FEM Source Effect Investigation with 2.5D Numerical Modelling

Land-based controlled-source frequency domain electromagnetic methods (FEM) using horizontal electrical dipole (HED) source create a lot of interpretational problems associated with the finite transmitter-receiver distance and with conductivity inhomogeneities in the vicinity of the transmitter. For elongated conductivity structure the developed 2.5D finite difference forward modelling code makes it possible to investigate non-planewave-, source overprint- and shadow effects. These source effects are investigated both in broadside (TE mode) and in collinear configuration (TM mode). Because the investigations covered the transition zone to present these effects impedance amplitude and impedance phase sections were applied. For the investigated models the shadow effect develops similarly in the two modes and greater source overprint and non-planewave effect were observed in TM mode than in TE mode. These source effects manifest themselves characteristically in case of simple 2D structures and numerical modelling may help the interpretation of the multi-receiver FEM measurements using HED sources in the two modes.