由大地电磁数据同时反演电阻率和磁化率参数的方法研究

【作者】 李学民；

【导师】 曹俊兴；

【作者基本信息】 成都理工大学 ， 地球探测与信息技术， 2005， 博士

【摘要】 大地电磁法(MT)是重要的深部地球物理探测手段,在油气、地热勘探,深部探测等领域有着广泛的应用。传统的MT数据处理通常假定地下介质无磁性,仅对介质的电阻率参数进行反演。本文以国家自然科学基金重大项目为依托,在分析计算岩石磁化率参数对大地电磁数据影响的基础上,提出了由MT数据同时反演介质电阻率和磁化率参数分布的方法,并用所发展的方法技术成功地进行了实测资料的处理。论文的主要内容与成果如下: (1) 推导了电阻率和磁化率同为参数时的二维大地电磁正演计算公式。利用Madden和Mackie等所提出的基于传输网络的有限差分算法,实现了其正演计算,并详细地阐述了电阻率和磁化率对大地电磁响应的影响,证实了磁化率主要是影响磁场信号的同相分量,而对磁场信号的异相分量和电场信号的影响比较弱。当磁化率k≥0.01SI时,在某些低频点处视电阻率值可以增加5Ωm左右,并且随着磁化率的增加,这种增加的幅度显著增大;而阻抗相位的变化则比较小。通过对比研究,认为在大地电磁勘探中,起决定作用的是岩石的电阻率;但是在高磁性物质存在的地区进行大地电磁观测时,还必需重视磁化率的影响。 (2) 提出了由大地电磁实测数据同时反演电阻率和磁化率分布的方法技术。考虑电阻率和磁化率同为参数变量,推导了由MT数据同时反演电阻率和磁化率参数分布的反演计算公式。在同时反演过程中,观测数据和物性参数之间以及电阻率和磁化率物性参数本身之间是一种复杂的非线性关系。本文对电阻率和磁化率之间的非线性关系利用一个线性函数来处理,重点是采用了Rodi和Mackie所发展的非线性共轭梯度算法来处理观测数据和物性参数之间的非线性关系,完成了新反演方法的程序代码编写。 (3) 推导了二维大地电磁法中TM和TE两种极化模式的观测数据关于模型参数的灵敏度表达式。利用Mora提出的计算灵敏度的思想,根据Rodi、Mackie和Madden发展的算法,给出了观测数据关于模型参数灵敏度的计算表达式和求解过程。 (4) 通过数值实验和实际观测资料实验,证明了本文研究发展的算法和计算程序的正确性与价值。使用所研发的由MT数据同时反演电阻率和磁化率参数分布的方法技术对内蒙古大杨树盆地9条MT测线的观测资料进行了重新处理,厘定了电阻率分布图像,并新获得了磁化率的分布图像。反演结果表明,在火山岩分布地区,由MT数据获得的磁化率分布信息更有助于推断火山岩的分布,并能为在火成岩覆盖地区寻找沉积岩的分布提供一种新的依据。利用新发展的方法技术同时还可以解决在某些大地电磁观测数据中出现负的同相分量值问题,使得这些出现在低频段的负的同相分量值不再是一种“污染源”,而是作为一种提供磁化率分布很重要的信息源。更多还原

【Abstract】 The MT has been a powerful geophysical tool in exploring the interior of the earth and widely applied in the oil and gas prospecting, geothermal prospecting and deep survey. It is assumed in traditional MT data processing that underground media is not magnetism, in which only the resistivity parameter. Sponsored by NSFC, we calculated and analyzed the effect of rock susceptibility on geoelectric and geomagnetic observations and proposed a new method to invert resistivity and susceptibility distribution simultaneously from the MT data. The application of this method in observed data processing is successful. The main contents and achievements of the thesis are as follows:(1) The 2-D forward modeling formula of the resistivity and susceptibility as model parameters is deduced. By use of the finite difference method, which is based on transmission network proposed by Madden and Mackie, we completed forward calculation and demonstrated in detail the effect of resistivity and susceptibility on MT response. It is conformed that the susceptibility mainly affects the in-phase components of magnetic signal, but a little effect on the quadrature components of magnetic and electrical signal. When the susceptibility is k≥0.01SI, the apparent resistivity amplitude may increase about 5Ωm at some low frequency, and the orders of change increase observably as increase of magnetic susceptibility. But, impedance phase is quite small. So in the MT survey, the resistivity of rock is the decisive factor. However in the region with high susceptibility, the susceptibility must be taken into account.(2) A new method to inverse the resistivity and susceptibility simultaneously from the MT data is proposed. In this inversion method, both resistivity and susceptibility are parameters. The relationship between the observed data and physical parameters of rock and physical parameters of rock themselves are nonlinear. We use a linear function to depict the nonlinear relationship between resistivity and susceptibility, and use the nonlinear conjugate gradient method proposed by Rodi and Mackie to deal with the nonlinear relationship between observed data and physical parameters. We accomplished the program codes of the new method.(3) The expression of observed data with model parameters under the condition of TM and TE polarization in 2-D MT sounding is given. Using the idea of calculation sensitivity proposed by Mora and the algorithm developed by Rodi, Mackie and Madden, we gave the expression of calculating sensitivity and the resolving procedures.(4) By numerical experiments and observation data experiments, the results show that it is feasible to inverse the resistivity ant susceptibility distribution simultaneously and that the program is right. Using this program, We processed the observed data from DaYangShu basin, and the results show that the output of resistivity and susceptibility can gave more information about the distribution of the lava and then can gave new information about sediment. Using this method we can solve the negative in-phase problem as well so that the negative in-phase component can also be used as valuable information.更多还原