本文關鍵詞： 瞬變電磁法 矢量有限元 三維正演　出處：《長安大學》2015年碩士論文　論文類型：學位論文

【摘要】：瞬變電磁法經過近些年國內外的發(fā)展與實踐,逐漸成為一種引人注目的地球物理勘探方法。但是由于瞬變電磁理論的復雜性,目前理論技術仍不夠完善,尤其是在三維正反演處理解釋方面遠沒有達到實用水平,主要原因是對三維空間中瞬變電磁的響應特征與規(guī)律認識不清,缺少能準確模擬三維空間中復雜結構地電模型的軟件。針對此種情況,本文著重研究瞬變電磁場的矢量有限元三維正演,希望給后續(xù)的瞬變電磁正反演研究工作提供一定的依據和參考。內容主要如下:首先推導了均勻半空間電偶極子在空間中任意位置的電場,然后利用異常場法,將回線源在均勻半空間中產生的響應作為背景場加載到電場的異常場控制方程中,采用Witney型插值基函數對求解區(qū)域進行離散、插值,實現了回線源頻率域的三維正演,其中稀疏矩陣的存儲采用CSR存儲格式,方程組的求解采用MKL庫中的并行求解器PARDISO。在進行時間域轉化時,先采用對數間隔對需要計算的頻率段進行離散,利用三次樣條函數插出需要求解的頻率域響應,再通過余弦變換實現時間域的轉化。采用Fortran語言編寫了瞬變電磁三維矢量有限元正演程序,并分別與典型的H型、K型模型的數字濾波解做了對比,進行了誤差分析,發(fā)現矢量有限元計算的瞬變電磁場響應較準確,垂直分量的誤差基本在百分之五以內。接著與WangHohmann的三維FDTD算法計算的三維模型進行了對比,進一步證明了本文矢量有限元算法的正確性。在保證程序正確的基礎上,分別計算了均勻半空間含低阻體和含高阻體的中心回線瞬變電磁響應,并分析了大回線源在均勻半空間、均勻半空間帶低阻異常體、均勻半空間帶高阻異常體中響應的分布特征,發(fā)現不同平面上的垂直分量響應與煙圈理論相符合。接著模擬了電性源地空系統(tǒng)探測地下傾斜礦體的地電模型,計算了不同方向斷面上的全域視電阻率,視電阻率斷面能較客觀的反映地下異常體的規(guī)模和賦存位置。在此基礎上試算了淺海條件下拖拽式海洋瞬變電磁在不同海水深度、不同接收深度的高阻油氣藏模型,分析了衰減曲線的形態(tài)和響應規(guī)律。最后發(fā)現,海水深度對油氣藏探測影響很大,對于相同規(guī)模、相同埋藏深度的油氣藏,海水越淺、觀測的瞬變電磁場越大,但是異常場與背景場的差異越小,海水越深、觀測的瞬變電磁場越小,但是異常場與背景場的差異越大。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.
【學位授予單位】：長安大學
【學位級別】：碩士
【學位授予年份】：2015
【分類號】：P631.325

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本文編號：1468994