Ever since the explosion in development of Ground Probing Radar (GPR) systems in the 1990s, the technology has become an increasingly integral part of the utility surveyor’s toolkit.

By collecting measurements using a single or dual channel GPR system, then comparing detected targets to those found with other technologies, a reasonable picture of the subsurface can be created. To achieve the best possible result, the interval between collected GPR measurements is decreased; however, the net effect of this is to increase the time required and subsequently the cost involved to complete a survey. The resulting data is still subject to an ‘Interpretation’ which is still heavily reliant on the surveyor’s knowledge of the site, the technology and its capabilities.

The answer is to deploy multiple antennas simultaneously in the form of an array that allows enough data to be collected to form an actual image of the subsurface, rather than an interpretation of one. Several manufacturers realized this some years ago and began developing the GPR array technology based systems. Once the hardware reached a satisfactory level of evolution, the focus then turned toward the software as given the huge volumes of data involved, this then became the bottleneck. The drive toward perfecting this approach became ever more focused as the industry began to require more accuracy and reliability in the survey information that was being procured. At the same time, with the recession biting, it was essential that these higher levels of accuracy could be provided in a more cost effective manner.

Having spent many years focusing on these issues, Macleod Simmonds Ltd (MSL) of Portsmouth, United Kingdom, now offers a GPR consultancy that provides both a survey service and/or a software package that takes the output from multi-antenna surveys to a whole new dimension.

SURVEY CAPABILITY

With over a decade of experience in the GPR survey field, staff at MSL can provide survey capability for almost any type of terrain, application or location that end users might require.

For the more usual survey location, i.e along an existing roadway or smooth surface terrain, MSL has its Carriageway System. Based on a multi-antenna set up from Italian GPR manufacturer IDS, the unit has 40 individual GPR channels operating at 200 Mhz and 600 Mhz (the higher the frequency the higher the resolution but to a shallower depth), which will provide a grid resolution of as little as 6 cm x 6 cm. The data is collected by traversing the array across the survey site at about 5 mph with the GPR system towed by a suitable vehicle. Up to 10,000 m2 of coverage per site shift can be achieved using this system.

Similarly, but for more difficult terrain, MSL has its Rough Ground System. This unit can be pulled across the survey site using either a quad bike or 4×4 vehicle. Working at 200 Mhz the system offers resolution to 12 cm x 12 cm at a vehicle ‘acquisition’ speed of 10 mph. Up to 20,000 m2 can be surveyed in one site shift.
For narrower locations or where vehicle access is not possible, MSL also has its Footpath and “Fill in” System. This unit has two operating modes. First, the 200 Mhz system has eight channels and provides a 12-cm GPR grid resolution. Second, the 600 Mhz system has 24 channels and a 4-cm GPR grid resolution.
Using the system requires no grids to be marked on the ground and up to 2.5 km x 3 m coverage is possible per site shift. The Footpath and ‘Fill in’ System can be operated either manually using a push frame or for faster surveys by attaching it to a small vehicle or lifting unit that is small enough to maneuver along the footpath or narrow section being surveyed.

The advantages of this type of survey approach are many-fold including:

•    No traffic management required for carriageway works
•    No need for personnel to work in the carriageway so safety concerns are reduced significantly
•    Large areas covered quicker than ever before
•    Equivalent of conducting orthogonal grids of GPR measurements at the resolutions stated below while travelling in one direction:

o    600 Mhz – 4 cm x 4 cm
o    200 Mhz Carriageway – 6 cm x 6 cm
o    200 Mhz Rough Ground – 12 cm x 12 cm

•    Dense data grids allow the subsurface to be ‘Imaged’ which results in actual plan images of the underground environment being created
•    Positioned by robotic total station or GPS
•    Penetration of up to 2.5 m depending on ground conditions

These GPR systems not only detect buried services but also have to capacity to detect voids, anomalies, structures and construction features. They can also be utilized for archaeological, as well as construction surveys.

