可控源海洋电磁勘探中空气波影响的理论分析和数值模拟

可控源海洋电磁勘探(MCSEM)中空气波对海底电磁响应的影响已为业界所熟知,且已提出了多种压制方法.然而,由于空气波与海水层中其它信号相互作用的复杂性,至今其作用机理及对有效信号的影响方式尚未完全清楚,这也阻碍了浅水域MCSEM油气勘探的应用.本文在前人研究的基础上, 基于一维层状介质的电磁位和电磁场分析,采用电磁场的模式分解理论导出了空气、海水层和海底地层之间的相互作用关系.分析了MCSEM电磁响应的影响因素和空气波的作用机理.以无限水深假想模型、三层介质模型和四层介质模型为基础,导出了水平电偶极子(HED)的横电(TE)和横磁(TM)模式的电磁场关系,分析显示了前人提出的上下行波分离进行空气波压制所存在的缺陷.通过理论和数值模拟知道, 利用横电(TE)和横磁(TM)模式受空气相互作用影响的差异和电磁场水平分量与垂直分量受空气层相互作用影响程度的不同可减弱空气波对有效信号的影响,这将有助于实现浅水域海洋电磁勘探数据的有效利用.

Theoretic analysis and numerical simulation of effects of air wave interaction on the marine controlled source electromagnetic exploration

The effects of the air wave interaction on electromagnetic(EM) responses recoded by EM sensors which have been deployed on the seafloor in the marine controlled-source EM exploration method(MCSEM) have been well recognized in EM survey community.However,because of the complexity of the physics of the interactions among air layer,sea water and the formation under the sea floor,the mechanism of the effects of air wave interactions and their presentations on the EM responses has not been fully understood yet.This holds back the application of the MCSEM in offshore hydrocarbon exploration in shallower water regions.Having extended the investigation carried out,in this study,we have derived the interaction relationships of the EM signals among the air layer,sea water zone and under seafloor formations in an one-dimensional layer model using the mode decomposition theory of the electromagnetic field.Moreover,the factors that affect the MCSEM responses and the mechanism of the air wave interactions within different media have been analyzed.On the basis of the presumed infinite sea water model,three-layer model and four-layer model,we have derived the EM field expressions of the TE and TM modes which were decomposed from the EM responses excited by the HED source.It is indicated that the method reducing the air wave effects by separating the recorded EM signal into up-going and down-going waves suggested by former researchers has inevitable drawbacks.Theoretical analysis and numerical simulation show that the ways that the TE and TM electromagnetic modes were contaminated by the air waves are very different,and the effects of the air wave interaction on the horizontal and vertical EM field components also present significant differences.All these characteristics can be applied to mitigate the air wave interaction,and this will be helpful for the effective use of the MCSEM data in shallower water regions.