An apparatus for measuring a pipe within a well structure comprises a tubular body extending between first and second ends and having an inner surface defining a passage therethrough and an outer surface formed of a non-magnetic alloy metal. The apparatus further comprises a pair of flanges each connected to one of the first or second ends of the tubular body having a passage therethrough corresponding to the passage of the tubular body, each of the pair of flanges being connectable inline with the well structure. The apparatus further comprises at least one sensor stack being locatable around an outside of the tubular body comprising a sensor adapted to measure magnetic field strength and at least one magnetic stack being locatable around the outside of the tubular body comprising at least one magnet.