利用K-SVD字典学习算法压制海洋大地电磁噪声

海洋大地电磁数据处理的结果会影响数据反演和解释的准确性.在浅水中,由海浪引起的强电磁干扰和大地电磁场叠加在一起,使得噪声所在频段内的视电阻率和相位曲线出现畸变.针对这种强干扰,本文将K-SVD字典学习算法应用于海洋大地电磁信号处理中,通过稀疏表示海浪感应磁噪声实现大地电磁磁场信号的重构,并结合视电阻率信息进行相位校正....     

瞬变电磁测深早期数据的修正

通过时间-频率转换关系,TEM数据可以转换成平面波场测深数据,从而可以对TEM资料进行拟平面波场处理解释.在对瞬变电磁视电阻率数据向平面波场测深视电阻率数据转换时,发现由于瞬变电磁使用晚期计算公式及装置问题,使测深曲线早期数据发生畸变.文中建立了视电阻率曲线进入晚期所满足的关系式,从理论上给出不同情况下瞬变电磁测深视电阻率曲线进入晚期的临界点.以瞬变电磁、大地电磁、CSAMT为例,对大量的模型进行正演计算,对计算结果进行对比分析,建立不同地表电性结构、不同时间延迟情况下,瞬变电磁早期数据误差的校正量板.     

大地电磁测深法在秦皇岛—唐山沿海地区地热资源调查评价中的应用

应用大地电磁测深法,对秦皇岛-唐山沿海地区进行地热资源勘查.在该测区布设6条测线,获得103个测点的野外资料.采用先进的数据处理和反演方法,得到大地电磁测深剖面二维反演电阻率模型,根据当地区域地质资料,对地层、断裂构造进行推断解释,并结合华北地区地热资源类型及其与地质构造特征的关系,圈定该测区地热资源远景区.     

南黄海海底大地电磁测深试验研究

针对南黄海盆地深层地震地质条件差,地震波受屏蔽,能量严重衰减,在海洋反射地震资料中很难识别反映中-古生界的反射地震波组,不能满足南黄海盆地前第三系油气资源前景评价的需要等问题,于2006年5月,开展了南黄海海域海底大地电磁测深试验,在盆地内五莲斜坡和胶州凹陷区布设了3个海底大地电磁测深点,取得了可靠的海底观测数据.本文详细阐述了海底大地电磁测深与常规大地电磁测深在数据处理方法上的差别,论述了海洋电磁噪声对海底大地电磁场观测的影响,提出了海底大地电磁场实测资料“水平姿态”校正、“方位”校正和“海洋电磁噪声”校正的方法.通过对试验观测资料的处理和反演,并结合地质、物性资料进行综合分析,结果表明在南黄海盆地海底以下深部还存留有古生代地层,这对于南黄海前第三系含油气前景的评价具有重要意义.试验结果也证明了海底大地电磁测深技术在研究古生代残留盆地方面的问题可以发挥相当好的作用.     

电偶源频率电磁测深二维阻抗视电阻率计算的源效应校正法

研讨了频率域电磁法中不同源装置的大地电磁测深、线源频率电磁测深和偶极源频率电磁测深阻抗视电阻率的源效应影响特征。在唯象分析的基础上，提出了几种电磁测深法阻抗视电阻率的相互换算法──源效应校正法（大地电磁测深二维ＴＥ极化视电阻率和其它两种电磁法的赤道装置二维阻抗视电阻车）。模型试验表明，利用这一源效应校正法可以由大地电磁二维视电阻率近似地计算出线源频率电磁二维阻抗视电阻率。这一方法被尝试应用于由线源频率电磁二维阻抗视电阻率估算偶极源频率电磁二维阻抗视电阻率。     

表层构造横向不均匀性在电磁测深响应中的影响

大地电磁测深结果受表层构造横向不均匀性的影响,其视电阻率曲线会发生畸变。为了分析表层构造横向不均匀性对测深曲线的影响,本文采用有限单元方法对几种野外常见的表层横向不均匀构造进行了模拟计算。研究结果表明,表层构造的横向不均匀性对于测深曲线的影响是严重的     

MT激电效应的模拟研究及在油气检测中的应用

本文引入Cole-Cole模型来模拟大地的激发极化效应，对三层水平地层且中间层为极化层的大地电磁测深的视电阻率进行了理论计算，分析了极化参数对视电阻率曲线的影响规律.采用广义逆方法对三层水平地层且中间层为极化层的模型进行了反演研究，结果表明该反演方法能够较好地确定地层电阻率的同时获得地层激电参数，应用于实际资料的反演时，反演结果与已知含油气地层的实际参数十分吻合.     

