Text of CCase study template - Slab Scanning Study - UST Locating(1).pdf · PDF fileCase Study:...
Case Study: Locating USTs (Underground Storage Tanks) Using Ground Penetrating Radar. This application note will show an example of locating large anomalies buried within the subsurface utilizing Ground Penetrating Radar (GPR) systems. Successful detection of known or unknown USTs can prevent severe environmental damage to the soil and its surroundings. In many cases old USTs have simply been left in the ground whilst a new surface is laid over the top. Whilst this may be deemed a structurally sound practice, the threat of any contaminants eventually leaking into the ground is a very serious and costly undertaking to resolve. Due to our legal right of obligation, some content from this survey eg: Client / Company name and the location this survey took place, cannot be disclosed.
Scope: To locate three possible USTs within locations specified by the client, one of these locations a UST has been confirmed by visual inspection however the full scale of the UST was still required to be identified. For the remaining two locations it was only speculated there may be USTs present and therefore a complete survey of each of the locations was required. Size of each location to be surveyed was approximately 60 Sq. meters. Results of any anomalies identified were to be presented to the client in a written report with the following relevant information:
Approximate depth of anomaly.
Approximate length & width of anomaly.
Location of anomaly measured from fixed reference points. All locations surveyed where an anomaly could be identified were marked out with reflective road cones and photographed for reporting purposes, images from the Ground Penetrating Radar were also included in the report for further reference of what was being detected.
Locations to be surveyed for USTs highlighted in green
Location number: 1 UST indicative only.
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The survey was conducted using a cart mounted Ground Penetrating Radar system attached with a 500 MHz antenna and a 3D modeling system built into the software. The 500 MHz antenna is capable of scanning down to depths of 4.8 meters in good ground conditions, the USTs in this particular project were deemed to be at a depth of no more than 1.5 meters (if there at all) which made the antenna chosen for this project more than adequate. Each location was then measured up in a grid formation in increments of 500mm spacings, this being the approximate width of the GPR system. Once this step was completed the scanning operation could then commence. Making use of the 3D imaging software available, a multiline (area) survey was setup in the system in accordance with the 500mm increments marked on the concrete surface, a specific distance was also marked on the concrete surface to ensure the data collected by the GPR system would be accurate. Note:
The surface conditions covering each location consisted of combination of asphalt over compacted soil and 150mm thick concrete slabs with 200mm center steel reinforcing mesh within.
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Data Collection: Once multiple line scans were completed in one direction at one of the locations over the grid formation the survey could then be finalized for that direction and the CMD technician could analyze the data collected. This process was then repeated in the perpendicular direction. This is not a process that is required to be reviewed offsite or in an office, the 3D imaging from the GPR used in this application can be reviewed in only a matter of seconds once the survey has been finalized, as well as the fact the technician can view the data being collected in real time on the LCD monitor attached to the GPR allows for immediate interpretation of all subsurface elements detected. Where a definitive parabolic arc could be identified in a consistent line throughout the area surveyed indicated there was in fact an anomaly of some description within the location. Using the filtering techniques within the GPRs software allowed for the CMD technician to filter out all the surrounding noise that was captured and focus directly on the anomaly. As the anomaly did not continue in length any great distance it would be fair to presume that this was in fact a UST. If this had continued in length in both directions it would be considered to be a possible storm water drain and would be mapped accordingly. To determine a more accurate length of the anomaly the center of the parabolic arc (UST) was marked on the concrete surface, from this point a scan could be conducted running directly along the length of the UST enabling the GPR to detect the point in which it begins and ends. Once the final analysis of the data has been carried out the CMD technician then marked the very outer perimeters of the anomaly identified on the concretes surface, photographed the location and then took measurements of the anomaly and measurements from fixed reference points so a plan could be produced and presented in the final report.
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Results: Location No: 1
As the location where the UST was deemed to be consisted of a man hole cover and a change in concrete structure surrounding the immediate area, only at one point could the presence of the UST be confirmed. This is represented by the shaded area at the base of the photo.
The image taken directly off the GPR system identifying the large anomaly within this location. The change in the soil structure also displays the edges of where the initial trench was excavated.
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Location No: 2
Upon scanning commencing in this location in an east to west direction a large anomaly was identified immediately. The image presented on the GPR system was very clear and required very little filtering. Once this anomaly had been marked on the slab surface further scanning was conducted around the immediate location to gauge the dimensions of the anomaly and to ascertain whether this could in fact be a buried UST. Once scanning was completed in the east to west direction, the center point of the anomaly was marked on the slab surface and then scanned over in the north to south direction to establish the length. The image displayed on the GPR confirmed the length of the anomaly and eliminated the possibility of it being a large pipe or other service. The dimensions of the anomaly were measured at approximately 2400mm in length, 1350mm in width and at an approximate depth of 850mm. The location of the anomaly in relation to a fixed point was measured and plotted on a CAD plan for the clients reference (See Following Page). The extent of the anomaly that was identified as indicated by the marker cones.
Image taken from the GPR displaying the length of the anomaly taken directly over its center point. It is visibly clear where the signal drops off at each end which confirms this is a standalone object and eliminates the possibility that it is a service pipe or other anomaly.
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Location No: 3
After multiple pass scanning being undertaken over the area of concern, no signs of a UST could be identified below the surface. There was also no indication of any previous trenches being excavated within this area which may have indicated that the UST had in fact been removed at some point in time.
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Summary: After multiple scanning in a grid formation over all three locations it could be verified that locations numbered one and two clearly displayed signs of buried objects. Neither of the anomalies identified at these two locations displayed any evidence that they could be possible services as each object identified had a start and finish point. The data collected in this survey allowed for immediate action to be taken in the removal of these two USTs. As the building itself was undergoing extensive renovations which were to completed within two months from the date of this survey, having precise locations and depths of the two USTs allowed for quick and minimal site disturbance whilst they were removed as well as eliminating the possibility of any soil contamination or unstable ground if they were left unattended to.