海洋可控源电磁法多参数正演响应特征分析
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摘要
鉴于介质的色散现象,充分考虑介质电磁参数(ε、μ、σ),获得各层介质中的电磁场和势函数。为了考察岩、矿石电磁参数、海水深度和海底地层厚度等变化时海洋电磁响应特征规律,给出典型地电模型和观测系统参数,借助高精度快速汉克尔滤波系数,采用30点高斯勒让德数值求积方法,计算水平电性源频率域可控源电磁法(CSEM)在海底各观测点电场E_z和磁场B_x的正演响应。结果表明,随海水深度变浅,电场E_z和磁场B_x振幅曲线变化幅度增大,空气波逐渐占据主导地位,当水深超过3 km时,观测区内可以忽略空气波的影响;电场E_z分量对海底高阻层引起的异常大,而中低阻层引起的异常小,磁场B_x分量幅值对目标层电导率变化与E_z相同,因此,水平电性源频率域CSEM法不适宜探测海底低阻目标。并且对厚层目标层的探测能力高于薄层。针对岩层中磁导率变化的情况,海底沉积物电性均匀或者呈层状分布时,电场E_z和磁场B_x幅值均受到影响。此外,电场E_z和磁场B_x的幅值基本不受介电常数变化而影响。海洋可控源的探测效果与偏移距的选择有密切关系,有利偏移距范围为3 000 m～12 000 m。
For the medium dispersion, the electric conductivity σ, the magnetic permeability μ, and permittivity ε should be taken into account when modeling marine controlled-source electromagnetic(MCSEM). Electromagnetic(EM) fields at each layer will be solved based on Lorentz potential or Coulomb-gauged potentials. To study how parameters describing seabed media to impact on the characteristic of EM fields, we utilize Kong's filter with lengths of 241 for fast Hankel transform and 30 points Gauss Legendre quadrature to calculate the electric field component E_z and magnetic field component B_x in layer media when a horizontal electric source with finite transmits EM field diffuse through the ocean, seafloor and air. From the responses we know that the amplitudes of E_z and B_x decrease while the depth of water becomes deeper. In the water where the depth is less than 3 km, air wave distorts the EM fields reflect seafloor geologic structure, while the interferences of air wave can be ignored more than 3 km. In addition, z and B_x are sensitive to resistive layer compared with conductive layer. Therefore horizontal electrical source MCSEM is not effective to explore the conductive. Meanwhile, E_z and B_x changes with μ, but both E_z and B_x are not affected by ε. The good offset of CSEM is 3000～12000 m.
For the medium dispersion, the electric conductivity σ, the magnetic permeability μ, and permittivity ε should be taken into account when modeling marine controlled-source electromagnetic(MCSEM). Electromagnetic(EM) fields at each layer will be solved based on Lorentz potential or Coulomb-gauged potentials. To study how parameters describing seabed media to impact on the characteristic of EM fields, we utilize Kong's filter with lengths of 241 for fast Hankel transform and 30 points Gauss Legendre quadrature to calculate the electric field component E_z and magnetic field component B_x in layer media when a horizontal electric source with finite transmits EM field diffuse through the ocean, seafloor and air. From the responses we know that the amplitudes of E_z and B_x decrease while the depth of water becomes deeper. In the water where the depth is less than 3 km, air wave distorts the EM fields reflect seafloor geologic structure, while the interferences of air wave can be ignored more than 3 km. In addition, z and B_x are sensitive to resistive layer compared with conductive layer. Therefore horizontal electrical source MCSEM is not effective to explore the conductive. Meanwhile, E_z and B_x changes with μ, but both E_z and B_x are not affected by ε. The good offset of CSEM is 3000～12000 m.
引文
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[13] 史增园.海洋可控源电磁法正演研究与实现[D].北京:中国石油大学(华东),2010.SHI Z Y.Forward research and implementation of ocean controlled source electromagnetic method [D].Beijing:China University of Petroleum (East China),2010.(In Chinese)
[14] ANDERSON W.L.A hybrid fast Hankel transform algorithm for electromagnetic modeling[J].Geophysics,1989,54:263-266.
