| 三维起伏地形条件下航空瞬变电磁响应特征研究 |
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| 引用本文： | 赵越,李貅,王祎鹏,郭建磊,曾友强.三维起伏地形条件下航空瞬变电磁响应特征研究[J].地球物理学报,2017,60(1):383-402. |
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| 作者姓名： | 赵越 李貅 王祎鹏 郭建磊 曾友强 |
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| 作者单位： | 1. 长安大学 地质工程与测绘学院, 西安 710054;
2. 中国科学院声学研究所, 北京 100091;
3. 中航勘察设计研究院有限公司, 北京 100098 |
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| 基金项目： | 国家重点基础研究发展计划（973计划）（2013CB036002）、国家重大科研设备研制项目（2011YQ030133）联合资助. |
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| 摘 要： | 航空瞬变电磁法以其速度快、成本低、通行性好等的优势能够有效的应用于地质地形条件复杂的地区.目前对于航空瞬变电磁法的研究主要基于平坦地形的理想情况,对于地形效应的研究相对较少,然而实际应用中地形不可避免,若忽略地形影响将对资料解释造成较大的误差,从而制约航空电磁方法的进一步发展.本文基于交错网格的时域有限差分方法对三维起伏地形条件下航空瞬变电磁进行正演模拟,在保证算法准确性的前提下给出大量模型算例.以经典地形模型为例,利用所给方法计算三维正演响应,结果显示起伏对于航空瞬变电磁数据有着显著的影响且影响主要集中在早期.而后,以实际地质资料为基础,构建起伏地形条件下包含多个异常体的三维复杂模型,计算了复杂模型的航空瞬变电磁响应,并给出三维全域视电阻率曲线,从而对地形效应的影响有了更加直观的认知.最后,通过大量模型讨论了地形的尺寸参数、电性参数、飞行轨迹与飞行高度等因素变化对于航空瞬变电磁数据的影响情况,并得出有价值的结论.
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| 关 键 词： | 航空瞬变电磁 地形效应 三维正演 时域有限差分 |
| 收稿时间： | 2015-10-10 |
Characteristics of terrain effect for 3-D ATEM |
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| ZHAO Yue,LI Xiu,WANG Yi-Peng,GUO Jian-Lei,ZENG You-Qiang.Characteristics of terrain effect for 3-D ATEM[J].Chinese Journal of Geophysics,2017,60(1):383-402. |
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| Authors: | ZHAO Yue LI Xiu WANG Yi-Peng GUO Jian-Lei ZENG You-Qiang |
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| Institution: | 1. College of Geology Engineering and Geomatics, Chang'an University, Xi'an 710054, China;
2. Institute of Acoustics, Chinese Academy of Sciences, Beijing 100091, China;
3. AVIC Institude of Geotechnical Engineering Co. Ltd., Beijing 100098, China |
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| Abstract: | Airborne data measured at areas with a rolling topography always contains terrain information, which is actually a noise to the measured data. To better understand the influence of the terrain to our survey, a modeling of 3D airborne transient electromagnetic (ATEM) considering terrain influence is implemented based on the finite different time domain (FDTD) method.
A FDTD algorithm for 3D ATEM problems with topography is developed in this paper. The unconditionally-stable Du Fort-Frankel method is used in time-stepping and an additional fictitious displacement current is introduced into the diffusion equations to form explicit difference equations. The solution calculates both electric and magnetic field responses inside the model. The Dirichlet condition is employed along each boundary. The tangential components of electric fields and vertical components of the magnetic fields at the six faces of the model are set to zero. This requires the model must be large enough, which is easier to be implemented by a non-uniform discretization. The "step-form" grid is used for the approximation of the terrain, and denser grids are made near the surface of the air-ground boundary to ensure a higher precision. The fictitious permittivity is allowed to vary during the computation to ensure the stability and optimize an efficient time step. Homogeneous full-space model is simulated and the result of which is compared with 3D terrain model from the literature.
Through a series of 3D synthetic modeling, terrain can be a significant source of distortion in ATEM surveys:(1) The topography has a great effect on ATEM responses, especially at early channels, the topographical effects of a hill model produces a high resistivity over its top and a low resistivity over its foot, and a valley model is completely opposite. (2) The topographical parameters such as the depth and width could affect the simulated ATEM responses, and the ATEM field is more sensitive to the terrain along the survey line. (3) The electrical parameters of terrain changes have a significant influence on ATEM response, especially the electrical parameters of the surface medium, low resistance surface influence is bigger than high impedance surface effect. (4) The ATEM terrain effect is also affected by the flight altitude and flight path. (5) The results confirm that topographical variations can have a significant impact on measured ATEM data, and the significance of the distortions varies with the details of the topography.
3D FDTD is capable of simulating the ATEM response of complicated terrain models, and the topographical variations can be treated flexibly by it. The results confirm that topographical variations can have a significant impact on ATEM survey. Thus, topographic effects should be taken into account when designing ATEM surveys and interpreting data. |
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| Keywords: | Airborne transient electromagnetic method Terrain effect 3D modeling FDTD |
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