Ground Penetrating Radar (GPR) is an important tool for utility locators around the world as it detects both metallic and nonmetallic targets at different depths. The method itself has been around for more than 50 years, but most systems in the market offer limited visualization, both onsite and offsite.
GPR is a compact device that scans the subsurface in a non-destructive way. It can penetrate the surface from just a few centimeters down to tens or hundreds of meters, based on soil conditions and antenna characteristics. GPR emits electromagnetic (EM) waves into the subsurface and calculates the time these waves need to travel through the various subsurface material and return to the GPR receiver. This two-way travel time, along with a few other parameters such as the dielectric, provide the user an estimate of the target depth.
Currently, two different GPR technologies exist in the market – pulsed systems and stepped frequency devices. The pulsed systems send tiny pulses of EM wave down to the subsurface during periodic intervals, while the stepped frequency devices send continuous parcels of wave at different frequencies at the same time. In the past, due to computing limitations, pulsed systems were the preferred choice in the market. Despite their popularity, they have several drawbacks, like the frequency dilemma, where a user has to select multiple antennas at different frequencies to carry on different applications. Pulsed systems are limited by the central frequency of the antenna, so they have specific limitations in terms of depth and resolution.
Nowadays, stepped frequency is gaining popularity because with a single device you can sweep through the complete frequency spectrum, from low to high frequencies, so you can see depth and resolve small objects at the same time.
A typical workflow for a utility locator includes the following steps:
a.Quick subsurface scan using electromagnetic locators (EML)
b.Identify areas of interest
c.Pass over these areas with a GPR to locate the targets
d.Identify coordinates of the targets with a GPS or GNSS
e.Post process data on computer offsite
f.Export data on maps (typically linear graphics that represent pipes)
We are working hard to make the workflow shorter and more effective.
Many utility locators, especially in North America, use EML as a quick and easy tool to scan large areas and get a rough image of what is beneath the surface. This gives an initial estimation of what is happening in the subsurface (step a), allowing you to pick smaller sub-areas where GPR will provide validation and accurate information (step b).
EML detect targets made from conductive materials and will need to energize non-conductive targets to trace them, which is not easy, nor flexible. Typically, GPR can detect almost all types of targets, gives a deeper penetration, can provide B-Scans of the subsurface, and export this info into maps. With traditional GPR technology, it is not feasible to eliminate step a or step b. However, when GPR is equipped with mapping capabilities, these steps can be eliminated.
If a GPR user can collect data in real-time and visualize a subsurface map in 3D without the need for grids or post processing, there will be no need to scan areas with both EML and GPR, as the latter will be an all-in-one tool for the subsurface mapping professional.
The software is an iPad application that allows users fast and easy 3D visualization on site. Steps d, e, and f are eliminated or shortened significantly. Here is why.
With built-in GNSS capabilities, the software assigns every target on the screen very accurate coordinates, so step d is gone from your workflow. You must do nothing other than turn on the integrated GNSS received and off you go. Since every target has accurate geographical coordinates, it is very easy to automatically export this data on GIS maps or CAD software, without the need to manually assign these coordinates. It is evident, that step f now takes less time.
Being able to see data on a linescan and on a map adds to the understanding of what lays beneath. You can skip any hyperbolas that are adding to the noise and concentrate only on hyperbolas that are repeating over the different linescans. You can annotate and determine the type of targets with a simple click so that you visualize different targets with different colors (Figure 1).
Knowing your exact position during a survey offers a plethora of benefits including easier target interpretations, better understanding of where the quality of data is good or bad offering on the spot adjustments of your parameters.
Data can be post-processed onsite easily and fast with the powerful application, all without exporting data first to a computer. Step e is almost eliminated.
The most interesting aspect of the software is the visualization of targets using augmented reality onsite.
The software gives you the option to augment the subsurface data in the real world (Figure 2), exactly on the place where they are, making your job easier and safer. You know exactly what the subsurface looks like, and you can show it to others too. These tools of visualization ensure that digging is now safer.
Dr. Michael Arvanitis is Head of Product Strategy and Marketing GPR for Screening Eagle Technologies. He holds a bachelor’s degree in mathematics and a PhD in Geophysics. Learn more at screeningeagle. com/en/products/260.