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

The GPR UtilityScan Module

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 GPR UtilityScan Module Collecting Data

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

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.

A GPR Scan Being Made with a 400 MHz Antenna

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.

A 400 MHz Antenna Collecting GPR Data

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.

Underground Utilities Displayed in the GPR Data Screen Profile

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.

The GPR UtilityScan Module

 400 MHz All-purpose Antenna

A 400 MHz GPR Antenna

 900 MHz Shallow Antenna

A 900 MHz GPR Antenna

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:

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.

An Alphanumeric Orthogonal GPR Survey Grid

Conducting a GPR Survey

A GPR survey is performed by using the following seven-step process shown below:

GPR Scan Starting Point on Survey Grid

1. The antenna is set up over the first grid line and the cart is pushed forward to begin the scan.

GPR Survey of First Coordinate of Survey Grid

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. 

Locating an Anomaly During Initial GPR Scan

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.

The Completion of First Scan on GPR Grid

4. After the target is marked, the scan is continued until another hyperbola is found or the run is completed.

The Completion of GPR Survey in One Direction of Grid

5. Once the run is completed, steps 1-4 are repeated on each grid point until all runs have been completed in one direction.

Marking Targets Running in Straight Line as Underground Utilities

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.

The Entire GPR Survey Grid Completed in Both Directions

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 

Limitations of GPR Technology for Utility Locating