the object reflects, refracts, and scatters the signal. The receiver detects the returning signals and records variations within them. The system has software that translates these signals into images of the objects in the subsurface. This is how it is used to map structures and utilities buried in the ground or in man-made structures.

Watch this short video to learn more about how ground penetrating radar is used for utility location.

What Does GPR Detect?

Ground Penetrating Radar signals can be used to find a wide range of items. This this subsurface tool is most effective when there is a large difference between the electromagnetic property of the target and surrounding material. GPR is often used to map items made of the following materials:

• Metal,
• Plastic,
• PVC,
• Concrete,
• Natural materials.

The possible applications are virtually endless. It’s frequently utilized to detect:

• Underground utility lines and pipes,
• Changes in ground strata,
• Geological features and rock obstructions,
• Air pockets or voids,
• Excavated and back-filled areas,
• Groundwater tables,
• Bedrock.

Utility Mapping Applications

Subsurface Utility Engineering (SUE) is a branch of engineering that manages the risks of utility coordination. The mapping of underground utilities is an essential part of identifying potential conflicts and communicating those conflicts to concerned parties. Electromagnetic (EM) location and ground-penetrating radar are currently the most common non-evasive methods of locating underground utilities. GPR provides a numbers of advantages over EM location, especially in specific SUE applications.
Subsurface utility mapping takes advantage of this technology to increase the precision of their work when combined with traditional locating methods. GPR provides assistance for discovering unmarked utilities and structures, subsurface mapping, and excavating projects.

For these types of applications, the advantages are significant.

Advantages of Ground Penetrating Radar

GPR is an extremely cost-effective and non-invasive way of surveying. It provides invaluable information before workers even break ground or start excavating.

1. It is safe for use in public spaces and a wide variety of project sites.
2. It detects metal and non-metal objects, as well as voids and underground irregularities.
3. It makes it possible to measure the dimensions, depth and thickness of targets.
4. Data is provided quickly and can cover a large site area.
5. Only one side of the surface needs to be scanned to provide data.
6. Frequencies can be regulated to deliver a range of resolution and penetration depths.
7. Data collected during the survey can be seen immediately or used in later projects.
8. No digging, excavating, or ground disturbance is necessary.
9. Landscaping, structures, lawns, etc. are be left undisturbed by the survey process.
10. It’s less expensive than other methods.

Find out When to Call a Private Utility Locator.

Where Can GPR Be Utilized?

As with all types of radar imaging, Ground penetrating radar delivers varying levels of accuracy depending on the conditions.

Soil Properties & Ground Material

GPR works by sending a tiny pulse of energy into the ground then recording the strength of reflected signals and time it takes them to return to the receiver. A scan consists of a series of pulses over a single area. While some of the GPR energy pulse reflects back to the receiving antenna, some energy continues to travel through the material until it dissipates, or the scanning session simply ends. The rate of signal dissipation varies widely, depending on the properties of the materials.

It can be applied to a variety of ground materials, including:

• Soil,
• Rock,
• Ice,
• Fresh water,
• Pavement,
• Concrete structures.

As the energy pulse enters a material with different dielectric permittivity or other electrical conduction properties, it produces a reflection. The strength, or amplitude, of the signal is the result of the contrast in the dielectric constants and conductivities between the two materials. A pulse moving from wet sand to dry sand will produce a very strong reflection, for example, in comparison to the relatively weak reflection produced by moving from dry sand to limestone.

Depth

The ground itself can limit how deep GPR signals penetrate up to 100 feet (30 meters) deep. The ground has electrical resistivity, which means it opposes the flow of electric current to some degree. As the signal penetrates deeper, it naturally gets less effective. This depends mostly on the type of soil or rock being surveyed and the frequency of the antenna used. For example, the maximum penetration depth in concrete is usually about 2 feet. In moist clays and other high conductivity materials, GPR signals depth is significantly shallower, reaching about 3 feet (1 meter) or less.

Water Content

Dielectric permittivity of the substrate is also a factor. Dielectric permittivity is the ease with which materials become polarized. The quantity of water present in the material greatly affects dielectric permittivity. Certain materials can become polarized in the presence of an electric field.

What Is the Difference Between GPR and Seismic Reflection?

