NEAR-SURFACE DISTORTION EFFECTS ON CONTROLLED SOURCE MAGNETOTELLURIC TRANSFER-FUNCTIONS

In this paper, we have studied the distortion effect. of 3-D near-surface inhomogeneities within the frame of recently developed concepts of controlled source tensor magnetotellurics (Qian & Pedersen 1991; Li & Pedersen 1991), in which a pair of horizontal dipole (electric or magnetic) sources are used to determine impedance tensor and tipper functions, that are independent of source orientations. The tensor description of the distortion of surface electromagnetic fields generated by horizontal electric and magnetic dipole follows. Wannamaker, Hohmann & Ward (1984) and Zhang, Roberts & Pedersen (1987). Reciprocity relations have been derived to relate electromagnetic field distortion matrices when the source and receiver are interchanged. These relations can be used to relate the distortion effect when either the source or the receiver is close to near-surface inhomogeneities. Based on thin sheet modelling we show, contrary to intuitive expectations, that the measurement of magnetic field for electric or magnetic dipole sources will not in general provide an undistorted estimate of the average conductivity below the surface. The characteristics of distortion effects for electric and magnetic dipole sources are discussed in detail both from analytical and numerical modelling points of view.