Analysis of the mechanism of air wave interaction and the effect of the sea water depth on the marine controlled source electromagnetic exploration

Abstract

In recent years, Marine Controlled Source Electromagnetic Method (MCSEM) has been gradually adopted as a routine geophysical technique in deep sea hydrocarbon exploration. However, up to now most of the published MCSEM cases are limited to deep water regions where their water depth is larger than about 300 m because of the significant effects on the electromagnetic (EM) signal of the air wave interference. Besides deep water areas, there are larger amounts of license blocks where water depth is shallower than 300 m and EM data acquired by present MCSEM equipments cannot be accurately processed and interpreted because of the contamination of the air wave interactions with the signals. In the study, the air wave interactions with the sea water, seafloor formations and air above the sea surface have been analyzed through the EM wave mode decomposition method. First, the solutions of the electromagnetic fields are expressed in terms of two potential functions that correlate with the transverse electric (TE) and transverse magnetic (TM) modes. Secondly, general potential functions of the horizontal electric dipole (HED) in sea water and layer models are derived from the Maxwell equations and corresponding boundary conditions. Next, the electric and magnetic fields which were deduced from two potential functions have been numerically computed by using the integrals of the Bessel function. Finally, the effects of the air wave interactions with sea surface air, sea water and sub-sea formation have been analyzed through multilayer models with different water depths. It is found that EM field components of TE and TM modes have significant differences in responding to the air wave interaction and we may mitigate or reduce the air wave interference through acquiring multi-components EM fields.