Foundations of the method of EM field separation into upgoing and downgoing parts and its application to MCSEM data

The renewed interest in the methods of electromagnetic field decomposition into upgoing and downgoing parts is generated by the practical problem of removing the effect of EM airwaves on marine controlled source electromagnetic data collected in shallow water. In this paper we consider the principles of solving this problem using the classical methods of EM field separation into external and internal parts. We demonstrate that the most general approach to upgoing/downgoing field decomposition is based on the theory of Stratton-Chu type integrals. This approach allows us to separate the field observed on an arbitrary surface. In the case of flat observational surface, the Stratton-Chu method is equivalent to the decomposition technique based on the Fourier transform in the spatial frequency domain. We present also a novel method of EM field separation using the method of horizontal gradients of the EM field. The new technique is tested on synthetic MCSEM data. We demonstrate also that the method represents a useful tool for a rapid qualitative interpretation of MCSEM data.