Lord Consulting
Power Industry Asset Management Consultants
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Power Industry Asset Management Consultants
Home » Technical Papers » Ground Penetrating Radar
Figure 1: A Noggin 500 Smart Cart during its recent appraisal at Power and Water Corporation Darwin prior to purchase for both power and water utility detection. Pictured from left: PAWA's Chris Pemberton, Christina Foo, and Paul Denniss study a displayed trace
The past 4 years witnessed the successful 'handover' of GPR technology to the utility industry from the traditional realm of the geophysical and geological scientist
It is barely 30 years ago since the first ground penetrating radar (GPR) units with footprints the size of a small modern car were dragged by powerful snow tractors across the frozen Canadian ice to determine the ice cover and moraine profiles below.
The past 4 years has witnessed the successful 'handover' of GPR technology to the utility industry from the traditional realm of the geophysical and geological scientist. Whilst there is never a substitute for operator experience, the equipment is now at a stage where 1-2 days of training can have a competent utilities technician on the road with GPR performing useful work.
Ground penetrating radars are indeed a true radar unit in the same sense that we all know for aviation applications and the like, the main difference being that a GPR unit is designed and specially shielded to couple directly into the ground virtually all the radar energy emitted. Once in the ground, the cell phone-strength radar waves travel at about 1/3 the speed they would in the air but the exact speed will vary in large part to the water content of the ground, water slowing the speed of travel to about 1/10th of the speed in air. As the radar waves encounter an interface where there is either a strong reflector (such as metal or concrete), or where the speed of propagation changes significantly (e.g: encountering a water-filled PVC pipe), a portion of the transmitted signal is reflected back to a receiver antenna in the GPR unit. The GPR measures elapsed time very precisely and, after performing a simple calibration for propagation velocity, the operator views a real-time display of all metallic and non-metallic subsurface ground stratigraphy, and buried utilities, down to a depth where the reflected signal is too weak to read.
Completing the picture, a typical modern GPR utility package with all the above features and a clear pedigree back to the origins outlined is the Noggin Smart Cart from Sensors and Software Canada, illustrated in Figure 1. Such sets feature a weather-proof backlit real-time display which also can store recorded data for later processing. An integral odometer is fitted, giving distance information and triggering the radar to take multiple 'snapshots' as the Cart is pushed. An on-screen cursor worked from the odometer permits one to stop and back up at any time to mark out a target that has been crossed; pushing the cart past the reversal point continues the data acquisition. A generous battery supply allows a nominal 8 hours of operation. For ease of transportation between sites the Carts are readily folded up or dismantled completely, a carbon-fibre tubular frame providing strength, rigidity, and light weight. The radar unit itself is suspended from the cart in a sealed package.
Typically, a 250 MHz radar unit is employed and will penetrate nominally 1.5 to 2.5 m, resolving target diameters of around 5-10% of the depth. Smaller and identically-constructed units of either 500 or 1000 MHz are also offered and are able to be interchanged on the Cart in minutes: these will penetrate to about 60% and 30%, respectively, of the distance of the 250 MHz unit but each will see twice the resolution of the other, by virtue of the doubled operating frequency in each case.
Hand in hand with the recent packaging of the GPR hardware for utility work, there has been very significant and on-going advances in post-processing software to provide enhanced and extremely useful visualisation tools such as 2 and 3D mapping of areas, GPS-tagging of data, and topographic correction.
What can be done with a modern GPR package? A very brief list would include: the location and verification of the depth, position, and orientation of buried metallic or non-metallic utilities; the determination of the orientation and shape of buried targets on archaeological sites; the determination of the ground stratigraphy (or 'layering') so as to plan excavation or horizontal thrust boring so as to miss hard ground or unknown targets); the imaging in almost life-like clarity of reinforcing bars in concrete structures; the GPS-tagged determination of the depth of snow cover in ski-fields; location of buried storage tanks; location of erosion behind tunnel walls; the location of graves; determining the nature of road beds or asphalt covering on bridge decks; and, as the radar travels very well through pure water, the depth and sedimentation in water supply reservoirs or the like.
