Concrete Scanning and X-ray using GPR

At Penhall Technologies, a division of Penhall Company, we specialize in using ground penetrating radar to scan concrete. We employ expertly trained analysts that use state-of-the-art GPR (Ground Penetrating Radar) equipment to find common embedded objects and other subsurface hazards that are hidden in the concrete.

GPR Concrete scanning used to detect voids in columns We can help you locate rebar, pipes, post and pre stress tension cables, conduits and many other subsurface hazards. Often, the data we collect is used to analyze and measure the appropriate remedial solutions for concrete structural deficiencies and building remodels. Our skilled GPR analysts can also detect voids within or underneath concrete slabs.

Penhall is a safety leader in the industry. For many years contractors, engineers, and government agencies have come to trust Penhall first for safe and precise scanning services. In addition to concrete scanning services we offer private utility locating and x-ray imaging.


  • Reduce your safety risk. Keep everyone on the job safe.
  • Reduce your financial exposure. Cutting a post tension cable can cost you thousands.
  • Save time. Know exactly where to cut or drill to avoid a hazard and delays.

Ground Penetrating Radar – How does it work?

In the GPR scanning process, radio waves are directed into the concrete using a transmitting antenna at a certain frequency.  Since radio waves are a form of electromagnetic energy, this energy bounces off of buried objects such as rebar, post tension cables, pipes and conduits and are then captured by a receiver in the same antenna.

GPR scanning equipment contains software that can quickly and accurately interpret the radio waves. The key to this interpretation is two-fold; first, the strength of the signal reflection, or the amplitude, and second the two-way travel time of the radar pulse. It’s important to note that the two-way travel time is how the scanning equipment will determine how deep the objects are embedded in the concrete. Any object that reflects the signal is displayed real-time on the screen as the GPR equipment moves along. In some cases, it can even be determined if the object is metallic or nonmetallic.

Since the equipment uses electromagnetic pulses, the depth of the scan is determined by the electrical conductivity of the material being scanned. Material with higher conductivity will allow the radar to penetrate deeper. Conversely, material with lower conductivity will limit the depth the radar can penetrate. The amount of water found in the reflective material primarily determines how conductive the material will be. The greater the depth of the scan the less accurate it becomes. Because of this, GPR is often conducted using high-frequency waves that may only penetrate several feet into the concrete but offer a much higher resolution and a clearer image of what’s inside.

Since GPR spreads out as it travels into the ground as opposed to a straight beam, the antenna can detect the buried object even if it is not directly above it. The reflected radio waves create an image where the buried objects show up in the form of peaks and hyperbolae. The shape of the hyperbola, whether narrow or broad, depends on the electromagnetic velocity and size of the subsurface material. If the signal does not hit a reflective object, it will gradually dissipate as it passes through the material.

Concrete Scanning – Benefits and Limitations

  1. Locating conduit using ground penetrating radarGPR provides a real-time image that displays detected objects
  2. GPR provides the depth of the objects within a few centimeters
  3. GPR can detect objects made of metal, plastic, and various other materials

While most GPR related research is focused on environmental and engineering applications, GPR can also be used in concrete construction, utility detection, urban planning, archaeological site characterization, military, security, and law enforcement. Whatever the application, GPR makes it possible to avoid subsurface hazards before drilling, cutting, digging, or where extra caution is necessary. According to World many construction outfits are coming around to the notion that concrete scanning services are vital for job safety.

Although recent advances in GPR hardware and software have improved the technology, some limitations still exist.

  • High-conductivity materials, such as clay soils and soils that are salt contaminated, reduce the depth of GPR ground penetration
  • Signal scattering in heterogeneous conditions (e.g. rocky soils) limit performance
  • Interpretation of GPR radargrams (the radar image of mineral deposits or a geophysical surface) is not intuitive
  • Considerable expertise is necessary to effectively design, conduct, and interpret GPR surveys
  • Relatively high energy consumption can be problematic for extensive field surveys

Concrete Scanning Tools

As part of our GPR scanning, Penhall analysts uses the GSSI 2000 MHz Palm Antenna to inspect concrete in tightly spaced areas that were previously inaccessible such as corners, against walls and around obstructions. Like the bigger 1600 MHz version, the Palm Antenna enables Penhall  to locate hazards such as rebar, post tension cables and conduits that may be embedded in concrete structures.

