Far-zone fields of electric and magnetic dipoles in a stratified laterally isotropic earth and mitigating the airwave in marine CSEM

Asymptotic expressions are derived for electromagnetic fields induced by arbitrarily oriented electric and magnetic dipoles in a laterally isotropic stratified medium. Considering that the leading terms, which describe the airwave, decay in accordance with the same geometric law whether the source is a horizontal electric or magnetic dipole, it is suggested to use a transmitter, which combines these sources. It is shown that the airwave greatly diminishes if the amplitudes and phases of the horizontal current and magnetic moments of this transmitter are chosen to minimize the far-zone vertical magnetic field. A properly tuned combined transmitter will induce the electromagnetic field with a relatively small transverse electric mode. Raw electromagnetic data acquired with the combined transmitter will be more sensitive to parameters of the resistive reservoir than data acquired using the traditional horizontal electric dipole type of the transmitter.