Accurate detection of underground infrastructure and buried anomalies (e.g., utilities, substructures and sinkholes) continues to be a significant issue for design and construction projects worldwide. Conflicts with unmapped or abandoned utilities/substructures can be deadly to field crews and the public. Even when not catastrophic, these incidents often result in significant delays in project schedule and added costs. KCI recently had the opportunity to pilot and is now utilizing a new innovative geophysical technique, AM Gradiometer (AMG) to detect and delineate underground anomalies.
Originally developed for the military to detect roadside bombs and more recently selected as an American Society of Civil Engineering innovation award winner, this patented non-destructive geophysical tool uses AM-Band transmissions from radio station towers to detect underground objects and anomalies. Tuned to a frequency in the area where it is being used, the device’s antennas detect signals bouncing off of these subsurface features. Because the AMG does not transmit any signal of its own and only receives them, users don’t have to worry about interfering with other communication systems. The device consists of multiple receiver antennas, which can be spaced accordingly along the metal frame to operate at specific depth ranges. Configuring the antennas in various positions allows detection of targets up to 40-50 feet deep.
An AMG survey, or transect scan, is performed in a grid pattern across and beyond an area of interest. Using the tool, a two-person crew can typically scan one acre in a day. Depending on the local terrain the device can be pushed on a cart or carried by hand. If an anomaly is present, a signal gradient will be recorded between two antennas. The tool will produce a peak/point that is displayed on the device’s screen and also stored in the system’s data collector. The AM gradiometer also includes a GPS and surveying total station allowing for mapping of detection points in a coordinated data file. AMG scanning can detect electrical conductors (any type), metal pipes (regardless of contents), non-metallic conduits (fluid-filled), liquid or air-filled void space (including subsidence/sinkholes), natural gas or fuel lines, brick or concrete sewer lines, shielded fiber optics cable, and hydrocarbon or saline moisture plumes.
Once all field scans are complete, data are fully processed and interpreted in the office. The GPS latitude and longitude location of detections, as well as the depth estimate range, can be easily converted into CAD for visualization and use in developing underground 3D models. Information is then compared with traditional SUE mark outs or as-built plans. AMG data can be presented in several forms including, a spreadsheet, a Google Earth map, and a CADD file. Confidence level outputs can then be used to determine where targeted test pits are needed to confirm the presence of underground assets.
Instead of having to dig a large number of test holes, AMG can help pinpoint the exact location where one is needed. This allows us to perform spot checks, ultimately reducing associated costs, time spent in the field, downtime, required traffic control and impacts to the community.
Charbel Khoury, PhD, PEPractice Leader
During the pilot phase, our team used the equipment at several project sites, allowing crews to explore the capabilities of the tool and evaluate how the technology compared to other geophysical techniques including electromagnetic and ground penetrating radar (GPR) systems. KCI engineers were able to provide input and recommendations to improve the tool and make it more efficient.
One pilot project included the use of subsurface utility engineering, AMG technology and LiDAR to create 2-D drawings and 3-D Building Information Models (BIM) for a gas regulator station. The project focused on locating and verifying the entire facilities footprint, including the pressure sensing or control lines. Unlike GPR that provided inconclusive data, the AMG survey was successful in detecting known and unknown underground utilities within the project site. Our team validated the findings by performing test holes at various locations. Surveys of each encountered utility location compared extremely well versus the AMG detection lines with a horizontal variance of less than a foot.
Although various methods of subsurface investigation have their own benefits and limitations, AMG offers some significant advantages over other geophysical techniques such as GPR, including:
Another benefit of the AM Gradiometer is its ability to identify unknown or abandoned assets in addition to known assets. With traditional SUE methods, a known utility must be present in order to locate it, whereas this technology allows workers to find something that is not detected by traditional SUE mark-out or shown on existing plans. This information is critical for design and construction planning and can help avoid costly and time-consuming delays.
AMG is an advanced technique used by the KCI team to address the complexity of detecting underground utilities/substructures in challenging geological soil and site conditions.