Ground-Penetrating Radar (GPR) Technology With The UtilityScan Module
A Brief Overview of GPR Technology
Ground-penetrating radar (GPR) technology is a non-invasive, non-destructive geophysical surveying technique that produces a two-dimensional cross-section image of the subsurface. When used in conjunction with the GSSI UtilityScan module, which is manufactured by Geophysical Survey Systems, Inc. of Nashua, NH, this technology makes it possible to locate underground utilities or other similarly sized objects. We currently use the SIR-3000 or SIR-4000 control units linked up to a 400 MHz UtilityScan Standard System.
Basic Principles of GPR
Quite often, non-metallic, inaccessible, unknown or abandoned utilities cannot be located with traditional cable and pipe locators. When this occurs, Ground Penetrating Radar (GPR) must be used in conjunction with it or in place of it. 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. All GPR units are made up of a power supply, control unit, and antenna, which sends a pulse of electromagnetic energy into the subsurface.
Power Supply, Control Unit & Antenna
GPR works through the interaction of a control unit module and an antenna (also called a transducer), which are both shown in the figure above.
Pulse of Radar Energy
The control unit contains electronics which trigger a pulse of radar energy that the antenna transmits into the ground. The antenna then receives this electrical pulse produced by the control unit, amplifies it, and transmits it into the ground or other medium at a particular frequency.
Antenna frequency is one of the major factors in depth penetration. The higher the frequency of the antenna, the more shallow into the ground the signal will penetrate and the easier it will be to resolve the targets. Conversely, the lower the antenna frequency, the deeper the signal will penetrate and the more difficult it will be to resolve the targets. Antenna choice is one of the most important factors in survey design. The following table shows various antenna frequencies, their approximate depth penetration, and their appropriate applications. For utility locating applications, the 400 and 900 MHz antennas will be cover almost all potential situations.
How GPR Technology Works
GPR technology works by sending a tiny pulse of electromagnetic energy from the antenna into a material and recording the strength and time required for the return of any reflected signal. A series of these pulses over a single area is called a scan. As shown below, a scan is performed by moving the antenna across the surface linearly to create a series of electromagnetic pulses over a given area.
During the scan, reflections are produced whenever the energy pulse enters into a material with different electrical conductivity or dielectric permittivity from the material it just left. The strength, or amplitude, of the reflection is determined by the contrast in the dielectric constants and conductivities of the two materials. This means that a pulse which moves from dry sand (dielectric of 5) to wet sand (dielectric of 30) will produce a very strong reflection, while moving from dry sand (5) to limestone (7) will produce a relatively weak reflection. Below is a list of some common materials and their dielectric values.
While some of the GPR energy pulse is reflected back to the antenna, some energy also keeps traveling through the material until it either dissipates (attenuates) or the GPR control unit has closed its time window. The rate of signal attenuation varies widely and is dependent on the properties of the material that the pulse is passing through. Materials with a high dielectric value slow the radar wave and decrease its penetration depth, and highly conductive materials attenuate the signal rapidly. For example, water dramatically raises the dielectric of a material, thereby inhibiting signal penetration; and highly conductive materials such as metals completely reflect the signal.
As shown in the figure below, radar energy is emitted from the antenna in a cone shape, and the two-way travel time for energy at the leading edge of the cone is longer than for energy directly beneath the antenna.
To understand how this physical process translates into a data image, imagine scanning perpendicularly across a pipe. Because it will take longer for energy at the leading edge of the cone to be captured, when the antenna first approaches the pipe, it will appear down 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 center of the antenna is located directly on top of the pipe, the minimum distance of separation is reached and the reflections reach their zenith in the profile--the hypotenuse of a right triangle in the figure above. As the antenna begins moving away from the pipe and the distance between them increases again, the reflections will once again appear further down in the profile. After the scan is completed, the center of the pipe will look like an upside down "U"--a hyperbola--as depicted in the data screen profile image shown below.
To conduct a GPR survey, a topic that will be discussed below, a series of scans are performed inside an orthogonal grid, the hyperbolas are marked on the ground with chalk or paint, and the dots are connected to represent the location of underground utilities or anomalies.
The GSSI UtilityScan Module
The UtilityScan module consists of the GSSI SIR-3000 control unit and a survey cart with an integrated survey wheel encoder attached to a either a 400 or 900 MHz antenna. This technology enables us to collect data in real-time, accurately pinpoint the location and depth of buried objects with a back-up and cross-hair cursor, it easy to transport, and it can withstand the toughest conditions.