GPR Scanning Equipment Basics

The GSSI SIR® 3000 is the industry’s number one choice for data accuracy and versatility in ground penetrating radar scanning. The Subsurface Interface Radar (SIR) System enables Penhall to save clients time, money and even lives. In the video below, we’ll outline the specialized equipment that Penhall uses, including the high-resolution, all-purpose 1600 MHz antenna, to locate embedded rebar, post tension cables and conduits in concrete structures.

Portable GPR Scanning System

The StructureScan™ Mini HR, manufactured by GSSI, is an all-in-one high-resolution GPR system for concrete scanning. Along with our other equipment, Penhall uses the Mini to scan concrete to locate rebar, conduits,  post-tension cables, and other obstructions.

What is the difference between GPR and X-ray?

It’s important to understand some key differences between GPR and X-ray before determining which to use on your next project. Both provide data on what subsurface objects may exist in the concrete. The decision about which option is best for the situation depends on a variety of factors, including the project parameters (e.g. objective, location, etc.) and how definitive the image needs to be.

Key Differences

Ground Penetrating RadarX-ray
Radiation ExposureNoYes
Interpretation of ScanSubjectiveDefinitive
Required Access to ScanOne side of a surfaceBoth sides of a surface
Scan TimeShortLong (approx. 5-6 hours)
Occur during business hoursYesNo
Real Time InspectionYesNo
Large Area ScansContinuous no limitMultiple exposures
Standard Scan Size24 x 2416 x 16
Slab on GradeYesNo
Electromagnetic InterferenceYesNo

Determining the Cost of Your Concrete Scanning Project

Pricing for ground penetrating radar (GPR) services is unique for each project. The ultimate cost is dependent on a number of factors such as:

  • Job location
  • Number and/or size of areas to be scanned
  • Depth of objects to be identified
  • Type of equipment needed to capture the image
  • Amount and/or complexity of the hazards believed to be in the concrete

Identifying cables in post tension concrete slabs


Is GPR reliable?

Penhall scan analysts have extensive training in both the science of GPR and the proper use of the instrument. We always use the best practices available and the latest technology. GPR can not completely eliminate the risk of avoiding rebar, post tension cables, conduits or pipes before cutting, coring, or trenching but it can greatly reduce your chances and save you a lot of money.

What type of markings are made at the site during the scanning process?

We mark objects identified during the scan directly on the concrete slab using tape, chalk, keel, or spray paint depending on the specific requirements and location of the scanning area. Rebar is marked in black and post tension cables and conduit or unknown objects are marked in red. Other colors, such as yellow or blue, may be used to mark the edge of the scanned area.

How close can I cut/core to the site markings?

As a general rule, you should cut, core, or trench as far away from a marked hazard as possible. For rebar, we advise leaving a 2 inch margin from the edge of the line. For post tension cable or conduit, we advise leaving a 3 inch margin from the edge of the line.

What type of report will I receive after scanning?

In addition to the site markings, we offer several different reports and/or drawings after scanning is complete. All scan analysts are required to complete a field report or basic report documenting the job details, what the GPR scan identified, as well as pictures and data screenshots to support the written conclusions when required. If requested, a field drawing or sketch of the site area can be included on the field report. More complex and thorough reports are available upon request and for an extra fee.

GPR Reporting Methods

In addition to the field report or basic report, there are four types of reports that can be produced to that detail the hazards identified during the scanning process and reference any unknown objects that were found.

  1. Complex Report – In addition to information included on the field report or Basic report,this report includes a drawing of the scanned area. The drawing will note the location and depth of rebar, post tension cables, or any unknown object.
  2. GPS Report – GPS reporting has become a popular option because it can help address specific questions and concerns about markings. The collected data can also exported into CAD and Google Earth.  A GPS report makes it possible to permanently keep track of the findings revealed from the GPR survey and easily reference the report at any time in the future to precisely locate a utility. These types of reports are usually associated with a full geophysical survey involving multiple instruments in what is called standard utility locating. Standard utility locating is typically done outside for trenching, excavating, coring, etc. This is not typically done for shallow, indoor concrete scanning.
  3. CAD Report – CAD reports are most commonly used where GPS would be difficult to determine. CAD reports include all the same information found on the field or Basic report, and often the GPS report information. The GPR data can be added as an additional layer to an existing CAD file.
  4. Laser Scanning Report – GPR markings are scanned by a laser and then exported into a 3D image. Once the 3D image has been created, the precise location of the identified objects can be mapped. This type of report can help map post-tension cables and conduit within large areas of concrete.

