Preliminary results: a three coil balanced system for imaging steel bars behind steel boundaries

This paper describes preliminary results obtained from a modified transformer coupled system for detecting and imaging steel objects on the distal side of a ferrous steel boundary up to 2 mm thick, using a balanced three-coil system. Typically, it may be applied for the imaging of steel reinforcing bars embedded within concrete, which includes a sheet steel layer located between the bars and the upper surface of the concrete. A transmitter, receiver and a dummy coil are placed on the upper surface of the specimen and moved across it. The dummy coil is used to cancel the common mode signal (cross-talk) between the transmitter and receiver, thereby enhancing the signal emanating from the steel bars. By exciting the transmitter with signals ranging between 125 Hz and 1 kHz, it is possible to generate a magnetic field that is not severely attenuated by the sheet, enabling the detection of a ferrous target located below the sheet. In addition to signal cancellation, a real-time digital signal processor is employed to perform very narrowband signal identification, amplifying the signal-to-noise ratio considerably. Both good quality and defective steel bars with diameters of 16 mm have been successfully imaged below mild steel sheets up to 2 mm in thickness.