浅水域可控源海洋电磁测量中空气波的渐近表达与波场分离方法

Methods of Air Wave Approximation and Decomposition for Marine Controlled Source Electromagnetic Measurements in Shallower Water Domain

可控源海洋电磁(MCSEM)勘探中空气波对海底电磁响应的影响已为业界所重视,它是由水平电偶极子源发射的电磁信号沿空气-海水界面传播与来自海底地层的有效信号相互作用产生的。在浅水域勘探时,空气波淹没来自地层的有效信号,使浅水域MCSEM实现油气层识别产生困难。基于层状介质模型,采用电磁场的模式分解理论导出了半空间电阻率模型的空气波表达式,利用该式将空气波在海水层的传播近似用海水-空气界面与海底地层之间的多次鸣震表达。结合源和接收器两边电磁场的传播特征,导出了有限水深时空气波近似关系,用于近似模拟空气波响应。为了对比不同方法压制空气波的效果,基于上行波场和下行波场的分解方法,获得了含油气高阻储层上上行波的异常幅度增大数倍的结果,显示了波场分解方法压制空气波的良好效果。最后,利用不含油气层的背景模型和含油气储层模型电磁响应的数值模拟结果,比较空气波渐近表达和波场分离2种空气波压制方法可知:对于水平层状模型后者效果更好;前者可适用于崎岖海底地层的数据处理,后者只适用于水平海底地层。

In marine controlled-source electromagnetic（MCSEM）exploration,the influence on the EM response of the airwave has been paid great attention by the industry and the air wave is one kind ofEM signal from the horizontal electric dipole（HED）source that is reflected from or travels along the air-sea interface and return back to receivers.In shallow water domains,the airwave dominates over the effective signal from seabed sediments,and makes it very difficult to identify the hydrocarbon reservoirs.On the basis of the layer model,we have derived airwave expression of half space resistivity model by using the mode decomposition theory of the EM field and the air wave in the sea water layer is approximately expressed as a series of wave reverberations that travel down and backward between the sea surface and the seabed.According to the propagations of EM field near sources and receivers,we have obtained airwave expression of a finite water layer.In order to compare the effectiveness of the various air wave mitigation methods,we have adopted the decomposition method of EM fields into upgoing and downgoing components,and have acuired larger anomaly of several times than that of the raw EM field above the resistive formation,and indicated the favorable effect of airwave attenuation by using of wave separation.Finally,simulation results of electromagnetic responses from different background and reservoir models have been used to check the effectiveness and adaptability of the two air wave mitigation methods of wave reverberations approximation and wave field decomposition,numerical results show that for model of horizontal layers,the latter is more effective,however,the former is applicable to rugged sea floor formation and the latter only fits for horizontal layers.