On small-scale near-surface distortion in controlled source tensor electromagnetics1

Abstract

Based on a Born approximation of a thin sheet integral equation, it is shown that small-scale surficial heterogeneity significantly distorts the electromagnetic field excited by electric dipoles only when either the source or the receiver are located on the heterogeneities. When a surface heterogeneity is beneath the source, the associated distortion of the electromagnetic field is manifest as a change in the effective electric dipole moment. Hence the magnetotelluric transfer functions and impedance relations remain undistorted in this case. When a surface heterogeneity is beneath the receiver, the electric field is severely distorted, but the magnetic field is only slightly distorted. The impedance tensor is therefore strongly distorted, but the tipper vector is almost unaltered. Since the controlled source tipper is a function of 1D earth conductivity, it is proposed that tipper data should be used in the first stage of 1D interpretation. For a 1D earth, the tipper vector must always point towards the source and, in the near-field limit, should have unit length. These two necessary conditions must be met by the measured tipper before it is interpreted one dimensionally.