Abstract:

Marine controlled-source electromagnetic method (MCSEM) is an effective technology for pre-drill reservoir appraisal. It is an important auxiliary technology for marine seismic to distinguish between oil-and water-bearing reservoir and consequently reduces the risk of dry wells and exploration cost. However, when surveying in shallow water using a horizontal electrical dipole (HED) as transmitting source, the airwave，propagating at the air-ocean interface，will almost totally mask the signal from the resistive reservoir. Due to the fact that the airwave doesn’t contain any information about the resistive layer, it seriously affects the MCSEM application in shallow water. Therefore, removal of the airwave from MCSEM data to get a good inversion result becomes a research focus. Based on Weidelt’s (2007) airwave theory, we first get the leading term of airwave by derivation of spectral kernels of TE mode. We then,discuss the reason for airwave removal before MCSEM inversion by elaborating the mechanism of its origin and effects. Finally, we divide the current technologies dealing with airwave problem into three categories: 1)Remove airwave based on its characteristics in frequency-domain and time-domain; 2)Airwave-free survey; 3)Direct interpretation. After analyzing  these technologies, we find that all current technologies cannot completely remove the airwave effect in MCSEM data. Each method has its own conditions and limitations for application. A combination of these methods is more helpful and effective.

Key words: marine controlled-source electromagnetic method (MCSEM), airwave, shallow water, resistive reservoir, electric fields