Since you specified iron, the recommended methods would either be magnetometry or electromagnetic induction (EMI). With magnetometry you can use either a single sensor or a gradiometer arrangement. Since magnetometry is a passive method, the signal falls off as 1/r^3 while EMI methods are active and the signal falls off as 1/r^6 with r being the distance between the target and sensor. There is not depth limit to magnetometry as it is purely a function of distance and the mass of the source. Since magnetometry is passive and the earth's magnetic field is the source, two sensors are normally used. You can have a base and a rover or two sensors set up as a gradiometer. The reason for two sensors is that the field is changing the same at both sensors. The spacing between sensors in a gradiometer arrangement can be changed to optimize the depth of investigation. For archaeological surveys the spacing is usually either 1.0 or 0.5 meters between sensors as most of the targets are in the top 2.0 meters.
Since there are multiple types of sensors that will meet your needs, the real question is what is your budget and how large of an area do you need to survey? Can you access the survey area of interest with a wheeled sensor or will it need to be man carried? Some possible sensors are the Geonics EM31 or EM61-MK2 as EMI sensors, or the Geometrics G-858 or G-856 as magnetometers. There are numerous other manufacturers as well.