尖锐边界反演在临江市六道沟可控源音频大地电磁测深数据反演中的应用

在实际大地电磁测深数据反演过程中,为了得到更加清晰的电性单元分界面,传统的Occam平滑反演方法遇到了挑战,本文讨论了sharp boundary inversion(SBI)反演的理论及其应用.通过模型试算,对比Occam和SBI的反演效果,证明了SBI反演划分层状介质的可靠性.本文对临江市六道沟地区实测可控源音频大地电磁测深(CSAMT)数据进行了全区视电阻率转换,然后对全区视电阻率进行Occam反演和SBI反演.最后对比已有地质资料,表明SBI反演能够用于CSAMT数据的处理中.     

正交水平磁偶源的电磁场分布规律

正交水平磁偶源是模拟天然场源的较好人工源,可以方便地实现可控源高频大地电磁张量测量.正交水平磁偶源的电磁场分布规律是野外工作布置的理论基础,为此计算了均匀大地模型正交水平磁偶源的电磁场.计算结果表明:电磁场水平分量在各个象限都有一相对低值带,对应的标量视电阻率形成了畸变带,但张量视电阻率畸变带消失;张量视电阻率曲线形态显示出近区的低阻、过渡区的高阻隆起和远区趋于真值的规律.通过野外试验验证理论计算结果,在无法准确确定地下介质电阻率参数的情况下,以天然电磁场计算的电阻率为参照对比研究了正交水平磁偶源电磁场的分布规律.试验结果表明:正交水平磁偶源与电偶源的电磁场同样的存在近区、过渡区和远区;在远区,正交水平磁偶源与测点的相对位置对张量测量结果几乎没有影响,即在远区可以在任何方位测量;正交水平磁偶源的布置要考虑收发距的影响,保证测量在远区进行.     

希尔伯特-黄变换在海底大地电磁测深数据处理中的应用

海水运动产生的电磁场,是海底大地电磁探测数据中的主要噪声之一,影响了数据处理与解释的精度.为提高海底MT探测的应用效果,本文在海底MT信号处理中引入HHT时频分析方法.结合海底MT噪声的特点与海底MT信号的特性,对南海实测数据进行分析,通过对比去噪前后信号的Hilbert时-频谱和边际谱,表明利用经验模态分解及其多尺度滤波特性,能够有效压制海水运动产生的电磁噪声.经HHT去噪后的测深曲线在一定程度上有所改善,仍存在"飞点"现象;再结合Robust估计,明显改善海底大地电磁探测数据质量,得到满意的观测结果.     

Deep structure of the northeastern margin of the Parnaiba Basin, Brazil, from magnetotelluric imaging

The magnetotelluric (MT) method has been applied to the determination of the deep resistivity structure of the northeastern margin of the Parnaiba Basin. Transient electromagnetic (TEM) and MT data were collected in early 1999 along a 95 km long N–S line, extending from the coast across the projected subcrop position of a discontinuous fault found to the west of the study area that is believed to be a possible basin‐bounding fault. The MT data were processed to yield the TE‐ and TM‐mode responses and then corrected for static shift using central‐loop and single‐loop TEM data, respectively. Regularized 2D MT inversion was subsequently undertaken using a structured initial model with the near‐surface constrained by TEM inversion results. As a consistency check, we performed another set of 2D inversions using different smooth initial models. The various optimal 2D inversion models show clearly the presence of a major basement trough, over 2 km deep, located about 70 km from the coast. We interpret it as possibly marking the main basin margin and suggest that it may have implications for groundwater resource development in the area.     

郯庐断裂带中段(沂沭断裂带)电性结构研究与孕震环境

穿过郯庐断裂带中段（沂沭断裂带,36°N）所做的大地电磁测深（MT）剖面长约150km.使用Robust技术和远参考道大地电磁方法处理观测数据.通过分析视电阻率、阻抗相位、Swift二维偏离度和区域走向,定性确定测区的电性结构.二维反演解释中选择非线性共轭梯度（NLCG）方法,使用TE、TM两种模式资料联合反演,沿剖面的二维电性结构显示：自西向东,鲁西隆起、郯庐断裂带、胶莱坳陷及鲁东隆起4个电性区块分别对应,鲁东和鲁西隆起区为高阻,郯庐断裂带电性结构复杂,高、低阻相间,胶莱坳陷为低阻（高导）区.沿MT剖面附近曾发生3个地震,其震源区处在电性变化剧烈部位,并在震源区附近存在高导体.     

