Understanding the applications and benefits of ground penetrating radar

August 20th, 2019, Published in Articles: PositionIT, Featured: PositionIT

The ground penetrating radar market in South Africa is gaining popularity. This paper will look at what it is, explore its complementing technologies, where it is used, some examples and case studies from around the world and what is happening in this market in South Africa.

Group penetrating radar (GPR) is a non-destructive geophysical method that uses electromagnetic radar to detect sub-surface objects. This is done by moving the radar across the surface under investigation and looking for patterns that repeat itself.

Fig. 1: Examples of GPR utility detection machines.

There is misconception that GPR shows changes in the density of objects, but it actually shows changes in the conductivity of objects. Different types of soil or rock have different conductivities, and the speed at which radar moves through the object is measured by the dielectric value of the material. Air has a dielectric of 1 and water has a dielectric of 81. Radar waves travel the fastest through air and slowest through water. Most other materials fit between these two values. The software on the GPR machine would interpret the data and show where the conductivities change.

Fig. 2: Objects detected by GPR.

The higher the frequency of the radar the more detail it picks up, but the shallower its penetration. The lower the frequency, the deeper it penetrates, but also the less detail it shows. So there is a constant compromise for either depth or detail.

For concrete, for instance, you would not want to scan much deeper than 500 mm into the concrete, and therefore a concrete GPR scanner would use a much higher frequency (above 2000 MHz) than that needed for utility detection, where the focus is typically 1 – 3 m blow the surface. For this depth an antenna with a frequency of 200 – 900 MHz would be used.

Limitations of GPR

The asset owner and material composition of the object cannot be determined with GPR. As operators we do in many cases indicate the type of utility that we have detected (like electric cables or water pipes), but we only do this if we are able to trace the line to a mini-substation or fire hydrant or other above-ground feature. When scanning concrete, the exact size of the rebar can also not be determined, only its position and depth.

Fig. 3: A metal pipe which could not detect by an 800 MHz antenna.

To give you an idea of the limitations of GPR, Fig. 3 shows a 1,6 m diameter steel pipe at 1,8 m below the scanner. Steel is one of the most conductive materials and therefore we expect to detect this easily. We were however unable to detect the pipe at one point, even though the pipe was at the same depth where we were able to detect it just 200 m prior. The difference between the two scans is the soil conductivity above the pipe.

Despite its limitations, GPR is very useful, as the below case studies will illustrate.

Complementing technologies

Fig. 4: An electromagnetic (EM) locator and transmitter.

The most common tools to use with GPR are electromagnetic locators. While GPR works by sending out an electromagnetic pulse, electromagnetic locators work by picking up the electromagnetic field around electrical cables. In some cases an electromagnetic field can be transferred to pipes, which enables us to detect a water pipe with this as well. We have even had cases where GPR were not able to detect the pipe which the electromagnetic locator was able to detect. This is because the material the pipe is made of is not conductive enough or the soil above it is too conductive, but the pipe’s electromagnetism could be detected by the electromagnetic locator.

Another useful instrument for utility detection is a sonde. This is an object that emits an electromagnetic signal that you push into the pipe or sleeve which you are detecting. You can then chase the sonde from the surface using an electromagnetic locator. This method also makes detection at much deeper depths possible, but it also takes much more time and effort, and off-course, you need an entry point into the pipe.

Two other methods used in utility detection is potholing and hydro excavation. Various countries including the USA, Canada, the UK, the Netherlands, Australia and New Zeeland have a one call system. In essence, it is a system where anybody that needs to do an excavation in a public or private space needs to use the one call system to inform all sub-surface asset owners, requesting them to send drawings showing the approximate position of their assets. A utility detection company then has to locate and identify all these pipes, cables or fibre.

Fig. 5: Using a combination of EM locator and a sonde.

The position and depths of the assets are then confirmed by making use of a method called potholing. This method employs vacuum excavation either through high pressure water or air being blown into the ground to loosen the soil, which is then sucked out. This exposes the utilities so that the exact depth is known and the position verified, should the GPR survey get it wrong. In South Africa the market is still uneducated about the limitations of GPR and EM location and therefore potholing is not a requirement. Client believe that if the GRP and EM found nothing that excavation can continue without care. From experience we know this is not wise.

Fig. 6: An example of hydro-excavation.

Various vendors have developed 3D GPR imaging software that work with all GPR data. It allows taking “slices” at specific depths and viewing the scanned area from all angles in 3D. The images can also be georeferenced and mapped with laser scanner point clouds to create an exact image above and below ground.

When conducting a concrete GPR scan, many different technologies come together to do a thorough investigation of the concrete. This includes terrestrial laser scanning, infrared thermographic scanning and core drilling with concrete strength tests on the cores. All the detected objects are then captured and mapped by conventional land surveying methods.

Industries using ground penetrating radar

Even though the use of GPR for sub-surface detection work dates back to the early 1900s, the first affordable system was produced in the early 1990s. From there it has been gaining popularity. In most developed countries utility detection is required before excavating in public spaces, although no formal standards for the use of GPR are in place.

GPR is predominantly used in utility detection. There are however many other industries where it is also useful. Archaeology has been using 3D grid scanning to detect buried building structures, while police forensics and commercial companies often use it to detect unmarked graves. The military uses GPR to detect tunnels or arms caches, and in mining GPR is used at lower frequencies to follow reefs and fault lines.

