浅部频率域电磁勘探方法综述

适用于近地表(2000m以内)勘探的频率域电磁法主要有音频大地电磁法(audio-frequency magnetotellurics, AMT), 无线电大地电磁法(radio-magnetotellurics, RMT), 可控源音频大地电磁法(controlled source audio-frequency magnetotellurics, CSAMT), 广域电磁法(Wide Field Electromagnetic Method, WFEM).本文拟从最新的数据采集技术、数据处理技术、正反演算法、实例等四个方面, 论述适用于浅部勘探的AMT, RMT, CSAMT和WFEM方法的国内外最新进展, 总结目前AMT, RMT, CSAMT和WFEM方法遇到的困难, 对潜在的发展方向提出建议.综述表明:(1)张量测量、多站阵列、多站叠加可提高AMT、RMT和CSAMT数据的质量.利用近区数据WFEM法可获得良好的效果.国产与国外仪器在质量方面的差距正在逐步缩小.(2)数学形态滤波技术、Hilbert-Huang变换等可有效分离出有用的数据, 局部畸变仍然是亟待解决的难题, 需要更为深入的研究.(3)矢量有限元与非结构网格的出现大幅度提高了有限元处理复杂电磁问题模拟的精度与应用范围, 成为目前电磁正演的首选工具.完全非线性反演算法仍然局限于1D、2D问题, 共轭梯度法和高斯牛顿算法等为解决3D问题的发展趋势.地质约束的引入和多数据联合反演可以减小反演的非唯一性.各向异性的反演为目前反演研究的热点之一.(4)野外数据解释的正确性严重依赖于对地下结构先期的维性判别, 在2D特性不明显、3D特性明显时, 需要采用3D进行反演解释.

Frequency-domain electromagnetic methods for exploration of the shallow subsurface:A review

Four electromagnetic(EM)methods are widely used in exploration of the shallow subsurface(above depth 2000 meters), which are the audio-frequency magnetotellurics(AMT), radio-magnetotellurics(RMT), controlled source audio-frequency magnetotellurics(CSAMT)and the wide-field electromagnetic methods(WFEM). The purpose of this review is to present the advances of these technologies from five aspects, i.e. data collection, data processing algorithms, forward modeling, inversion algorithms and case studies. We also point out their current difficulties and suggest the potential development trends.To address the issues above, we have made a detailed investigation to the recent literature and chosen the most significant papers published.(1)Data acquisition and instrument. Along with the growing power of EM data collection instruments, the tensor measurement has been commonly adopted. In order to obtain more reliable data, instrument array or multi-instruments have been widely used in the field. By large funds for developing instruments, the quality of domestic instruments is gradually approaching that of the foreign equipment.(2)Data processing algorithms. The time series of AMT data usually contains different types of noise, such as artificial square waves and industry noise around target zones. Therefore, the signal-to-noise ratio of AMT data is usually low. In order to obtain useful signals, we can utilize the robust digital morphological filtering technology. In transforming the time domain signal into the frequency domain, the Hilbert Huang transform is a suitable choice. Recently, the main reliable approach to estimate the tensor impedance is simultaneously using multi-channel multi-frequency data. As for the CSAMT and RMT methods, the state of art in the data acquisition is to identify near field and far field zones. A new idea is the wide-band electromagnetic acquisition technique, which presents a unified way to compute the apparent resistivity in both near field and far field zones. Compared to the AMT, RMT and CSAMT methods, the WFEM method can work better in the near field zone.(3)Forward modeling and inversion. In order to compute the EM responses for complicated AMT/RMT/CSAMT problems, the finite element method has become the indispensable tool. The edge-based finite-element methods have replaced the nodal-based finite-element methods. To have a capability of dealing with complicated topography, the unstructured grids instead of traditional structured grids have become more important. The high efficiency of direct solvers such as MUMPS and Pardiso have played an important role in solving multi-source EM problems. As for the inversion, to simulate 3D cases, two inversion algorithms, which are the nonlinear conjugate gradient method and Gauss-Newton algorithm, have been applied to successfully performing 3D inversion of AMT/RMT/CSAMT data. To further improve the inversion quality, the joint inversion scheme becomes more popular.(4)Applications. The above AMT/RMT/CSAMT methods are widely used in mineral resources exploration, geothermal resources or groundwater surveys, and environmental and engineering geophysics. In inverting the AMT/RMT/CSAMT data, we should analyze the possible dimensionality of the subsurface structure. When they show a 2D structure, the results by 2D inversion are reliable. However, for 3D cases, inversion results and geological interpretations must be based on 3D inversion algorithms.