Ground Penetrating Radar or GPR, is a non-invasive investigation tool that utilises high frequency radio waves to capture images from below the ground surface. The major advantage of GPR is that it allows us to pinpoint the location of underground assets including utilities and other points of interest without the need for digging.
HOW DOES GPR WORK?
We’ve all seen the quintessential ‘Radar’ screen depicted in war movies. The circular rings on an otherwise blank screen with a green line moving around on the axis, and the ghostly green ‘blips’ that pop up to signify ‘the enemy is coming’. That’s Radio Detection and Ranging (RADAR) and it’s the same technology used in GPR.
GPR technology utilises energy waves in the microwave band (yes also just like your Microwave Oven), ranging in frequency from 1 to 1000 MHz (Mega-Hertz). A GPR unit is comprised of three main parts – a transmitter, a receiving antenna, and a control unit. The transmitter sends electromagnetic pulses into the ground and the receiver records the echoes that are bounced back from subsurface objects. If the electromagnetic impulse hits an object, the density of the object reflects, refracts, and scatters the signal. The receiver detects the returning signals and records variations within them. The control unit then translates this data into images and displays them in the form of a Radiogram (pictured above). And don’t worry – there will not be a test!
The skill with using GPR is not in its operation, but in the interpretation of the data displayed. Therefore, the effectiveness of any GPR survey is directly linked to the experience of the person providing the interpretation.
WHAT IS GPR USED FOR?
Ground Penetrating Radar technology can be used to identify a wide range of buried targets and is applied across a range of specialist fields from archaeological surveys to forensic crime scene investigations. In our industry though, GPR is most commonly used to locate buried assets made of Metal, Plastic, PVC, Concrete or other Natural materials, and the application possibilities are virtually endless.
Here at Ground Scan NZ, we utilise GPR to detect:
Underground Utility Lines and Buried Pipes,
- Geological features and rock obstructions,
- Air pockets or voids,
- Excavated and back-filled areas,
- Changes in subsurface strata,
- Groundwater tables,
- Bedrock.
UTILITY MAPPING USING GPR
Ground Scan NZ take advantage of advanced GPR technology to increase the efficacy and precision of our Utility Mapping projects by combining it with our traditional locating methods. GPR provides valuable assistance for discovering unmarked or otherwise undetectable utilities and structures, and the post-survey processing software allows us to graphically display the subsurface environment in a format that is not only highly accurate and accessible, but easy to understand.
ADVANTAGES OF USING GPR
GPR is an extremely cost-effective and non-invasive way of surveying. It provides invaluable information before workers even break ground – thus it is the safest method of exploration and an important safety step to any construction project. In addition:
1. GPR is safe for use in public spaces and a wide variety of project sites.
2. No digging, excavating, or ground disturbance is necessary.
3. It detects metal and non-metal objects, as well as voids and underground irregularities.
4. It makes it possible to measure the dimensions, depth and thickness of targets.
5. Data is provided quickly and can cover a large site area.
6. Frequencies can be regulated to deliver a range of resolution and penetration depths.
7. Data collected during the survey can be seen immediately or used in later projects.
8. Landscaping, structures, lawns, etc. are be left undisturbed by the survey process.
9. Only one side of the surface needs to be scanned to provide data.
10. It’s less expensive than other methods.
WHEN CAN GPR BE USED
As with all types of radar imaging, GPR delivers varying levels of accuracy depending on the conditions, and only a fully trained and experienced GPR operator knows how to work these variables. These conditions can include:
Soil Type and Ground Conditions – As noted, GPR works by sending a tiny pulse of energy into the ground then recording the strength of the reflected signals and time it takes them to return to the receiver. A scan consists of a series of pulses over a given survey area. While some of the GPR pulses reflect back to the receiver, some energy continues to travel through the material until it dissipates. This rate of signal dissipation varies widely, depending on the properties of the materials. For example, a pulse moving from wet sand to dry sand will produce a very strong reflection, in comparison to the relatively weak reflection produced by moving from dry sand to limestone.
GPR technology can be applied to a variety of ground types, including Soil, Rock, Ice, Fresh Water, Pavement and Concrete structures
Depth – GPR signals can theoretically penetrate up to 30 meters deep, but the ground itself can limit how deep we can carry out an effective survey. The ground has ‘electrical resistivity’, which means it opposes the flow of electric current to some degree. As the signal penetrates deeper, it naturally gets less effective. This depends mostly on the type of soil or rock being surveyed and the frequency of the antenna used. For example, the maximum penetration depth in solid concrete is usually around 600mm. In dry sand and other low-conductivity materials, the GPR signals depth effectiveness is significantly increased.
Ground Water Content – The moisture content of the substrate is also a big factor in GPR surveys. The quantity of water, or the underground moisture level, present in the subsurface material greatly affects the transmission of energy waves through the ground – thus effecting the clarity of the imaging obtained.
NON-INVASIVE PRE-DIG SURVEYS WITH GROUND SCAN NZ
At Ground Scan NZ we leave the ground just as we found it. Our crew provides exceptional imaging results for the location of utilities across a variety of media – and without ever disturbing the ground. If you have a job that could benefit from concrete scanning or GPR services, contact us for a no-obligation quote.