GSSI, a company which sells between 30% to 40% of global GPR systems, give the following market breakdown of their sales:

  • 50-60% utility detection
  • 10-20% concrete scanning
  • 5% road scanning (surface/subgrade condition)
  • 5% geotechnical (foundations, tunnel lining, etc.)
  • 1-2 % research and teaching
  • 1% forensics (police and commercial unmarked grave detection)

It is typically used in mining safety, forestry (root biomass calculations), agriculture (soil conductivity mapping), military/police (tunnel detection/arms caches/unexploded ordinance), civil works (detecting and removing hollow trees which may fall during tornadoes etc), and even for treasure hunting.

Local and international uses of GPR

In South Africa the use of GPR and other utility detection technology are still in their infancy. It appears to be because of the availability of cheap labour which means many clients would rather dig by hand for five days than get a utility detector out.

In the telecommunications industry, directional drilling is often used to do road crossings. When conducting scans for these operations it is critical to determine the exact depth and position of all services to avoid damaging them. Subscan has conducted more than 3000 scans on road crossings. Before using our services, our clients had hit rates of between 50% and 60%. Since employing GPR this has gone down to 2% (i.e. average success rate of 98%). This clearly shows the benefit of using this technology, but also that it is not failproof. We work with highly experienced drilling companies who know GPR is not the final answer and that they still need to work with care.

I’ve been involved with factories, schools, shopping centres and hospitals that need their full infrastructure detected and mapped. Although the final product can never be called an as-built drawing, it makes the design phase of any new work much more accurate since the new infrastructure can be connected to the existing infrastructure.

GRP is being used more widely across Africa as well, and in the last year we have had three projects in the DRC and Sierra Leone. This is still mostly required by private international companies and no municipalities yet.

The US Federal Highway Administration did a study on 71 road projects with a total value of over $1-billion. They found that using utility detection during a project cost less than 0,5% of the total value of the project, but the saving it resulted in was 1,9% of the total cost. If an overall cost saving of more than triple the cost of the detection work is possible, as well as a decrease in delays, then this is something we should to take note of in South Africa.

Fig. 7: The Dutch one-call system, called KLIC.

In 2004, Belgium’s biggest gas pipeline explosion occurred about 50 km south of Brussels, killing 24 people and disabling the 1 m diameter, 80 bar pressurised line for six weeks. Following this, their neighbour, The Netherlands, passed a law that made it mandatory for both excavators and asset owners to register assets in a one-call system, called Klic. Over the years this system has developed into arguably the best “dial before you dig” system in the world.

The system runs 24/7 and has a response time (with all the required detail) of less than one day. Users can file a request and view all maps on a smartphone or tablet. Measurements from a wall or road are provided, meaning the system’s users know exactly where objects runs. The accuracy of pipe and cable locations are also continually improved. Whenever an excavation finds a pipe or cable in a position different from what the Klic system shows, it can be logged with photos and measurements. The owner of that asset then has a week to investigate and update the correct position. The next phase of the system development aims to have all maps in augmented reality to enable users to walk with the app in-hand and view the sub-surface objects on screen. All of this is possible because the Dutch government forced all asset owners to save their information in the same format. Before we can hope to achieve this in South Africa we would need buy-in from all stakeholders – government and private.

During 2016, Japan (a country with 124-million people and a paved road network of 973 000 km, compared to 158 000 km in South Africa), only recorded 134 instances where underground utility damage occurred through excavation. This contrasts with the 400 000 instances recorded in the US every year.

Geoff Zeiss, an expert in the geospatial software industry, writes how they implemented a one call system in Calgary in the US, and this system reduced the average number of hits to 0,25 hits per 1000 excavations.

Every construction project requires investing time and effort prior to and during excavation to locate underground utilities. Construction bids are routinely inflated by 10% to 30% to accommodate risk associated with unknown or poorly located underground utilities  In the US this has engendered what is estimated to be a $10-billion per year industry. Every state in the US and most provinces in Canada have a one-call centre and legislation that requires anyone planning to excavate to contact the centre. Utilities and telecoms are required to send crews to locate and indicate with paint or pin flags the location of their underground infrastructure. Because they are considered liable for any underground damage, construction contractors do not rely solely on this information but increasingly use expensive vacuum and hydro excavation equipment to detect and expose underground infrastructure. Unfortunately the information captured by these techniques is rarely shared and the location of underground infrastructure is recaptured repeatedly.

A tool with diverse applications

GPR is the most common and widely used method for utility detection and in South Africa there are many industries already making use of this technology with great success. For better results and wider adoption of GPR, all role players – including asset owners, municipalities, consulting engineers and utility detection companies – will have to acknowledge its benefit and start applying it in their work.

References

[1] Federal Highway Administration: Avoiding Utility Relocations. 19 April 2018. Online at https://www.fhwa.dot.gov/utilities/utilityrelo/4.cfm
[2] French Ministry of Sustainable Development: / Rupture and ignition of a gas pipeline 30 July 2004. Online at https://www.aria.developpement-durable.gouv.fr/wp-content/files_mf/FD_27681_Ghislengheinv_2004ang.pdf
[3] City regulation reduces underground utility damage during excavations. 19 May 2019. Geoff Zeiss. Online at https://geospatial.blogs.com/geospatial/2019/05/city-regulation-reduces-underground-utility-damage.html
[4] Role of surveyors in digitally mapping underground utilities. 1 May 2019. Geoff Zeiss. Online at https://geospatial.blogs.com/geospatial/2019/05/alberta-land-surveyors-opportunities-for-surveyors-in-locating-underground-utilities.html

Contact Hennie le Roux, Subscan, hennie@subscan.co.za

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