We must first explain the appearance of a trace of 'raw' data, illustrated in Figure 2. As the GPR unit traverses a buried utility, the physics of the reflection process trace an inverted hyperbola whose apex fixes the depth and position of the utility. In the illustration, the horizontal footpath stratigraphy is evident, as is the single dipping lines denoting each side of a trench. At the bottom of the trench lies a service (sewer) and a smaller water pipe above it to the right. These raw data traces are generally clearly interpreted by a practiced eye and generally sharply represented on the display.
Figure 2: 'text book' raw data GPR trace of a trench containing a sewer and water service to a school.
Certainly one of the most enticing enhancements in utility GPR work in the past 18 months has been the introduction of simply-used 3D visualisation software and GPR carts capable of acquiring data in suitable formats to accommodate it. Employing either an XY or a single axis survey grid at typically 0.5 m line spacing, a very accurate and clear representation of the depth and orientation of a utility profile under a road, for example, can be obtained.
Figures 3-6 show selected plan view highlights of an X-Y grid survey of a two-lane road at 0.5m line spacing. With this package, one may nominate the depth slice thickness when processing, this example being 120 mm. In Figure 3 we see a Telecom service laid into a bollard at 0.37-0.50 m, the reflected signal strength being shown by colourisation (blue weak, red strongest).
Figures 3-6: An X-Y survey of buried utilities at selected depths in the same section of road
Continuing to look deeper into the same section of road, we next strike (Figure 4) a 225 mm diameter RCRR storm water link pipe running diagonally between two sumps on either side of the road. At 0.75-0.85 m (Figure 5) is a pair of 100 mm ducts with a 70 mm diameter NS power cable, run diagonally from a substation to a service pillar on the opposite side of the road.
Figure 6 shows a sewer in the middle of the road, amongst other items.
Figures 7 and 8 show a fascinating survey done to determine the remaining extent of an historic 1884 horse-drawn tram system that was the first constructed in Christchurch. Some 50 m of the remaining section was surveyed and from it was determined that the track partially survives under the road and that one rail is more intact than the other. By processing the data, an assessment of the condition of the wooden sleepers was also possible, large holes in them clearly showing advanced decay except in the final 20% or so of the route.
In a related project, a fully-intact tram passing loop buried for 50 years under a main arterial road was mapped quickly with 1 m line spacing. A portion of this text-book trace is illustrated in Figure 9. Laid in concrete, no sleepers are expected in this trace. The red portions of higher intensity at the points are the result of the stronger reflections from the sizeable spring mechanism in the points.
Moving into building archaeology, several impressive projects were undertaken. Perhaps the most fascinating of these was the determination of the remaining extent of the foundations of the former chemistry department building at the former University of Canterbury site. Known as the Tin Shed (Figure 10), this huge temporary building was where the famous Lord Rutherford did his early chemistry studies before proceeding to England and his experiments that succeeded in splitting the atom. Demolished in 1916, the unique symmetrically-angled foundation walls gave the impression of two parallel lines running horizontally in the final map (Figure 11). The survey showed foundations remaining for much of the side of the building visible in the photo above, including part of the wall through the middle that supported the central chimneys (shown as a short “T” shape in mid picture). Other traces showed that some drains may also survive.
Locating graves is always a fascinating use for the equipment and the survey of Figures 12 and 13 shows the surveying and location of graves in a now unmarked portion of an early Christchurch cemetery. It was possible to locate children’s graves and multiple graves at differing depths in the one family plot, all through 5 quick lines at 0.5 m spacing.
Finally, buried fuel tanks are often a location problem and a trial survey was done on a truck stop to image the tanks there. Working on a simple single axis grid at 0.5 m line spacing, the position and extent of three tanks were still clearly shown (Figures 14 and 15). The tank in the foreground was partitioned into three and the filling covers clearly seen.
Figure 9: Part of a buried tram passing bay
Figure 12:Multiple graves at differing depths.
In summary, GPR offers the utility and regional authority customer a powerful new tool to conduct with ease and clarity much of the tasks that were previously either time-consuming or virtually impossible. It remains still one of many tools but offers potential for large savings in route planning, location of lost services, and in avoidance of damage to unknown or unmarked services in the course of other work. It also offers no shortage of challenge and latent satisfaction to the inquiring operator!!