The Bartington Grad 601-2 is a dual sensor fluxgate gradiometer, and can detect variation of around 0.1 nanoTesla (nT). It is generally used in a gridded way, by walking up and down in grids of 20 or 30m square, although larger or smaller grids are possible. The Bartington instrument was introduced in the early 2000;s and has undergone a number of enhancements between now and then (click the link below to go directly to the Bartington website for more information).