瞬变电磁法矢量有限元三维正演研究

发布时间：2018-01-27 17:23

本文关键词： 瞬变电磁法矢量有限元三维正演　出处：《长安大学》2015年硕士论文　论文类型：学位论文

【摘要】：瞬变电磁法经过近些年国内外的发展与实践,逐渐成为一种引人注目的地球物理勘探方法。但是由于瞬变电磁理论的复杂性,目前理论技术仍不够完善,尤其是在三维正反演处理解释方面远没有达到实用水平,主要原因是对三维空间中瞬变电磁的响应特征与规律认识不清,缺少能准确模拟三维空间中复杂结构地电模型的软件。针对此种情况,本文着重研究瞬变电磁场的矢量有限元三维正演,希望给后续的瞬变电磁正反演研究工作提供一定的依据和参考。内容主要如下:首先推导了均匀半空间电偶极子在空间中任意位置的电场,然后利用异常场法,将回线源在均匀半空间中产生的响应作为背景场加载到电场的异常场控制方程中,采用Witney型插值基函数对求解区域进行离散、插值,实现了回线源频率域的三维正演,其中稀疏矩阵的存储采用CSR存储格式,方程组的求解采用MKL库中的并行求解器PARDISO。在进行时间域转化时,先采用对数间隔对需要计算的频率段进行离散,利用三次样条函数插出需要求解的频率域响应,再通过余弦变换实现时间域的转化。采用Fortran语言编写了瞬变电磁三维矢量有限元正演程序,并分别与典型的H型、K型模型的数字滤波解做了对比,进行了误差分析,发现矢量有限元计算的瞬变电磁场响应较准确,垂直分量的误差基本在百分之五以内。接着与WangHohmann的三维FDTD算法计算的三维模型进行了对比,进一步证明了本文矢量有限元算法的正确性。在保证程序正确的基础上,分别计算了均匀半空间含低阻体和含高阻体的中心回线瞬变电磁响应,并分析了大回线源在均匀半空间、均匀半空间带低阻异常体、均匀半空间带高阻异常体中响应的分布特征,发现不同平面上的垂直分量响应与烟圈理论相符合。接着模拟了电性源地空系统探测地下倾斜矿体的地电模型,计算了不同方向断面上的全域视电阻率,视电阻率断面能较客观的反映地下异常体的规模和赋存位置。在此基础上试算了浅海条件下拖拽式海洋瞬变电磁在不同海水深度、不同接收深度的高阻油气藏模型,分析了衰减曲线的形态和响应规律。最后发现,海水深度对油气藏探测影响很大,对于相同规模、相同埋藏深度的油气藏,海水越浅、观测的瞬变电磁场越大,但是异常场与背景场的差异越小,海水越深、观测的瞬变电磁场越小,但是异常场与背景场的差异越大。文章最后指出了一些不足和今后的工作方向。
[Abstract]:Transient electromagnetic method has gradually become a kind of attractive geophysical exploration method through the development and practice at home and abroad in recent years. However, due to the complexity of transient electromagnetic theory, the current theoretical technology is still not perfect. In particular, the interpretation of three-dimensional forward and inverse processing is far from practical level, the main reason is that the response characteristics and laws of transient electromagnetic in three-dimensional space are not clear. Due to the lack of software to simulate the geoelectric model of complex structures in three-dimensional space, this paper focuses on the vector finite element 3-D forward modeling of transient electromagnetic fields. The main contents are as follows: firstly, the electric field of uniform half-space electric dipole in the space is derived, and then the anomalous field method is used. The response generated by the loop source in the uniform half-space is loaded into the abnormal field governing equation of the electric field as the background field. The Witney interpolation basis function is used to discretize and interpolate the solution region. Three-dimensional forward modeling in frequency domain of loop source is realized, in which sparse matrix is stored in CSR format. The parallel solver PARDISOin the MKL library is used to solve the equations. In the time domain transformation, the frequency bands to be calculated are discretized by logarithmic intervals. The frequency domain response needed to be solved is interpolated by cubic spline function, and the time domain is transformed by cosine transform. The transient electromagnetic 3D vector finite element forward program is compiled by Fortran language. Compared with the digital filtering solution of typical H-type K model, the error analysis shows that the transient electromagnetic field response calculated by vector finite element method is more accurate. The error of vertical component is less than 5%. Then, it is compared with the 3D model calculated by WangHohmann's 3D FDTD algorithm. The correctness of the vector finite element algorithm is further proved. On the basis of ensuring the correctness of the program, the transient electromagnetic response of the center loop with low resistive body and high resistive body in uniform half space is calculated respectively. The distribution characteristics of large loop source in uniform half-space, homogeneous half-space with low-resistivity anomaly body and uniform half-space with high-resistivity abnormal body are analyzed. It is found that the vertical component response in different planes is in agreement with the smoke ring theory. Then the geoelectric model for detecting underground inclined orebodies by electrical source space system is simulated and the global apparent resistivity in different directions is calculated. The apparent resistivity section can objectively reflect the size and location of the underground anomaly. On this basis, the towed ocean transient electromagnetic at different depths in shallow water is calculated. The shape and response law of attenuation curve are analyzed in the model of high resistivity reservoir with different receiving depth. Finally, it is found that the depth of seawater has a great influence on the exploration of oil and gas reservoir, and for the same size and same burial depth, the oil and gas reservoirs with the same size and the same burial depth are found. The shallower the sea water, the greater the observed transient electromagnetic field, but the smaller the difference between the anomalous field and the background field, the deeper the seawater, the smaller the observed transient electromagnetic field. But the difference between anomalous field and background field is greater. Finally, some shortcomings and future work direction are pointed out.
【学位授予单位】：长安大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：P631.325

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