Ground Penetrating Radar

HAVE you ever had the unfortunate experience of being told by your senior surveyor that today’s job is to get togged up and be prepared to lower yourself down into a sewer, in order to provide accurate survey data for the client? Wastewater and stormwater manhole surveys are where boys become men and no matter what excuse you make to the boss, you simply have to put on your nose clip, wear the full waterproofs (hopefully with no leaks), check your gas detector works and demonstrate to the council inspector that your confined space training certificate is current, clip your harness to the tripod winch-line and lower yourself down into the darkest hole imaginable. You’re also comforted that your tetanus inoculation jabs are up-to-date and you have your Weil’s disease card in your top pocket, just in case you get bitten by a furry friend and have to get checked out at the local infirmary. Never mind the beautiful aromas, the scurrying of various rodents, floating objects (some still warm) and the sound of rushing water which washes your tripod legs away when you think you’ve just found decent anchor points for the spikes. That expensive new total station you purchased last week has sunk without a trace beneath the rippling brown waves, never to be found again.

Not to worry, you still have your Disto, your waterproof notebook, together with China graph pencil and state-of-the-art micro digital camera that will capture all the data you need in order to provide the client with its ‘picture’ of what lies beneath the busiest road in Dublin.

The client requires a detailed record of the existing state of the manhole and intersecting pipes which are flowing into the supersize manhole, 3m deep, from all directions and at all levels. Your sketch and approximate directions of the pipes is to be accompanied by several high resolution digital images from the new and expensive camera you’ve just purchased at the same time as the departed total station. You tug on the cable and shout to your assistant above on the surface to haul you up, because you think you’ve got enough information and you just can’t handle the rodents gnawing on your wellie boots before the slurry enters through the developing toe hole. Unfortunately, it’s as you almost reach the surface, see the sun streaming through the small access manhole cover above, when the winch jars and the new camera jolts from your pocket and splashes into the murky torrents below, lost forever...

Now all of the above may sound like the survey from hell, but this is the way Coastway used to carry out these surveys and many survey companies still do. The number of complaints were rising rapidly and surveyors were handing in their notice rather than be subjected to sub-surface surveys of the nature just described. We simply had to come up with an alternative that involved little or no access by a surveyor with survey equipment into the manhole chambers. The answer was literally staring us in the face. We could use our laser scanner (a Leica HDS Scanstation 2) to survey roads, buildings and surface manholes during the day to provide the surface topographical survey, all connected to Ordnance Survey Ireland’s National Grid with a Leica 1200 SmartNet GPS.


That expensive new total station you purchased last week has sunk without trace beneath the rippling brown waves, never to be found again.

3D pipes and point cloud data software image.
We could then use the laser scanner for the survey of the manholes with the lid removed and the “Low” tripod setting and scan down into the hole from above. Our partner geophysicists Minerex, was intending to carry out ground penetrating radar(GPR) surveys of the same area, a busy road junction in Dun Laoighaire Dublin, which could only be surveyed by GPR in the middle of the night(read more later). That survey would complete part of the ‘hidden detail’ below the ground by locating buried utilities. However, the problem still arose with the large manhole chambers and surveyed detail required. Jeff Hott, the survey and laser scanning manager, pondered on the use of a laser scanner for the job, but the Scan station 2 was just not suitable, we needed the scanner to be physically inside the manhole. Then in a ‘Eureka’ moment Jeff shouted out, why not use the Leica HDS 6000, turn it upside down, drop it down the manhole, turn on the scanner and ‘hey presto’ you’ve got a 3D model point cloud of the whole manhole chamber and associated interconnecting pipework. Not only that but, you’d get a laser scan survey of at least 25m down each pipe, giving you the direction of each, the quality and condition, all at the same time.

Jeff was laughed out of the room not least by his director, who wasn’t willing to drop a €80,000 scanner down a black hole. There might be a chance it could produce what was required, but it might also be lost in the murky waters, only to be found on a distant shoreline some months hence. Think again his director said, but no, Jeff had a vision and he knew he had seen something at a laser scanning conference which caught his eye previously and just might work and provide the answer to the steady but continual departure of surveyors who refuse to enter a confined space manhole ever again – “not for all the tea in China” was muttered.

After scouring through the information brought back from a four-day meeting, the answer to all the underground scanning survey problems presented itself in two pictures. The inverted tripod! This tripod specially designed for the HDS 6000 would solve all our problems.

The tripod was manufactured and Coastway set about the initial test. After connecting the HDS 6000 to the tripod the first scan was carried out without problems. The second scan was carried out in a live utility chamber.

Moments later the scanner was in place, the computer at the ready and the scanner lowered into the open manhole which was 3.5 meters deep. Junior surveyor Chris Byrne fired up the scanner and computer, the utility chamber was scanned at multiple levels and everything worked perfectly!
In the middle of the night, using the HDS 6000 scanner and tripod system, we scanned three separate wastewater manhole chambers below ground, in a three-hour time span in Dun Laoghaire town centre. At the same time the Scan station 2 scanned the entire street scenes.

Ground Penetrating radar

GPR allows detailed images of the sub-surface to be seen in real time. A geophysical operator pushes or pulls the GPR often mounted in a buggy. The baby in this buggy allows the squiggles and wiggles to be interpreted into pipes and cables and other sub¬surface features, which can then be physically traced across the ground. This can all be done without the need for excavation or dipping your head or equipment into dark holes. The position of the identified services can either be marked on the ground to be surveyed in later, and their position joined together — like a giant game of dot-to-dot — or the data can be processed depending on the acquisition parameters back in the office to provide a 3D/2D cross-section view of the services crossing the site.

Combining laser scan and GPR Surveys

With the scan data from both scanners back in the office, our modelling team was able to create a 3D sub¬surface pipe network. We achieved fantastic results, as the images in this article demonstrate.
GPR data of the same project was provided by Minerex to be combined with the laser scan surveys. Wastewater pipes were identified and extruded into 3D objects, all maintaining the same spatial geometry as the surveyed scan data. The final combination of the three surveys provided the full 3D picture we required and had been striving to achieve. Existing and new clients are now able to work with an accurate and highly detailed 3D model of existing water/wastewater pipework within the hidden world of sub-surface utilities. New pipework can be brought into existing manhole chambers, aware of the spatial positioning of existing pipework. Reconditioning of manhole chambers and connecting pipes can be analysed, programmed and marked up for upgrade works.

All of above highlights the advances we have developed with the inverted laser scanner for use in this highly inhospitable territory, to provide a far superior end product to that provided previously. Health and safety issues are vastly improved by not having to send our surveyors underground and requests to leave the company have ceased...

For more information on Ground Penetrating Radar contact Coastway