Concrete Scanning Report Examples

Here are some examples of what types of reports you may received after Penhall completes your scanning project. All reports will come with a checklist, an emailed or texted photo of the scanning if desired, and a waiver form.

GPR report markingsGPR field report with scanning data

Understanding GPR Data

The individual reflected signals, or waves, that are received from within the ground are digitized into a reflection trace. In GPR, a trace is the basic measurement for all time-domain GPR surveys. When multiple traces are stacked next to each other (as shown in the image below), a two-dimensional vertical profile can be produced along the transect grid and then analyzed to produce both two- (2D) and three-dimensional (3D) images of what lies below a surface.

Regardless of the acquisition method, all recorded reflection traces are displayed with the two-way travel time of the reflected waves plotted on a vertical axis and the surface location (or trace number) plotted on a horizontal axis. The computer records these and displays them as black, white, and gray horizontal bands (when in grayscale mode). Strong reflections generate distinct black and white bands, while medial reflections produce less distinct gray bands.

Scanning data displayed on GPR equipment

The objective of GPR data presentation is to provide a display of processed data that closely approximates an image within the subsurface, along with the proper spatial positions of the anomalies that are often associated with the subsurface object. Therefore, data display is essential to effective data interpretation.

There are three types of displays of surface data:

  1. A one-dimensional trace
  2. A two-dimensional cross section, and
  3. A three-dimensional display.

A one-dimensional trace does not provide much value until several traces are placed side-by-side to produce a two-dimensional cross section, or placed in a three-dimensional block view. For this reason, one-dimensional traces are never used alone in modern GPR scanning.

Three-dimensional GPR data can be represented in one of three ways:

  1. 3D alignment of 2D traces – A 3D alignment of 2D traces involves almost no post-processing, so it requires less time to produce and the least amount of velocity variation assumptions.
  2. One or more depth slices – Depth slices require that the medium being investigated has an accurate representation of the velocity. In some cases, velocity can be measured. Other times it’s determined through a combination of trial and error, experience, and educated guesses. The more consistent the material (e.g. concrete), the quicker and easier it is to achieve an accurate model of velocity, which is why depth slices tend to be effective for modeling linear features, such as rebar and conduit.
  3. Isosurfaces – Isosurfaces require the most amount of post-processing and filtering. The result can be good for modeling more complex features, but there is a tendency for smaller, fainter features to be filtered out in the data display, which isn’t always desirable.
3D alignment of 2D tracesOne or more depth slicesIsosurfaces

Multiple viewing options allow the user to move around in the data, which can help reveal features that are not very visible in traditional vertical data profiles.

History of GPR

The first Ground Penetrating Radar (GPR) survey was conducted in Austria in 1929 to determine the depth of a glacier. GPR was largely forgotten until the late 1950’s as the U.S. Air Force began setting up bases in Greenland. Unknown to the operators of the time, radar penetrates ice sheets which caused radar operators to misread the altitude. This error caused planes to crash land on the ice as they attempted to land on the bedrock below the ice sheets. GPR was even used on the moon during the Apollo 17 mission! However, GPR did not become commercially available until the early 1970s; before that GPR was a build your own instrument. It was not until digital signal processing was invented in the 2000’s that GPR systems began to truly resemble the instruments used today.

Applications of GPR

Today, Ground Penetrating Radar is considered to be an effective geophysical method for non-destructively detecting and investigating the presence and continuity of subsurface objects for a variety of applications and industries, including:

