| 电性源瞬变电磁地空逆合成孔径成像 |
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| 引用本文： | 李貅,张莹莹,卢绪山,姚伟华.电性源瞬变电磁地空逆合成孔径成像[J].地球物理学报,2015,58(1):277-288. |
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| 作者姓名： | 李貅 张莹莹 卢绪山 姚伟华 |
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| 作者单位： | 长安大学地质工程与测绘学院, 西安 710054 |
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| 基金项目： | 国家自然科学基金面上项目(41174108)、国家重点基础研究发展计划(973计划)项目(2013CB036002)、国家自然科学基金重点项目(51139004)联合资助. |
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| 摘 要： | 电性源地空瞬变电磁法具有工作效率高、勘探深度大、采集信号信噪比高、适用于地形地质条件复杂地区等优点.但是,到目前为止,由于尚未建立起该方法的解释系统,大大制约了该方法的发展.本文旨在建立起完整的地空电磁探测系统,丰富整个探测系统的理论.本文围绕地空瞬变电磁法全域视电阻率定义、瞬变电磁虚拟波场的克希霍夫偏移成像、逆合成孔径成像方法三个科学问题进行了系统研究.提出了用磁场强度定义全域视电阻率的迭代算法,理论模型试验结果表明计算出的视电阻率曲线首支趋于第一层电阻率,尾支趋于最后一层电阻率,实现了全空域、全时域视电阻率的计算;在先前研究的基础上,实现了适合电性源地空装置的瞬变电磁虚拟波场的克希霍夫偏移成像;采用相关迭加技术,实现孔径内多测点数据合成,将传统的单点处理方式发展成为逐点推移多次覆盖的逆合成孔径处理方法.层状模型试验表明:(1)全域视电阻率能够光滑、完整、渐变地反映出模型的电性信息变化;(2)当改变三层模型中间层电阻率时,全域视电阻率曲线随着参数的改变分异明显,对电性层的识别容易且直观;(3)由于在电阻率计算中同时考虑了接收机高度、偏移距、时间等各参数的影响,全域视电阻率可实现全空域、全时域的视电阻率计算.含水采空区的复杂模型算例表明:(1)根据不同测线的全域视电阻率结果可以看出,在靠近采空区的位置,全域视电阻率断面可以清晰地反映出采空区的空间位置,随着测线离采空区越来越远,采空区异常越来越弱直至消失;(2)波场变换和偏移成像的结果显示存在两个电性差异较大的界面,上界面指示地表,由于空气和大地之间的电性差异较大,故该界面波场信号反映强烈,遍布整个区域,下界面异常信号则主要集中在中部,向外逐渐减弱,指示采空区;(3)逆合成孔径成像结果表明地表界面在合成前后没有变化,而采空区异常合成后范围明显变小,且异常边界清晰,指示的采空区位置与模型吻合很好.本文借助于逆合成孔径雷达成像的基本思想,建立了一套电性源瞬变电磁地空逆合成孔径成像方法.基于反函数思想结合迭代算法提出的电性源地空瞬变电磁法的全域视电阻率定义方法,实现了全空域、全时域的视电阻率计算;借鉴瞬变电磁拟地震偏移成像算法,实现了瞬变场的三维成像;借鉴逆合成孔径雷达的思想,提出电性源瞬变电磁地空逆合成孔径算法,进一步提高了成像的分辨率.采空区模型算例表明相关叠加合成确实具有增强有用信号、提高信噪比、提高分辨率的诸多优点,证实了瞬变电磁地空逆合成孔径成像方法的有效性.
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| 关 键 词： | 瞬变电磁 逆合成孔径成像 全域视电阻率 克希霍夫偏移成像 |
| 收稿时间： | 2013-12-11 |
Inverse Synthetic Aperture Imaging of Ground-Airborne transient electromagnetic method with a galvanic source |
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| LI Xiu,ZHANG Ying-Ying,LU Xu-Shan,YAO Wei-Hua.Inverse Synthetic Aperture Imaging of Ground-Airborne transient electromagnetic method with a galvanic source[J].Chinese Journal of Geophysics,2015,58(1):277-288. |
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| Authors: | LI Xiu ZHANG Ying-Ying LU Xu-Shan YAO Wei-Hua |
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| Institution: | College of Geology Engineering and Geomatics, Chang'an University, Xi'an 710054, China |
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| Abstract: | Ground-Airborne transient electromagnetic method with a galvanic source has such advantages as high efficiency, great exploration depth, high signal to noise ratio and good performance in mountainous areas. However, so far its interpretation system has not been established, which restricts the development of this method. This paper aims at building up a complete Ground-Airborne electromagnetic detection system to enrich the theory of this method. This paper consists of three parts: (1) full field apparent resistivity definition; (2) Kirchhoff immigration of TEM virtual wave field; (3) inverse synthetic aperture imaging. Magnetic field intensity is selected for full field apparent resistivity definition. The results of theoretical models show that the first and last part of the apparent resistivity curve can reach the resistivity of the first and the last layer, respectively. Thus, full space and full time apparent resistivity definition is achieved. Based on the previous study, Kirchhoff migration of Ground-Airborne transient electromagnetic method with a galvanic source is realized. Correlation method is adopted for multiple survey point synthesis, which pushes traditional single point processing forward point-by-point multi-coverage inverse synthetic aperture processing.#br#The results of layered models show that: (1) Full field apparent resistivity can tell the underground electric change smoothly, completely and gradually. (2) When the resistivity of the middle layer changes, the differentiation of full field apparent resistivity is obvious, which can easily and intuitively identify electric layer. (3) As receiver altitude, offset and time are considered simultaneously, full field apparent resistivity can achieve full space and full time apparent resistivity calculation. The results of the complex model with a water bearing gob shows that: (1) The further the offset from the gob is, the weaker the resistivity anomaly on full field apparent resistivity section is. (2) There are two interfaces from the Kirchhoff immigration results, the upper one indicates surface. As the electric difference between earth and air is large, the wave field signal of this interface is strong and spreads over the whole area. While the anomaly of the lower interface mainly distributes in the middle area and weakens outwards, which indicates the water bearing gob. (3) The upper interface changes little after inverse synthetic aperture imaging, while the gob anomaly narrows and has a clear boundary. The anomaly boundary matches well with the designed model. Inverse synthetic aperture imaging of Ground-Airborne transient electromagnetic method with a galvanic source is realized on the basis of Inverse Synthetic Aperture Radar (ISAR). Full field apparent resistivity is put forward with inverse function and iteration, which can realize full zone and full time apparent resistivity calculation. Pseudo-seismic immigration of transient electromagnetics is used in 3D transient field imaging. Inverse synthetic aperture radar is introduced in inverse synthetic aperture imaging of Ground-Airborne transient electromagnetic method with a galvanic source, which can help improve resolution. The results of designed models show that correlation can strengthen useful signal and improve signal to noise ratio and resolution, which prove the efficiency of inverse synthetic aperture imaging of Ground-Airborne transient electromagnetic method with a galvanic source. |
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| Keywords: | Transient electromagnetic method Inverse synthetic aperture imaging Full field apparent resistivity definition Kirchhoff immigration |
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