A complete instrumentation system is described that is capable of launching alternating magnetic fields through a large mild steel plate that is 2 mm in thickness and detecting them on the face opposite from the transmitter with remarkable signal-to-noise ratios. Results for signal frequencies ranging between 4.5 and 13 kHz are reported. The skin depth at 9 kHz for the steel used is approximately 137 mum. The detection of the minute fields arriving at the receiving coil is made possible by the use of digitally synthesized input signals, low-noise amplification, and, in particular, the use of a powerful real-time digital signal processing system that isolates the signal of interest using a super narrow-band filter and very high levels of distortion-free gain. Although traditional methods of weak signal detection, such as lock-in amplification, may also be applied in this context, the digital approach discussed here is both more cost effective and flexible, allowing the simultaneous detection of multiple frequencies