Coast effect distortion of marine magnetotelluric data: Insights from a pilot study offshore northeastern Japan

We report on strong coast effect distortions observed for broadband marine magnetotelluric (MT) data collected on the forearc offshore northeastern Japan. Eight days of horizontal electric and magnetic fields recorded at eight seafloor stations and the horizontal magnetic fields from a land remote station were processed with a robust multiple-station algorithm, yielding good MT responses and inter-station transfer functions at periods of 7–10,000 s. Transverse electric (TE) mode responses have cusps in apparent resistivity and negative phases at periods around 1000 s, while the transverse magnetic (TM) mode responses are galvanically depressed below the TE responses. An analysis of inter-station transfer functions confirms that the apparent resistivity cusps are a magnetic field, rather than electric field, phenomenon, consisting of an amplitude minimum and rapid phase change around a characteristic frequency. Poynting vectors for a TE coast effect model study illustrate that the anomalous phases are associated with energy diffusing back up to the seafloor from below, after being turned around from its usual downward propagating trajectory by inductive coupling between the conductive ocean and the resistive seafloor along the continental margin. We show that the characteristic frequency and position of the TE mode apparent resistivity cusps are determined by a relatively simple combination of the electrical resistivity of the seafloor, the depth of the ocean, and the distance from the coastline. By including coastlines and bathymetry in 2D inversion, we recover the seafloor conductivity structure along the forearc, demonstrating that broadband data can constrain the thickness of conductive forearc sediments and the underlying high resistivity associated with the mantle wedge and subducting oceanic lithosphere.     

Out‐of‐plane effects in 2D borehole‐to‐surface resistivity tomography and applications in mineral exploration

In this paper, we discuss the effects of anomalous out‐of‐plane bodies in two‐dimensional (2D) borehole‐to‐surface electrical resistivity tomography with numerical resistivity modelling and synthetic inversion tests. The results of the two groups of synthetic resistivity model tests illustrate that anomalous bodies out of the plane of interest have an effect on two‐dimensional inversion and that the degree of influence of out‐of‐plane body on inverted images varies. The different influences are derived from two cases. One case is different resistivity models with the same electrode array, and the other case is the same resistivity model with different electrode arrays. Qualitative interpretation based on the inversion tests shows that we cannot find a reasonable electrode array to determine the best inverse solution and reveal the subsurface resistivity distribution for all types of geoelectrical models. Because of the three‐dimensional effect arising from neighbouring anomalous bodies, the qualitative interpretation of inverted images from the two‐dimensional inversion of electrical resistivity tomography data without prior information can be misleading. Two‐dimensional inversion with drilling data can decrease the three‐dimensional effect. We employed two‐ and three‐dimensional borehole‐to‐surface electrical resistivity tomography methods with a pole–pole array and a bipole–bipole array for mineral exploration at Abag Banner and Hexigten Banner in Inner Mongolia, China. Different inverse schemes were carried out for different cases. The subsurface resistivity distribution obtained from the two‐dimensional inversion of the field electrical resistivity tomography data with sufficient prior information, such as drilling data and other non‐electrical data, can better describe the actual geological situation. When there is not enough prior information to carry out constrained two‐dimensional inversion, the three‐dimensional electrical resistivity tomography survey is the better choice.     

Integrated interpretation of the magnetotelluric and magnetic data from Mahallat geothermal field,Iran

Magnetotelluric (MT) and ground magnetic surveys were conducted on the Mahallat geothermal field situated in Markazi province, central Iran, as a primary part of the explorations and developments of a geothermal energy investigation program in the region. Mahallat region has the greatest geothermal fields in Iran. MT survey was performed in November 2011 on an 8 km profile crossing the hot springs with a total of 17 stations. The 2D inversion of the determinant MT data was performed using a 2D inversion routine based on the Occam approach. The 2D resistivity model obtained from the determinant data shows a low resistivity zone at 800-2000 m depth and a higher resistivity zone above the low resistivity zone, interpreted as geothermal reservoir and cap rock, respectively. It also revealed two major concealed faults which are acting as preferential paths for the circulation of hydrothermal fluids. To obtain more geophysical evidence, a ground magnetic survey with 5000 stations was also performed over an area of 200 km² around the MT profile. Magnetic measurements show a main positive anomaly of about +1000 nT over the study area, which could be interpreted as an intrusive body with the high magnetic susceptibility (i.e. mafic and ultramafic rocks) into the sedimentary host rocks. We interpret the body as the heat source of the geothermal system. Structural index and depth estimation of the anomaly indicate that the intrusive body is similar to a cylinder extending from about one kilometer depth down to greater depths. The results of MT and magnetic investigations indicate a geothermal reservoir which proves the preliminary geological observations to a great extent.     

