一维电阻率各向异性对海洋可控源电磁响应的影响研究

Effects of the electric anisotropy on marine controlled-source electromagnetic responses

地下介质的电阻率常常表现为各向异性,海底裂隙地层和层状沉积序列可能形成宏观电阻率各向异性.在解释海洋电磁资料时,电阻率各向异性的影响不应该被忽略,否则可能会得到错误的海底地电模型.作者编写了电阻率任意各向异性一维层状介质海洋可控源电磁场计算程序,计算了电阻率各向异性层状模型的海洋可控源电磁响应,讨论了覆盖层和高阻储层分别具有电阻率各向异性时的电磁场响应特征.

Recently,the marine controlled-source electromagnetic（CSEM）method has emerged as a useful exploration technique for mapping offshore hydrocarbon reservoirs and characterizing gas hydrates.The frequency domain marine CSEM system usually uses a deep-towed horizontal electric dipole（HED）source to transmit low frequency energy to an array of seabed receivers.The receivers detect and record the electromagnetic signals from the source.The resistivity distribution below the seabed may be determined by interpreting the recorded electromagnetic fields as a function of source-receiver offsets.When interpreting the marine CSEM data,the seabed is usually assumed to be electrically isotropic.However,it is commonly known that the electric conductivity in the earth′s subsurface shows anisotropic effects.Fracture zones and layered sedimentary sequences are capable of producing anisotropy on a macroscopic scale.Ignoring anisotropy may lead to a distorted image of seabed conductivity structures,even misinterpretation.In this paper,we present a method for simulating marine CSEM fields from an electric dipole source in stratified media with general anisotropy.We simulate the marine CSEM responses of layered models with three types of anisotropy：Vertical Transverse Isotropy（VTI）,Horizontal Transverse Isotropy（HTI）and Tilted Transverse Isotropy（TTI）.We study the effects of anisotropy in both the sediments and the reservoir on CSEM fields.The model studies show that the CSEM fields are affected considerably by anisotropy in both the sediments and the reservoir target,but to different extent.The horizontal and vertical resistivity of the surrounding sediment,and the vertical resistivity of the reservoir have a significant effect on CSEM responses,whereas the influence of the reservoir′s horizontal resistivity is hardly to be observed.The influence of the horizontal resistivity of the sediment on CSEM fields is much larger than that of the vertical resistivity.The electromagnetic responses are affected considerably by the anisotropic dip angle.