Electromagnetic Near Field Scanning has formerly been proposed to guide side channel and fault injection attacks. However very few studies support its use for reverse-engineering. This absence could be explained by difficulties linked to the diffusion of currents in the power supply network, which are the root of EM radiations. This diffusion has for consequence that a local electrical activity in an IC can be observed quite far from its origin point, thus limiting the interest of EM near field scans for reverse engineering. This paper proposes a solution to this problem by describing a method to extract the source areas of an IC where electrical activity is occurring from EM near field scans. Experimental results are given for an ARM based microcontroller designed in a 90 nm process.