  • Concrete Scanning – GPR is commonly used to locate rebar and reinforcing steel, and identify conduits and post-tension cables in concrete. GPR is also used for detecting the presence and extent of voids in the subgrade below a concrete floor in addition to the structural integrity of the concrete itself. Trained technicians can use GPR to locate these problem areas without any damage to existing structures, which allows for focused repair and remediation.
  • Utility Locating – GPR makes it possible to identify a variety of utilities, including: fiber optic lines, plastic or PVC conduit, cables and wires, water boxes, metallic and non-metallic objects, duct banks, vaults and manholes, abandoned lines, concrete pipes, missing valves, septic tanks and systems, subsurface utility mapping and engineering (SUM/SUE), and transit pipe.
  • Road inspection – Highway professionals, engineers and transportation departments depend on GPR to safely, reliably, and non-destructively evaluate roads for pavement preservation, planning, and rehabilitation. GPR can evaluate base and sub-base layers with data collection densities not obtainable by traditional labor-intensive methods, such as coring. GPR can also detect potential problem areas (i.e. voids and air pockets) beneath asphalt and pavement; identify reinforcing, cracking, and water infiltration; and determine slab and asphalt layer thickness
  • Military and Security – Military, security personnel and border patrols use GPR in various configurations to detect the location of tunnels, bunker locations, unexploded ordinance (UXO), weapons caches, movement in buildings or near exterior walls, etc.
  • Environmental – GPR is used for determining soil saturation levels, landfill and rubble limits, soil contamination, as well as for detecting where underground storage tanks have been left underground or removed.
  • Archaeology – Archaeologists and remote sensing specialists around the world rely on GPR for non-invasive site investigation, structure mapping, locating cemeteries and unmarked graves, and excavation or location of sensitive cultural resources for preservation or avoidance.

Science Behind Scanning Using GPR

The detectability of a subsurface feature depends on the contrast in electrical and magnetic properties of the substrate and the feature, the frequency of the antenna, and the geometry of the objects in relation to the antenna. The electric and magnetic properties of a substrate are known as relative permittivity. Relative permittivity is the dielectric permittivity of a material expressed as a ratio. In the case of GPR, this ratio will be designated as a number between 1 and 81. Historically the term for addressing wave propagation through a medium was known as the dielectric constant. Although dielectric constant is still commonly used, especially by GPR manufacturers, the scientifically accepted and preferred term is relative permittivity.

As discussed above, the electrical and magnetic properties of the medium being investigated determine the speed of propagation and attenuation (amplitude decay and frequency broadening) of radar waves. At the frequencies used in GPR, the amount and type of attenuation is dependent on the frequency being emitted; also known as frequency dependent attenuation. In the field, electrical properties tend to be much more important than magnetic properties in terms of GPR wave propagation. Rare exceptions to this would be high soil concentrations of magnetically active rocks and minerals.

At GPR frequencies (10 MHz to 3000 MHz) electrical properties are affected by substrate density, chemistry makeup, state of the medium, distribution, and water content. Of the noted factors, water content is the most important influence in wave attenuation for GPR systems due to the fact that water content determines the other four factors in soils. The largest determinant of wave propagation in typical soils, (including concrete and asphalt) especially at frequencies above 1000 MHz, are ionic charge transport through water filling pore spaces in rocks and soils.

The Dielectric Constant

It should be noted that GPR manufacturers use the term dielectric constant despite its ambiguity for several reasons. It is a simpler and more familiar concept than relative permittivity or dielectric permittivity, and it is literally used as a constant in the equations of the GPR computer during data interpretation and digital signal processing. Because dielectric is a dimensionless unit, different mediums have been assigned different numbers based on their permittivity relative to a vacuum. For the purposes of GPR, air is considered to have the same permittivity as a vacuum. The values assigned to mediums by the GPR computer range from 1 for air which GPR energy propagates through easily, to 81 for water which GPR energy does not propagate through. Thus, everything encountered by a GPR unit will be assigned a dielectric constant value from 1 to 81.

Soil types and objects whose dielectric constant varies considerably show greater contrast in the radar profiles then soil types that are closer in their dielectric number. Radar waves reflect off of the interface between objects with different dielectric constants. Essentially, the larger this difference, the more energy is reflected and the brighter the image is on the radar profile. If these constants are very similar, differentiation might not be possible.

As the radar pulses are transmitted through various materials on their way to the buried target, their velocity will change. The change is velocity is dependent on the physical and chemical properties of the material through which they are traveling. Once the travel times of the energy pulses are measured, and their velocity through the ground is known, distance (or depth in the ground) can be accurately measured.

