Locating Objects Below Reinforced Concrete

                Here at Delta Geophysics we feel it is an important part of our business to inform our clients about proper geophysical survey techniques and practices so that you can maximize survey results. The following is a brief discussion explaining how a geophysical survey should be conducted to identify anomalies beneath metal-reinforced concrete. 

                Reinforced concrete routinely encountered in the field is a major difficulty for geophysical instruments. It is usually found covering tank nests, beneath pump islands, at entrance ways, sidewalks, and inside buildings. In older gas stations upgrades often included abandoning fuel tanks beneath the pump dispenser slabs or nearer the road beneath the entrance aprons and sidewalks. Many abandoned UST’s have been missed at gas stations – locating these tanks can be difficult but not impossible if you use the right techniques and equipment.  Typically the signal from Ground Penetrating Radar is shielded by the metal reinforcement in the concrete and provides little, if any, real information beneath the reinforcement. The basic metal detector only detects the metal reinforcement in the concrete; it cannot discriminate the signal produced by a reinforced slab or a UST beneath a reinforced slab. An EM-61 survey is the answer.

                 The Geonics EM-61 is a time-domain metal detector which detects both ferrous and nonferrous metals. A powerful transmitter generates a pulsed primary electromagnetic field in the earth, which induce eddy currents in nearby metallic objects. The eddy currents decay producing a secondary electromagnetic field measured by the receiver coil. Because of the strength of the signal and the broad-band recording capabilities of the data logger, much information is contained in the EM-61 signal response. The proper recording and processing of this signal information is key to discerning anomalies beneath reinforced concrete.

                Specialized field procedures and adaptive filtering techniques used by Delta reduce the near-surface influence of the concrete surface slab allowing the detection of targets underneath. Delta’s field geophysicist further processes the signal using interactive target editing and grouping tools for final target list refinement. Processed and contoured data can then be displayed on site maps for visualization and field referencing.  Delta maintains state-of-the-art mapping capabilities in various software platforms to provide seamless integration with your existing engineering drawings, or to provide turnkey site mapping solutions plans for you. We have attached an example of an EM-61 survey at a gas station with the site features mapped by Delta and displayed in graphic format.

                Like all geophysical methods an EM-61 survey has limitations and may not provide conclusive evidence for anomalies below reinforced concrete.  But this instrument is an effective way of locating a metallic mass beneath reinforced concrete.

               Thank you for taking the time to read this brief description and if there are any questions or advice that you may have about geophysical surveys please feel free to contact us.