The principles of GPR are similar to those of seismology. The main difference is that ground-penetrating radar uses electromagnetic energy, rather than acoustic energy of seismic waves, to detecting subsurface structures.

Seismology refraction surveys record signals that bend within the ground and arrive back at the surface. Increasing seismic velocity in the ground, related to the ground’s elastic properties and density, bends these acoustic signals back towards the surface. Seismic imaging is popular for mapping horizontal structures beneath the ground, but not very effective for characterizing vertical features.

GPR uses electromagnetic energy in the form of high-frequency radio waves, which effectively detect changes in electrical properties below the surface. Seismic energy, on the other hand, detects changes in subsurface mechanical properties.

Related article: What’s the difference: X-ray vs. GPR Concrete Scanning?

What Is the Difference Between GPR and EM Location?

GPR uses a single unit that’s both a transmitter and receiver. It transmits high-frequency EM signals in the microwave range, typically between 10 megahertz (MHz) and 2.6 gigahertz (GHz). Signals in this frequency range are the most effective in detecting changes in the soil, such as those caused by utility lines. These signals reflect off objects in the ground and are displayed on the operator’s screen, with disturbances generally appearing as hyperbolic patterns.

The operator must then interpret these patterns to identify the ones that indicate utility lines. An experienced operator can determine the depth and position of utility lines, which are then marked on the surface. This information will then be incorporated into a three-dimensional map. Operator skill is a key factor in determining the effectiveness of GPR, even though it doesn’t have the limitation on materials that EM location does.

GPR requires significantly more training than EM location because a GPR display is much more subject to interpretation. For example, GPR results can be affected by factors such as the moisture content and specific materials comprising the soil. Water reflects the signals differently from soil materials, which can mask the presence of utility lines. Post-processing software can improve the technician’s interpretation, although this step requires additional time.

EM location uses a transponder that transmits a very low-voltage alternating current (AC) into an electrically conductive probe, typically a steel pipe. This current creates an EM field around underground utility lines, which is detected by a wand-like receiver. These lines can then be located and traced onto a map for later reference.

The specific methods of EM location include conductive location, inductive location and passive location. Conductive location requires the operator to connect the transmitter directly to a utility line, allowing it to create the EM field outside the line’s conductive material. Inductive locating involves applying the signal to the utility line, typically by placing the transmitter into the ground over the utility line. Passive location detects EM signals that the line already produces. so it doesn’t require a transponder to detect utility lines for electrical power and communication.

For SUE Applications

The primary benefit of GPR over EM location is that GPR provides better imaging for utility lines, regardless of their composition. GPR primarily detects ground disturbances rather than the lines themselves, so it doesn’t matter if the lines are metallic or nonmetallic. EM location, on the other hand, detects the EM signals produced by a flow of electrical current. This method therefore requires the utility line to be made of an electrically conductive material when using passive location. EM location can use a conductive drain rod to locate pipes made of nonconductive material in some cases, although a drain rod is frequently impractical or at least inconvenient.

The frequency of the signal emitted by GPR greatly affects its performance with respect to the size and depth of the object. For example, a higher frequency signal provides better resolution but penetrates the ground less deeply than a low-frequency signal. This property means that a high-frequency signal is better for detecting small, shallow objects, while a low-frequency signal is better for detecting large, deep objects. Older systems only use signals with one frequency, so their effectiveness varies greatly depending on the size and depth of the utility lines. However, more advanced models use multiple frequencies to maximize their effectiveness for a particular application.

These factors have resulted in an increase in the practice of combining GPR and EM. SUE companies often perform a sweep with an EM locator first to detect the utilities made with conductive materials. They can then follow up with GPR to detect all the other utilities. This combination of technologies allows engineers to discriminate between conductive and non-conductive utilities while improving overall mapping performance.

For Other Applications

GPR measures differences in the density of materials, which makes it useful for detecting a variety of other subsurface objects besides utility lines, including large rocks, tanks and void spaces. It’s therefore useful in many industrial activities that require information on subsurface conditions. For example, GPR is also used in bridge and road construction, which require knowledge of soil density. It also has applications in building inspections to determine if foundations are sound. Rail networks frequently use GPR to monitor the density of ballast used to balance the loads in rail cars.