[15] JOHANSEN H.K,SORENSEN K.I.Fast Hankel transforms[J].Geophysical Prospecting,1979,27:876-901.
[16] KONG F.N.Hankel transform filters for dipole antenna radiation in a conductive medium[J].Geophysical Prospecting,2007,55:83-89.
[17] D.Guptasarma,B.Singh.New digital linear filters for Hankel J0 and J1 transforms[J].Geophysical Prospecting,1997,45:745-762.(In Chinese)
[18] 张伟,王绪本,覃庆炎.汉克尔变换的数值计算与精度的对比[J].物探与化探,2010,34(6):753-755.ZHANG W,WANG X B,QIN Q Y.Research and application on numerical integration of Hankel transforms by digital filtering[J].Geophysical and Geochemical Exploration,2010,34(6):753-755.
[2] 何继善,鲍力知.海洋电磁法研究的现状和进展[J].地球物理学进展,1999,14(1):7-39.HE J S,BAO L Z..The situation and progress of marine electromagnetic method reseach[J].Progress in Geophysics,1999,14(1):7-39.(In Chinese)
[3] 刘国兴.电法勘探原理与方法[M].北京:地质出版社,2005.LIU G X.Principles of geoelectricity[M].Beijing:Geological Publishing House,2005.(In Chinese)
[4] 莫杰.探索海洋奥秘开发海洋资源[M].北京:地质出版社,1998.MO J.Marine exploration,development of marine resources [M].Beijing:Geological Publishing House,1998.(In Chinese)
[5] CHAVE A D,COX C S.Controlled electromagnetic sources for measuring electrical conductivity beneath the oceans:1.Forward problem and modeling study [J].Journal of Geophysical Research,1982,87:5327-5338.
[6] COX C S.On the electrical conductivity of the oceanic lithosphere [J].Physics of the Earth and Planetary Interiors,1981,25(3):196-201.
[7] CONSTABLE S,SRNKA L J.An introduction to marine controlled source electromagnetic methods for hydrocarbon exploration [J].Geophysics,2007,72(2):WA3-WA12.
[8] MACGREGOR L,SINHA M.Use of marine controlled- source eleetromagnetie Sounding for sub-basalt exploration [J].Geophysical Prospecting,2000,48(6):1091-1106.
[9] KEY K.Marine Electromagnetic Studies of Seafloor Resources and Tectonics [J].Surveys in Geophysics,2012,33(1):135-167.
[10] ELLINGSRUD S,EIDESMO T,JOHANSED S,et al.Remote sensing of hydrocarbon layers by seabed logging (SBL):Results from a cruise offshore Angola [J].The Leading Edge,2002,21(10):972-982.
[11] KEY K,RAY A.Bayesian inversion of marine CSEM data with a trans-dimensional self parametrizing algorithm [J].Geophysical Journal International,2012,191(3):1135-1151.
[12] SASAKI Y.3D inversion of marine CSEM and MT data:An approach to shallow-water problem [J].Geophysics,2013,78(1):E59-E65.
[13] 史增园.海洋可控源电磁法正演研究与实现[D].北京:中国石油大学(华东),2010.SHI Z Y.Forward research and implementation of ocean controlled source electromagnetic method [D].Beijing:China University of Petroleum (East China),2010.(In Chinese)
[14] ANDERSON W.L.A hybrid fast Hankel transform algorithm for electromagnetic modeling[J].Geophysics,1989,54:263-266.
[15] JOHANSEN H.K,SORENSEN K.I.Fast Hankel transforms[J].Geophysical Prospecting,1979,27:876-901.
[16] KONG F.N.Hankel transform filters for dipole antenna radiation in a conductive medium[J].Geophysical Prospecting,2007,55:83-89.
[17] D.Guptasarma,B.Singh.New digital linear filters for Hankel J0 and J1 transforms[J].Geophysical Prospecting,1997,45:745-762.(In Chinese)
[18] 张伟,王绪本,覃庆炎.汉克尔变换的数值计算与精度的对比[J].物探与化探,2010,34(6):753-755.ZHANG W,WANG X B,QIN Q Y.Research and application on numerical integration of Hankel transforms by digital filtering[J].Geophysical and Geochemical Exploration,2010,34(6):753-755.