GPR is a non-invasive, non-destructive geophysical surveying technique that is used to produce a cross-sectional view of objects embedded within the subsurface.
Quite often, non-metallic, inaccessible, unknown or abandoned utilities cannot be located with traditional cable and pipe locators. When this occurs, GPR must be used in conjunction. In addition, GPR can also be used as a safe alternative to x-ray for determining the location and depth of objects embedded within concrete structures.All GPR units consist of three main components: a power supply, control unit and antenna. To understand how these components interact, we must first understand the definition of a scan. A scan is performed by moving the antenna across the surface linearly to create a series of electromagnetic pulses over a given area. During a scan, the control unit produces and regulates a pulse of radar energy, which is amplified and transmitted into the subsurface at a specific frequency by the antenna. Antenna frequency is inversely proportional to penetration depth, which makes antenna selection the most important step in the survey design process. A table showing various antenna frequencies and their corresponding depth ranges is shown below.
During a scan, the control unit records the strength and time required for the return of any reflected energy. Reflections are produced in the data screen profile (on the control unit) whenever the energy pulse enters and exits contrasting subsurface materials. The way it responds to each material is determined by two physical properties: dielectric constant and electrical conductivity.
The dielectric constant is a descriptive number that indicates how fast electromagnetic energy travels through a material. Energy always moves through a material as quickly as possible, but certain materials slow down the energy more than others. The higher the dielectric, the slower the energy will move through the material, and vice versa.
To determine the location of a subsurface target in the data screen profile, there must be a contrast between the dielectric values of the material one is scanning through and the target one is searching for. For example, a pulse moving from dry sand (dielectric of 5) to wet sand (dielectric of 30) will produce a strong, highly visible reflection, while moving from dry sand (5) to limestone (7) will produce a weak one. In addition, if one knows the dielectric value of the subsurface material one is scanning through, the control unit can measure the amount of time required to receive the reflected signal and convert this to depth. Since the GPR emits electromagnetic energy, it is subject to attenuation (natural absorption) as it moves through a material. Energy moving through resistive (less conductive) materials such as dry sand, ice or dry concrete will penetrate much further than energy moving through absorptive (more conductive) materials such as salt water or wet concrete. As a result, the greater the contrast in electrical conductivity between the material one is scanning through and the target one is searching for, the brighter the reflection; high conductive materials such as metals produce the brightest reflections.
To understand how dielectric and electrical conductivity differences translate into visual data requires an understanding of how the antenna emits energy. For example, imagine the antenna scanning perpendicular to a pipe (or rebar). Energy emits from the antenna in a cone shape, not in a straight line as one might think. The two-way travel time for energy at the leading edge of the cone is longer than for energy directly below the antenna. Because it will take longer for energy at the leading edge to be captured, when the antenna first approaches the pipe, it will appear low in the data screen profile. As the antenna moves closer to the pipe and the distance between them decreases, the reflections will appear higher in the profile. At the point where the antenna is located directly above the pipe, the minimum distance of separation is reached and the reflections reach their zenith. As the antenna moves away from the pipe and the distance between them increases, the reflections appear lower in the profile once again. After the scan is completed, the center of the pipe will appear in the data screen profile as an upside down U, which is referred to as a hyperbola.
To gather, organize and present the data, a series of scans are performed within an orthogonal survey grid. At the end of each scan, the data screen profile is reviewed for the presence of hyperbolic targets. If present, the antenna is moved backward to place a cursor (which depicts the center of the antenna in the data screen profile) on the center of the targets. The location and depth of the targets are then marked on the surface with chalk, paint and/or flags. Once the entire survey grid has been scanned, the marks are reviewed to search for patterns similar to that of the desired targets, in this case a pipe. Any marks that run in straight line and whose hyperbolas appeared as highly conductive metal targets are then connected, thereby displaying the location and depth of the pipe.
To see how ground penetrating radar (GPR) technology is used to perform an underground utility locating survey, please visit the underground utility locating services or underground utility locating projects page.
To see how ground penetrating radar (GPR) technology is used to perform a concrete imaging or concrete inspection survey, please visit the concrete imaging and inspection services or concrete imaging and inspection projects page.
