A sensor system for a sub-sea oil and gas well includes a casing, a transmit coil, a receive coil, and a processor. The casing defines an interior space through which a drilling pipe string transits. The transmit coil is coupled to the casing and is configured to conduct a current pulse and induce an electromagnetic field within the interior space. The electromagnetic field corresponds with the current pulse and interacts with the drilling pipe string. The receive coil is coupled to the casing and is configured to detect the electromagnetic field, including perturbations of the electromagnetic field due to the drilling pipe string's interaction therewith. The processor is coupled to the transmit coil and the receive coil. The processor is configured to compute a diameter of the drilling pipe string based on the current pulse and the electromagnetic field detected by the receive coil.