Other industrial sectors that use GPR include law enforcement, which often requires non-invasive testing in forensic investigations. The study of archaeological sites also benefits from GPR’s ability to examine subsurface structures without disturbing them. The operator’s skill is particularly important for these applications, both for the high degree of accuracy required and use of GPR in conjunction with other techniques. The number of applications is therefore quite large, although the precision needed is highly dependent upon the application. The most common use of this technology is likely to remain in SUE, where GPR’s superior resolution and reliability will make it the preferred choice over EM location. GPR innovations will include producing GPR signals that can be accurately interpreted by multi-skilled operators rather than GPR specialists.

Why Is Regulation Needed for GPR?

Most countries regulate devices that emit EM radiation, largely due to their potential for interfering with other devices on the same frequency. However, the specific laws and their level of enforcement vary greatly by jurisdiction. The manufacturers and operators of GPR need to stay abreast of these regulations, which change regularly.

Governments throughout the world have closely managed and regulated the use of the EMS for decades. The primary reason for all these rules is that powerful EM broadcasters like radio and TV stations could render each other’s broadcasts useless if they operated on the same frequency, especially if they’re in close proximity. International coordination is needed because two such stations can be on opposite sides of a national border, even when they’re near each other.

Furthermore, governments routinely license the EMS, meaning they sell the rights to use a particular frequency range within a certain geographic area. Once someone pays the license fee, they essentially own that frequency band within their designated area. These slices typically cover only a few kHz or MHz each, but they can cover the entire EMS from 100 kHz to 100 MHz. These licenses are extremely lucrative for governments, as some of the frequency bands used by cell phones have sold for over a billion dollars. Governments, therefore, have a strong financial incentive to ensure the EMS remains free of interference.

What Bodies Are Responsible for GPR Regulations?

The multinational nature of EM regulation makes its organizational and administrative structure complex. For example, the International Telecommunications Union (ITU) is the international coordinating body for EM spectrum regulation, although spectrum managers in the individual countries are the primary regulators.

The Federal Communications Commission (FCC) regulates public use of the EMS in the United States, while the National Telecommunication and Information Administration (NTIA) manages the U.S. government’s use of the EMS. The FCC generally requires an approved organization to test devices that emit EM radiation. The FCC then reviews the results and issues a unique identifier if it approves the device. This identifier must be displayed on the product and its manuals, along with FCC warnings regarding its proper use. All EM devices approved for use in the U.S. and Canada are listed on government web sites.

Industry Canada (IC) performs a role similar to the FCC in Canada, and the European Telecommunications Standards Institute (ETSI) is an independent organization that manages telecommunications standards in Europe. Manufacturers must indicate the specific ETSI standards that a device complies with and provide the documentation needed to verify these claims.

Tips on GPR Compliance

A GPR device must comply with its country’s regulations before it can be legally sold or operated. Furthermore, the penalties for violating these regulations can be severe. These regulations have applied to GPR for the last 10 to 20 years in most countries, which generally follow FCC or ETSI standards. However, jurisdictions typically have variations on these standards, which can change at any time.

Seemly minor changes in technology can pose major challenges in regulating these devices. For example, a revised ETSI standard with a series of minor technical changes in GPR devices have been awaiting adoption for the past four years, keeping these devices in regulatory limbo in Europe.

Users need to adhere to the current regulations, with the understanding that these rules are currently very much in flux. It’s particularly important that they only buy certified equipment to avoid causing significant interference on a heavily used frequency. Such an incident could prompt a review of the current standards, resulting in greater restrictions on GPR operations.

The most convenient way to do this is to locate the required sticker on the GPR device or search the applicable website for the manufacturer’s product. You can also request the FCC or IC identification number from the vendor. In Europe, you would ask for the ETSI declaration of conformity. Take your business elsewhere if the GPR doesn’t comply with your country’s EMS regulations.

Find out How Drone Technology Is Revolutionizing GPR Applications.

SoftDig® Uses GPR – Leaving a Minimal Worksite Footprint

At SoftDig®, we leave the ground just as we found it – no destruction, no despair. Our crew provides exceptional imaging results for the location of utilities across a variety of media – and without ever disturbing the ground. If you have a job that could benefit from concrete scanning or GPR services, contact us by sending an estimate request.