GPR: Using Electromagnetic Pulses to Find What Lies Beneath
Anyone who’s ever used a fish finder can appreciate how Ground Penetrating Radar (GPR) works.
A fish finder sends an electrical pulse that is converted into a sound wave, into the water. As the boat moves, the finder collects the sound wave recordings, which are displayed on a monitor side by side in a cross-section. When the sound wave strikes a fish, the wave is reflected back, showing the fish’s size and shape. The distance to the fish can be determined because the sound wave travels through the water at a known speed and the two-way travel time is known.
Ground penetrating radar sends electro-magnetic pulses into the ground. The pulses travel through the ground and reflect when they encounter different materials, due to a change in velocity or when they cannot penetrate materials such as metal.
GPR can save an underground utility contractor costly delays and mistakes. With urban redevelopment of central business districts at an all-time high, this is becoming more important.
The amplitude and phase of the reflected pulse, along with the two-way travel time is recorded and used to identify the location of the targets.
One of the earliest uses of GPR was to map the thickness of ice sheets and glaciers. Today the technology is utilized in a variety of applications, including archeological digs, road and bridge inspection, searching for unexploded ordnance, and of course, locating buried utilities.
A utility-locating GPR system usually consists of three elements: a GPR controller, a mid-frequency (200 to 400 MHz) antenna, and a cart with an integrated encoder that provides distance information. As the antenna is moved over the survey area, point targets such as pipes, appear as hyperbola shapes due to the increasing then decreasing two-way travel time as the antenna moves over the target. These hyperbola shapes may be processed to better determine the exact depth of the utility.
How far can GPR penetrate? Typical depth penetration for GPR is up to 30 meters. However, it depends on two variables; the antenna frequency and the material. In general, the lower the antenna frequency, the “deeper” the depth penetration. However, all materials are not created equal. Non-conductive dry materials, such as sand, are normally easiest to penetrate while conductive wet materials, such as clay, are typically more difficult for radar. Radar works well in fresh water but very poorly in salt water due to the high conductivity.
An integral part of Subsurface Utility Engineering (SUE), GPR is a type of geophysical survey that typically occurs after relevant engineering schematics have been sourced (Level ‘D’) and all visible utility appurtenances have been identified and plotted on a composite drawing (Level ‘C’). GPR is in accordance to the Subsurface Utility Engineering Process, codified in 2002 by the American Society of Civil Engineers (ASCE).
Benefits of GPR
“It's an effective product to confirm the existence of structures mentioned at Levels C and D,” says Jan Kesik, Canadian territory manager for Geophysical Survey Systems, Inc. (GSSI), a manufacturer of ground penetrating radar technologies.
In an interview with CUI, Kesik noted that GPR identifies both metallic and non-metallic structures.
“Therefore it can detect piping, water mains and multiple levels of metal piping if conditions are favourable. That said, GPR is a value-added tool for utility locating and complements frequency meters and other geophysical methods used to obtain a detailed SUE.”
GPR can also save an underground utility contractor costly delays and mistakes, which can occur if the contractor relies on old and sometimes unreliable information. With urban redevelopment of central business districts at an all-time high, this is becoming more important.
“New technology, such as fibre optic cable, is increasing the demand for trench work and non-destructive testing,” says Kesik. “Just the cost of hitting a fibre optic cable alone is motivation for proper SUE prior to construction.”
“The longer that fibre optic cable is down, and we’re talking potentially thousands of phone and Internet connections, the higher the cost to the communications provider. As well as the cost to businesses and the general public from having to cut services,” Kesik elaborated. “It’s in their best interest to never hit one.”
While GPR has a number of benefits under the SUE rubric, there are limitations. One is the type of soil or material being surveyed. Conductive soil containingsalt, or salt-saturated pavement, willattenuate the signal and therefore limitthe depth of penetration. Water, on theother hand, slows the signal down. “Dry,non-conductive environments are ideal,”says Kesik.
For utility contractors, knowing the type and depth of targets they are searching for will govern decision-making about the type of GPR instrumentation to invest in. GSSI for instance offers a range of GPR systems from an entry-level locating unit to advanced systems such as the UtilityScan DF (Dual Frequency).
Shown at the recent Canadian Common Ground Alliance’s 2014 Damage Prevention Symposium in Banff, UtilityScan DF offers a dual-frequency (300 and 800 MHz) antenna with the ability to locate both shallow and deep targets.
Kesik said having the dual-frequency antenna is an advantage because it eliminates the need to re-scan an area if the GPR provider doesn’t bring the right antenna. Shallow and deep targets can also be read on the same screen, using the “blend” mode.
“It’s saving a lot of time because you’ve got both shallow and deep, or low and high frequency antenna, all in one shot,” concluded Kesik.