Illuminating Above and Below: LiDAR and GPR Form a Dynamic Data-Collection Duo
Mobile mapping platforms that capture detailed 3D views of infrastructure with LiDAR sensors are on the rise as an accepted best practice, particularly for new projects that alter or add to existing infrastructure. The ability to survey and capture what exists at engineering-grade accuracy helps inform modeling and design processes. Now there’s a powerful combination of technology to map what’s above ground and marry that with a detailed 3D view of what’s below ground via GPR.
GPR use for underground mapping goes back decades, but traditional units were walk-behind sensors that required a second pass in the opposite direction, which resulted in slow collection. But new high-density-array GPR solutions towed behind moving vehicles can collect up to 50 linear kilometers per day vs. roughly 500 meters per day with a walk-behind approach.
“With an array radar, you can drive back and forth, and map huge areas in a fraction of the time it would take to collect it conventionally.”
— Craig Simmonds
GPR captures as-built conditions that couldn’t be viewed without digging. Similar to LiDAR’s bouncing light, the GPR sensor bounces an electromagnetic energy signal that penetrates the subsurface. The intensity of the return is measured to discern the location and condition of pipes and cables, roadbed conditions (including the ability to classify aggregate material and soil) and any voids.
“The world is always changing, and what happens underground greatly impacts what happens above ground,” says Stuart Woods, vice president of geospatial solutions at Leica Geosystems. “You have water tables that go up and down, with California as a worst case right now, and the density of the soil changes, impacting utilities and changing the elevation of railroad tracks.”
Merged LiDAR point clouds and GPR data generated from the Leica Pegasus:Stream provide an immersive dataset that can be exported to CAD, BIM or GIS to visualize, inform design and record conditions.
“If we’re capturing images or LiDAR point clouds above ground, we know where a telephone pole is and that a cable runs down,” adds Woods. “We can now quickly align the cable or pipe we are seeing below ground via GPR and relate that to the utility we’re seeing above ground. It allows us to connect the dots a lot better.”
The evolution of GPR has followed a steady progression. Shallow and smaller-diameter pipes are detected with higher frequency, and larger pipes at great depth are discovered with lower frequency. Single-channel receivers required selecting high or low frequency, and the earliest versions involved placing an antenna on the ground—essentially snapping an image—and then moving it to the next spot. Dual-channel receivers pushed capacity with two different frequencies, and faster electronics allowed sensors to be pushed along at a walking pace. The latest Stream family of products from IDS features four antennas mounted side-by-side for more-detailed underground understanding and much faster electronics to be able to capture while moving at a driving pace. The 40-channel dense array solution with two different frequencies provides a higher level of productivity as well as better quality 3D results.
The Pegasus:Stream combines Leica Geosystems’ Pegasus:Two mobile mapping LiDAR sensor with the IDS Stream EM high-density-array ground-penetrating radar (GPR). The solution returns detailed 3D data about underground features via GPR as well as detailed point clouds from the LiDAR system. The new offering is optimal at a speed of 15 kilometers per hour for roadway applications and as high as 250 kilometers per hour for railway applications.
Hitting the Road
Roads in urban areas are one of the highest-demand applications for GPR technology, because details on the “where, what and when” of pipe and cable installation rarely are documented. Even when some plans are available, they’re rarely aggregated into a view that combines more than one company’s infrastructure assets, or found in a format that can be shared for planning, design or maintenance reasons.
“When you map utilities, you end up with what looks like a bowl of spaghetti,” says Craig Simmonds, managing director at Macleod Simmonds. “The ducts are literally wrapped around each other. You have water pipes and gas pipes threaded through there, and it’s quite incredible. You’ve had years and years of utilities putting in as many services as they can and not removing them. With the resolution we get from the Stream, it really does help us to unravel that ball of wool.”
Traditional GPR utility surveys pushed antennas across the roadway to detect what’s underground. Surveyors moved in parallel lines three feet apart and then repeating in the other direction, because the radar detects targets up to 45 degrees from its direction of travel. This creates an orthogonal grid of measurements that pick up positions and patterns of the networks and structures underground.
To use older systems, the road (or at least half) needed to close, but high-density-array GPR can travel along in live traffic and collect data. With the close array of antennas, large data samples help create high-accuracy models rather than just looking at cross sections. Back in the office, a profile and plan image can be assessed to put the pieces together more reliably and quickly for a detailed underground model.