400 MHz All-purpose Antenna
Used for a wide variety of utility applications
Penetrates to a depth of 15 feet
900 MHz Shallow Antenna
Used mainly for concrete slab-on-grade surveys
Penetrates to a depth of 3 feet
Determining the Feasibility of Conducting a GPR Utility Locating Survey
Before conducting a GPR survey, the following four factors must be analyzed to determine whether or not conditions are optimal:
Topography - Make sure it is physically possible to move the antenna over the ground surface in a fairly smooth fashion.
Ground Cover - Always attempt to keep the antenna flat on the ground surface. If the antenna floats on top of thick grass or a layer of gravel, errors may occur because the signal will take too long to couple (penetrate) with the ground, causing the signal to bounce off the ground instead of going through it. A good rule of thumb when using the 400 MHz antenna is to survey over no more than one inch of grass or gravel, and to never survey through standing water, regardless of how shallow the puddle is.
Subsurface Conditions - Attempt to gather information regarding the survey area’s soil and water content. Knowing the soil grain size (sand, silt, clay, etc.) will help determine the most accurate estimate of the dielectric constant value, enabling one to determine survey parameters and to make time to depth estimations. In general, clay and water cause attenuation, thereby impeding penetration. Please visit the US Department of Agriculture soils website to gain access to soil maps throughout most the United States.
Site Accessibility - Determine the feasibility of working within a given area. The area must be large enough to collect a sufficient amount of continuous data to make interpretation of the data possible. This is important because the success of GPR or other geophysical survey techniques often depends on one’s ability to see contrasts in the data. If the survey area is located within a thicket of shrubs, underneath parked cars or outside a tall building, collecting sufficient data may not be possible.
Running Test Scans & Calibrating the Depth Scale Using the Ground Truth Method
If the field conditions appear optimal, test scans are performed across known targets to determine the quality of the data. If the results are good, a scan is performed across a target of known depth such as a drain pipe or a utility exposed during excavation to calibrate the depth scale on the y-axis of the data screen profile.
Setting Up an Orthogonal GPR Survey Grid
To set up an orthogonal survey grid, the location of known utilities (from an as-built or the cable and pipe locating survey) are used as a reference point. Specifically, the grid is oriented so that it allows for the greatest number scans to be performed perpendicular to the known utilities, the direction they must be crossed in order to produce the narrowest hyperbolas possible. In most situations the orthogonal grid can be imaginary, but if documentation is required, an alphanumeric grid must be created with chalk or pink paint. An example of typical GPR survey grid is shown below. The spacing of grid line intersections is dependent upon the purpose of the survey and the size of the survey area, and this decision can only be made through an analysis of multiple factors and field experience. A brief rule of thumb to determine the spacing of an imaginary grid is to perform the scan, stop at the end of the grid, pivot the wheels, turn 180° until the wheel overlaps the track of the previous scan, and then perform a scan parallel to the first.
Conducting a GPR Survey
A GPR survey is performed by using the following seven-step process shown below:
1. The antenna is set up over the first grid line and the cart is pushed forward to begin the scan.
2. During the scan, the cart is kept running in a straight line and the screen is watched for the presence of hyperbolas or other anomalies.
3. If any appear, the cart is backed up until the cursor is located over the center of the hyperbola or the edges of the anomaly, and the ground is marked with chalk or paint alongside the center of the antenna.
4. After the target is marked, the scan is continued until another hyperbola is found or the run is completed.
5. Once the run is completed, steps 1-4 are repeated on each grid point until all runs have been completed in one direction.
6. Any markings that run in a straight line and appear to be the size and depth of underground utilities are marked with paint and/or flags, using the appropriate APWA color codes. Any targets that run through the survey area in a straight line that cannot be identified as underground utilities are marked as unknowns with pink paint and/or flags.
7. Steps 1-6 are repeated in the opposite direction to complete the survey grid and any additional targets are marked (in this example there were no others).
Advantages of GPR Technology for Utility Locating
It is non-destructive and safe: the electromagnetic energy does not harm the subsurface, organisms, or the environment.
Can find non-metallic and non-conductive utilities incapable of being located with a cable and pipe locator.
In favorable soil conditions, it can sometimes detect the location and depth of all underground utility types and materials.
Resolves duct bank utilities detected as one target with the cable and pipe locator technology into multiple targets.
Detects underground storage tanks (USTs), septic tanks, and leach fields.
Detects the location of voids.
Limitations of GPR Technology for Utility Locating
The signal cannot penetrate through high-conductivity materials such as reinforcing, clay soils, and soils that are salt contaminated, especially areas where de-icing salts have been applied generously.
Due to signal scattering, targets are difficult to confirm or resolve in heterogeneous conditions (e.g. rocky soils).
Interpretation of data is generally non-intuitive to the novice.
Considerable experience and expertise is needed to properly design, conduct, and interpret GPR surveys.
Difficult to use in inclement weather.