SOFTWARE PROCESSING AND OUTPUT

According to Craig Simmonds FCInstCES, MSL senior GPR consultant, what makes the service offered by MSL different from anyone else in the GPR market today is the unique hardware systems together with the software that has been developed in-house by the company to provide the various output options sought by modern engineers.

Simmonds said of the software packages on offer, “Our processing software does not just provide a plan picture of what lays beneath the surface. It offers a 3D image of the whole ground structure which can be quickly and efficiently manipulated, reviewed and interrogated   in a way that  has not been possible before now.”

Once data has been collected by the survey equipment, it is transferred into the software system. Summarizing what the software generates in simple terms, it generates a plan image of the survey site. This plan is multi-layered in that plans of the site at different depths can be seen from the surface to the maximum depth of penetration. Panning through these layers on screen highlights not only where utilities structures or other buried items lay within the substructure of the ground and along what route they run but also what depth range they cover. For installations such as large diameter pipes, this could be a significant factor depending on what construction operations etc., will follow the survey operation.

For more detailed analysis, at any point on the plan a single section line or multiple section lines can be requested by the analyst. It is also possible to request a series of section lines that run across the whole or part of the survey site at a preset interval. The software then automatically generates the depth cross section at these requested locations for display. The section images are dynamically linked to the plan image allowing the analyst to immediately identify which detected pipe in the section relates to which detected pipe in plan. This information enables the analyst/site engineer to establish a clear understanding of the hidden environment and the services, anomalies or structures that are present within it. Using this information, not only can an existing plan be updated to show these positions but also where there might be open corridors that could be used to install new build services or structures with minimal interference with existing services etc.

Both plan and section analysis also enables the engineer to view potential void sites beneath or around existing structures and services and where structures may have failed, without the need for full site excavation.

An example of this capacity to potentially find the unexpected occurred during one survey along a roadway where at the edge of the road a series of unexpected anomalies occurred in the data output. Further investigation on site showed that what had been found was in fact a series of old cellars that extended from the nearby building, out into the roadway, with some having had reinforcing or protection caps place over the cellar roof arches. Without this information it may have been possible for future construction work to have occurred along the road side with trench bottoms collapsing into these ‘unknown’ voids or drilling equipment unexpectedly boring into them.

A VIEW OF THE FUTURE

With digital imaging of surface structures becoming an increasing requirement among the construction fraternity, MSL is also looking at how a combination of both surface and subsurface structures and networks could be established and how the relationship between them might be imaged for the use of construction engineers of all fields not just the utility sector.

By marrying high resolution multi-camera filming techniques with multi-laser scanning capability, the intention is to create a view not just of the surface structures along a survey route but also to be able to highlight ‘terrain’ variations such as road humps or troughs or other anomalies. By completing a GPR multi-channel survey at the same time along the same route, data from both surveys can be combined to show how the surface and underground environments fit together and how they interact. Output can again be exported for display purposes to Cad systems.

While this latter technique, known as ‘Mobile Mapping’ or ‘Surface Based Lidar’ which takes its positional data via Inertia GPS, is still in development, it just goes to show the future potential that is becoming a reality with the use of multi-channel array GPR survey systems.

Commenting on this potential, Simmonds said: “It has taken many years to get to the point where our equipment and, in particular, our software has the capacity to achieve the levels of output now offered. Yet, we still view this as only part of the future potential for our systems. We are actively working toward a full BIM (Building Information Modeling) integration as part of our standard output options and continue to look at new output criteria according to our customers’ needs. Our main focus is to provide data that will make understanding of what lies beneath our feet and the relationship between the visible and hidden environments much easier to fully understand. To this end we will continue to work with the industry and provide minimal impact, time efficient and cost effective solutions to construction industry engineers and planners. With our services, we will assist in minimizing the disruption to everyday life, business and traffic flows that have become a serious concern to engineers and the population at large.”

This article was submitted by MacLeod Simmonds Ltd., headquartered in Portsmouth, United Kingdom.