“The technology is capable of identifying and locating pipes and cables, and can distinguish material (between metallic and nonmetallic pipes and cables),” says Alberto Bicci, GeoRadar Division director with IDS. “The radar determines the position of the pipe in horizontal and vertical dimensions, and you can infer the diameter of the pipe from the curvature you detect.”
The ideal time to collect roadway GPR is at night, when traffic is lighter. The GPR has no issue with darkness, but using RGB cameras to colorize the LiDAR point cloud requires light. Large camera sensors help combat this issue, as the sensor-to-pixel ratio allows more light and responds faster to light changes. High-illumination LED floodlights also can be used to ensure the best exposure.
“To make a 3D reconstruction of the subsoil, you need to sample the soil with a very fine resolution, sampling the subsurface every few centimeters,” adds Papeschi. “You need large arrays of antennas and a high velocity of sampling while you are moving, so the electronics that drive each of the antenna must be very fast.”
With IDS One Vision software, the GPR system is capable of providing a real-time view of the subsoil, which helps guide collection. Back in the office, post-processing software provides more detail, taking the tomography (time slices similar to CT-Scans of the body) and, with image analysis, distinguishing the composition of materials found in the soil. The software has automatic and semi-automatic features that help process data, but the operator is always in the loop, controlling some parameters and compiling final results.
Riding the Rails
GPR also is being deployed for railway maintenance to examine ballast (the material under railroad ties) for fouling, where ballast gets contaminated because soil mixes with the grains of the ballast or train loads have broken down the ballast. Ballast condition affects track geometry, which impacts safety, so regular maintenance is required, and ballast needs to occasionally be refreshed. The array of antennas for the GPR railway system mounts in front, behind or underneath a locomotive to identify the zone of the ballast that must be replaced.
“Within a couple of days of collection and a few weeks of processing, we can survey a big rail network,” says Giuseppe Staccone, managing director, Ground Control Geophysic + Consulting. “We can run the measurements very fast, offering high-accuracy results quickly.”
Geotechnical conditions of railway tracks previously were examined by drilling, which provides a single point of information. Drilling still is important, but GPR offers continual information.
“GPR monitoring along the rail line measures the thickness of the different layers present on the ground, and the status of the ballast and subballast,” notes Staccone. “We suggest drilling every kilometer, because we can improve the accuracy by calibrating against local conditions. We are able to achieve a good accuracy of the thickness of the ballast if we know the correct propagation velocity of the electromagnetic wave along the track line.”
Survey results are delivered to customers via software that allows them to view the geotechnical conditions of the track as well as pictures, video or point clouds taken along the way. They’re provided with a cross section of the ballast bed, a display of any drillings, and details of trackbed layers up to two or three meters below the top of the sleepers.
“The customer also can view humidity all along the track,” adds Staccone, “which is an important item if there is any settling along the track. Together with track-geometry information, this gives rail managers the opportunity to decide the optimum course in terms of track renewal, planning and budgeting.”
The fusion of ground-based sensors follows a similar trajectory to aerial sensors. A combination of sensors can be customized to the objective at hand and fine-tuned to deliver results more quickly via automation.
“We at Hexagon (the parent company of Leica Geosystems), and in this case working with IDS, see mobile mapping as creating a sensor platform, which allows you to collect a variety of data and then use that to understand the environment,” says Woods. “This example allows users to see below ground with the IDS system and above ground with our mobile mapping system, Pegasus:Two. We can also attach other external sensors like gas sensors, thermal imaging, or pollution or sound sensors. So as we’re mapping the world, we collect as much data as we can to then make better decisions about urban environments.”
This ability to quickly capture data matches a need to quickly assess conditions to reduce costs, improve safety and respond to changing conditions such as California’s drought or increasing weather impacts on coasts.
Although developers are starting to build things underground in a way that’s more discoverable, such as installing utility subways where they can be jointly maintained, this is an extremely costly exercise and won’t happen broadly any time soon.
“At the end of the day, the main questions are how can I improve my knowledge about the assets that I need to maintain, and how can I save money by optimizing the maintenance and renewal procedures,” says Staccone. “These are the most important things for our customers.”