The influence of sampling parameters on extremely low frequency DCIM

The two-level discrete complex image method (DCIM) is successfully introduced into the extremely low frequency (ELF) electromagnetic wave propagation calculation in marine controlled-source electromagnetic (MCSEM) survey. The original path does not hold valid for the low frequency situation. The reason can be ascribed to the change of the wave number at different frequency. The new integral path suitable for the ELF situation is proposed. This paper mainly focuses on the selection of the sampling space, sampling number and number of exponential terms. The study exhibits the influences of these parameters on different paths. The appropriate ranges of these parameters are given. The electric field of a five-layer model is calculated under different frequency situations to validate the DCIM. The results show that the electric field attenuates faster with frequency increase. The influence of oil-layer conductivity is studied and the results show the electric field is stronger with lower conductivity. The influence of the source height and the receiver height are negligible. All the results accord to the physical meanings of MCSEM.