Higher frequencies do not penetrate into the subsurface as deeply as lower frequencies. However, higher frequencies give better resolution. Optimal depth penetration for concrete, for example, is up to 18 inches in dry, cured concrete. If the concrete is moist and uncured and has high electrical conductivity, penetration is sometimes only a few inches.

An important reason why GPR is frequently used to detect metal objects, such as pipes, conduits, and rebar, is that radar energy will not penetrate metal; so buried metal objects tend to be easy to see in GPR reflection profiles. When struck, the metal object usually creates multiple reflections, stacked one on top of one another, below the metal object, as shown in the image below. This is especially true if the object is located very close to the surface.

GPR Scanning Readout

Point source hyperbolas (location A, above) are generated from buried objects of a limited size. In this case, the hyperbola on the right was generated from a metal pipe and the lower amplitude hyperbola on the left from a plastic pipe. The series of high amplitude reflections that are stacked vertically at location B were generated by a large piece of metal near the ground surface. The grayscale colors shown in the photo above are assigned based on the amplitude, frequency, velocity, and polarity of the reflected wave signal.

Amplitude, Frequency, Velocity, and Polarity Explained

Amplitude, frequency, velocity, and polarity are a summary of all the information contained in a reflected GPR wave. These items are often correlated and/or directly related.

Amplitude is the maximum extent of a vibration or oscillation. The larger the amplitude, the brighter the image on the GPR display. Interfaces with large dielectric differences produce reflections with large amplitudes (i.e. a metal pipe in concrete). The amplitude of the reflected signal depends on the velocity created by contrasting dielectric constants, the geometry, and the polarity.

Amplitude Graph

Frequency, as it relates to GPR, is defined by the total number of oscillations per a given amount of time. The frequency of a reflection is determined by the center point frequency of the antenna chosen by the operator, and the frequency dependent attenuation of the subsurface being investigated. The frequency of the antenna determines the maximum theoretical distance that the GPR can penetrate. This penetration depth is heavily dependent on the properties of the medium being investigated.

Frequency Graph

The dielectric constant generally determines the velocity of the GPR waves as they travel through the subsurface. Wave velocity is generally independent of frequency for GPR, whereas the conductivity of the substrate generally controls frequency attenuation. The velocity of the wave is used by the GPR computer to determine depth of the object being investigated. Low velocity means a high dielectric number, and high velocity means a low dielectric number. Homogenous materials, especially those with a low dielectric constant, (i.e. dry sand) propagate waves better than heterogeneous material (i.e. rocky soils). The image below shows a type of attenuation known as signal scattering via heterogeneous material.


Polarity, as it relates to GPR, can be viewed as a positive or negative wave emitted by the GPR antenna. GSSI manufactured antennas alternate positive and negative waves with each pulse. Changes in dielectric constants between two mediums can cause a phase inversion which reverses the polarity of the wave. Thus a positive wave that was emitted by the antenna can reflect back negative, or vice versa.

The velocity of GPR waves and their polarity are correlated concepts. They are displayed on the GPR screen as black, white, and gray bands. Black bands are considered “fast”, and are negatively polarized, whereas white bands are considered “slow” and have a positive polarity. Gray bands are considered average reflections and make up the bulk of the background of the image. Both the terms fast and slow are relative to the medium the wave was traveling in before it encountered a change in dielectric constant, and how “bright” those bands appear on the screen is a function of the amplitude of the reflected wave.

GPR Terminology, Definition and Equations

Ambient and geologic noise sources – Boulders, animal burrows, tree roots, or anything else that may cause unwanted reflections or scattering of the radar waves. Lateral and vertical variations in EM properties can also be a source of noise.

Amplitude – The maximum extent of a vibration or oscillation, measured from the position of equilibrium.

Attenuation— The gradual loss in intensity of any kind of flux through a medium. Attenuation occurs when electromagnetic energy is converted to thermal energy or when the electromagnetic energy scatters in undesirable directions below the surface. How deep a GPR scan can penetrate is determined by the attenuation of the signal.

Back Scattered Energy – The reflection of waves back in the direction in which they were transmitted via various scattering methods.

Bistatic Antenna Arrangement – An arrangement where the transmitting and receiving antennas are apart from each other at a considerable distance.

