Possibility for an enhanced 3D imaging sensitivity in electromagnetic methods: keyhole imaging

We present a phenomenon which has not been reported until now It has been f ound using two independent methods (controlled-source audiomagnetotelluric (CSAMT) analogue modelling experiments and magnetotelluric (MT) numerical c omputations) that, in the transition zone between the so-called 'overshooti ng' and the 'normaI' period ranges, the anomalies over thin sheet-like 3D m odels having a complicated geometry are closely connected to the model patt ern. This phenomenon and the corresponding enhanced imaging sensitivity hav e been detected using several different interpretation parameters, in a ver y narrow period window towards the end of the overshooting period range of the corresponding sounding curve. This so-called keyhole period range can b e determined using simple two-layer MT and CSAMT relationships. Since the e ffect is small but measurable, the keyhole images could be used as an addit ional tool in practical problems where the main objective is the detailed g eometry of buried targets.