APPLYING 3D INVERSION OF SINGLE-PROFILE MAGNETOTELLURIC DATA TO IDENTIFY THE SHADE AND YUNONGXI FAULTS

Many synthetic model studies suggested that the best way to obtain good 3D interpretation results is to distribute the MT sites at a 2D grid array with regular site spacing over the target area. However, MT 3D inversion was very difficult about 10 years ago. A lot of MT data were collected along one profile and then interpreted with 2D inversion. How to apply the state-of-the-art 3D inversion technique to interpret the accumulated mass MT profiles data is an important topic. Some studies on 3D inversion of measured MT profile data suggested that 2D inversions usually had higher resolution for the subsurface than 3D inversions. Meanwhile, they often made their interpretation based on 2D inversion results, and 3D inversion results were only used to evaluate whether the overall resistivity structures were correct. Some researchers thought that 3D inversions could not resolute the local structure well, while 2D inversion results could agree with the surface geologic features much well and interpret the geologic structures easily. But in the present paper, we find that the result of 3D inversion is better than that of 2D inversion in identifying the location of the two local faults, the Shade Fault(SDF)and the Yunongxi Fault(YNXF), and the deep structures. In this paper, we first studied the electrical structure of SDF and YNXF based on a measured magnetotelluric(MT) profile data. Besides, from the point of identifying active faults, we compared the capacity of identifying deep existing faults between 2D inversion models and 3D models with different inversion parameters. The results show that both 2D and 3D inversion of the single-profile data could obtain reasonable and reliable electrical structures on a regional scale. Combining 2D and 3D models, and according to our present data, we find that both SDF and YNXF probably have cut completely the high resistivity layer in the upper crust and extended to the high conductivity layer in the middle crust. In terms of the deep geometry of the faults, at the profile's location, the SDF dips nearly vertically or dips southeast with high dip angle, and the YNXF dips southeast at depth. In addition, according to the results from our measured MT profile, we find that the 3D inversion of single-profile MT data has the capacity of identifying the location and deep geometry of local faults under present computing ability. Finally, this research suggests that appropriate cell size and reasonable smoothing parameters are important factors for the 3D inversion of single-profile MT data, more specifically, too coarse meshes or too large smoothing parameters on horizontal direction of 3D inversion may result in low resolution of 3D inversions that cannot identify the structure of faults. While, for vertical mesh size and data error thresholds, they have limited effect on identifying shallow tectonics as long as their changes are within a reasonable range. 3D inversion results also indicate that, to some extent, adding tippers to the 3D inversion of a MT profile can improve the model's constraint on the deep geometry of the outcropped faults.     

起伏地形下大地电磁L-BFGS三维反演方法

为推进大地电磁三维反演的实用化,本文实现了基于L-BFGS算法的带地形大地电磁三维反演.首先推导了大地电磁法三维反演的Tikhonov正则化目标函数以及Hessian矩阵逆矩阵近似表达式和计算方法,然后设计了一种既能保证空气电阻率固定不变又能保证模型平滑约束的协方差矩阵统一表达式,解决带地形反演问题.在反演算法中采用正则化因子冷却法以及基于Wolf条件的步长搜索策略,提升了反演的稳定性.利用开发的算法对多个带地形地电模型（山峰地形下的单个异常模型、峰-谷地形下的棋盘模型）的合成数据进行了三维反演,并与已有大地电磁三维反演程序（ModEM）进行对比,验证了本文开发的三维反演算法的正确性和可靠性.最后,利用该算法反演了华南某山区大地电磁实测数据,得到该区三维电性结构,揭示了研究区以高阻介质为基底,中间以低阻不整合面和相对低阻介质连续分布,浅部覆盖高阻介质的电性结构特征,进一步验证了本文算法的实用性.     