Converting Two-Way Times to Depths – To find the depth it is first necessary to determine the propagation velocity of the EM pulses. Once propagation velocity is calculated, one simply uses the formula Vm=(2D)/t where Vm is propagation velocity, t is two-way travel time, and D is depth. As referenced in, Standard Guide for Using the Surface Ground Penetrating Radar Method for Subsurface Investigation

Critical Distance – The distance that the receiver must be from the transmitter to receive a refracted wave.

Cultural Noise Sources — Reflections from nearby vehicles, buildings, fences, power lines, lamp posts, and trees.

Data Display — Where the reflected radar pulses are displayed on a horizontal and vertical axis. The distance covered during the scanning process is displayed on the horizontal axis. The time it takes for the radar pulse to travel to and from the reflective material, also known as the two-way travel time, is represented on the vertical axis. The data display is commonly configured to depict this information in two ways. First, the wiggle trace display will highlight the intensity of the wave in proportion to the amplitude of the trace. Second, when the color display is set to grayscale, the display will highlight show high intensity as black and low intensity as white.

Electromagnetic (EM) Waves — Synchronized oscillations of electric and magnetic fields that propagate at the speed of light.

Frequency – The rate at which something occurs or is repeated over a particular period of time or in a given sample.

GPR Antenna – A device that transmits electromagnetic waves which propagate into a material. GPR antennas also convert electromagnetic waves to currents on an antenna element, acting as a receiver of the electromagnetic radiation by capturing part of the electromagnetic wave.

GPR Equipment – Tools for measuring subsurface conditions; normally consists of a transmitter and receiver antenna, a radar control unit, and suitable data storage and display devices.

GPR Trace – The recording of both the sending and receiving pulses over a period of time; a two-way travel time history of a single pulse from the transmit antenna to the receiver antenna, including all the different travel paths. The trace is the basic measurement for all time-domain GPR surveys.

Monostatic System – When the same antenna is used for transmitting and receiving the signal.

Polarity – Movement of electric charge carriers produced by the composite motions of electrons in geometrically aligned atom. Can be simplified to positive and negative waves for GPR understanding.

Radar Control Unit — Synchronizes signals to the transmitting and receiving electronics in the antennas. The synchronizing signals control the transmitter and sampling receiver electronics located in the antenna(s) in order to generate a sampled waveform of the reflected radar pulses.

Ray – A straight line drawn from the transmitter to the edge of the wavefront. Rays are used to show the direction of travel of the wavefront in any direction away from the transmitting antenna.

Scan – A trace where a color scale has been applied to the amplitude values.

Scattering –when a portion of the wavefront encounters an object with a permittivity different from the surrounding material (host media), then that portion changes direction. Scattering at the interface between an object and the host material is of four main types

  1. Specular Reflection Scattering
  2. Refraction Scattering
  3. Diffraction Scattering
  4. Resonant Scattering

GPR radio wave scattering

Specular Reflection Scattering – Specular scattering is based on the Law of Reflection, where the angle of reflection is equal to the angle of incidence.

Refraction Scattering – The deflection of waves as they travel through a substance. The wave generally changes the angle of its direction upon contact with the object. Uncommon as a propagation path for GPR, since the electromagnetic wave velocity tends to decrease with depth.

Diffraction Scattering – The bending of electromagnetic waves.  Electromagnetic waves will scatter in different directions depending on the shape of a reflective object or material and where the pulse of the radar initially reflects off of the object.

Resonant Scattering – Scattering of the GPR wave by a system in which the system first absorbs the energy, then reemits the energy. Occurs when a wave impinges on a closed object (e.g., a cylinder), and the wave bounces back-and-forth between different points of the boundary of the object; closed objects are said to have a resonant frequency that is based on the size of the object, and the electrical properties of the object and the surrounding material. However, the ability of an object to resonate depends on the wavelength with respect to dimensions of the object.

Trace Spacing – spacing between measurement points is called the scan

Travel Time – The interval of time that it takes for the wave to travel from the transmit antenna to the receiver antenna. The basic unit of electromagnetic wave travel time is the nanosecond (ns), where 1 ns = 10-9 s.

Velocity – the speed of the GPR wave in a given direction.

Wavefront – surface surrounding the advancing wave.

We Scan For

  • Pipes and Conduits
  • Post-Tension Cables
  • Utilities and Live Wires
  • Rebar and rebar patterns
  • Voids within the Concrete
  • Structural Integrity of the Concrete