考虑电流型畸变的大地电磁三维反演

大地电磁测深(MT)的观测数据易受到由近地表小尺度非均匀体或地形起伏引起的电流型畸变干扰,消除或压制这种干扰对获取可靠的深部电性结构至关重要.当区域结构为二维时,电流型畸变可采用张量分解等方法予以消除或压制.当区域结构为三维时,畸变问题更加复杂和严重,传统张量分解方法往往效果不佳或无效,严重地制约了MT三维反演技术的实用性.对此,本文提出一种考虑电流型畸变的MT三维反演算法,将完整的电流型畸变参数引入到目标函数,并采用非线性共轭梯度法与电阻率参数同时反演,从而达到压制畸变的目的.该算法有两个关键点:一是通过分析实测数据所遭受畸变的分布特征,在目标函数中对其进行有效约束;二是在迭代过程中,通过自适应地调整双正则化因子保障算法的稳定和效率.理论模型测试结果显示,常规三维反演算法不能合理解释数据中的畸变成分,而只能通过引入虚假异常体强制地拟合受畸变数据,从而造成电阻率模型严重失真.与之相比,本文算法能够在反演中自动求解各测点所受到的畸变,获得更接近真实的电阻率模型.     

1D joint multi‐offset inversion of time‐domain marine controlled source electromagnetic data

The accurate estimation of sub‐seafloor resistivity features from marine controlled source electromagnetic data using inverse modelling is hindered due to the limitations of the inversion routines. The most commonly used one‐dimensional inversion techniques for resolving subsurface resistivity structures are gradient‐based methods, namely Occam and Marquardt. The first approach relies on the smoothness of the model and is recommended when there are no sharp resistivity boundaries. The Marquardt routine is relevant for many electromagnetic applications with sharp resistivity contrasts but subject to the appropriate choice of a starting model. In this paper, we explore the ability of different 1D inversion schemes to derive sub‐seafloor resistivity structures from time domain marine controlled source electromagnetic data measured along an 8‐km‐long profile in the German North Sea. Seismic reflection data reveal a dipping shallow amplitude anomaly that was the target of the controleld source electromagnetic survey. We tested four inversion schemes to find suitable starting models for the final Marquardt inversion. In this respect, as a first scenario, Occam inversion results are considered a starting model for the subsequent Marquardt inversion (Occam–Marquardt). As a second scenario, we employ a global method called Differential Evolution Adaptive Metropolis and sequentially incorporate it with Marquardt inversion. The third approach corresponds to Marquardt inversion introducing lateral constraints. Finally, we include the lateral constraints in Differential Evolution Adaptive Metropolis optimization, and the results are sequentially utilized by Marquardt inversion. Occam–Marquardt may provide accurate estimation of the subsurface features, but it is dependent on the appropriate conversion of different multi‐layered Occam model to an acceptable starting model for Marquardt inversion, which is not straightforward. Employing parameter spaces, the Differential Evolution Adaptive Metropolis approach can be pertinent to determine Marquardt a priori information; nevertheless, the uncertainties in Differential Evolution Adaptive Metropolis optimization will introduce some inaccuracies in Marquardt inversion results. Laterally constrained Marquardt may be promising to resolve sub‐seafloor features, but it is not stable if there are significant lateral changes of the sub‐seafloor structure due to the dependence of the method to the starting model. Including the lateral constraints in Differential Evolution Adaptive Metropolis approach allows for faster convergence of the routine with consistent results, furnishing more accurate estimation of a priori models for the subsequent Marquardt inversion.     

Three-dimensional inversion of magnetotelluric data including sea effects obtained in Pohang, Korea

Magnetotelluric (MT) surveys were conducted in Pohang, Korea, for low-temperature geothermal exploration in 2002 and 2003. Pohang is located in the southeastern part of the Korean Peninsula and close to the East Sea. In the interpretation of MT data from a coastal environment, sea effects must be correctly included because seawater is a strong conductor. We first constructed a five-layered earth model with a realistic coastline and bathymetry to investigate sea effects on MT data measured in Pohang. This model clearly shows that the Pohang data are significantly influenced by sea water at frequencies blow 1 Hz at the whole measurement sites. Next, we utilized a three-dimensional inversion algorithm based on the Gauss–Newton approach to produce a reliable resistivity model. Seawater is excluded from the inversion domain to fix the resistivity, while included in the modeling domain to simulate sea effects on MT responses. Blocks for the sub-seafloor are included in unknown parameters since they are sufficiently close to the survey area to affect MT responses in Pohang. Static shifts are also considered in inversion for more accurate interpretation. The rms data misfit is smoothly reduced from 11.2 to 1.87 after 7 iterations. The resulting resistivity model shows a pattern of low–high–low resistivity with depth. The model is compatible with resistivity logs obtained from four boreholes in the survey area, and can explain major geological features in Pohang.