Seismic-impedance inversion with fuzzy clustering constraints: an example from the Carlin Gold District,Nevada, USA

The seismic reflection method provides high-resolution data that are especially useful for discovering mineral deposits under deep cover. A hindrance to the wider adoption of the seismic reflection method in mineral exploration is that the data are often interpreted differently and independently of other geophysical data unless common earth models are used to link the methods during geological interpretation. Model-based inversion of post-stack seismic data allows rock units with common petrophysical properties to be identified and permits increased bandwidth to enhance the spatial resolution of the acoustic-impedance model. However, as seismic reflection data are naturally bandlimited, any inversion scheme depends upon an initial model, and must deal with non-unique solutions for the inversion. Both issues can be largely overcome by using constraints and integrating prior information. We exploit the abilities of fuzzy c-means clustering to constrain and to include prior information in the inversion. The use of a clustering constraint for petrophysical values pushes the inversion process to select models that are primarily composed of several discrete rock units and the fuzzy c-means algorithm allows some properties to overlap by varying degrees. Imposing the fuzzy clustering techniques in the inversion process allows solutions that are similar to the natural geologic patterns that often have a few rock units represented by distinct combinations of petrophysical characteristics. Our tests on synthetic models, with clear and distinct boundaries, show that our methodology effectively recovers the true model. Accurate model recovery can be obtained even when the data are highly contaminated by random noise, where the initial model is homogeneous, or there is minimal prior petrophysical information available. We demonstrate the abilities of fuzzy c-means clustering to constrain and to include prior information in the acoustic-impedance inversion of a challenging magnetotelluric/seismic data set from the Carlin Gold District, USA. Using fuzzy c-means guided inversion of magnetotelluric data to create a starting model for acoustic-impedance proved important in obtaining the best result. Our inversion results correlate with borehole data and provided a better basis for geological interpretation than the seismic reflection images alone. Low values of the acoustic impedance in the basement rocks were shown to be prospective by geochemical analysis of rock cores, as would be predicted for later gold mineralization in weak, decalcified rocks.     

一种基于亥姆霍兹分解的大地电磁测深有限元正演预条件解法

本研究针对大地电磁测深法有限元数值模拟中,迭代法求解线性方程组效率较低的问题,利用亥姆霍兹分解原理,将电场矢量双旋度方程的预条件问题转化为基于矢量位的泊松问题和基于标量位的拉普拉斯问题,并在四面体非结构化棱边元离散的情况下,借助节点元辅助网格离散上述预条件问题,进一步利用代数多重网格方法（AMG）实施求解,最终实现预条件算法.利用经典的COMMEMI理论模型进行试算并与前人的积分方程解进行对比,验证了本文数值模拟程序与预条件方法的正确性和可靠性.此外,利用不同自由度规模的实验模型对这一预条件算法的效率进行了测试.结果表明,这一算法可以有效地提升大地电磁测深法棱边有限元数值模拟迭代法的收敛性,计算效率较通用的不完全LU分解预条件算法明显更高;在较大自由度网格（>1000万）数值模拟计算中,其算法效率及内存占用相对直接解法有较大优势,也使小型工作站上利用较大自由度的有限元网格进行大地电磁测深数值模拟计算成为可能.     

大地电磁相位超象限现象的各向异性模型研究——以上下结构为例

大地电磁观测数据中的相位超象限现象可以由不同的电性结构产生.本文在已实现大地电磁三维任意各向异性有限差分正演的基础上,以具有上下结构关系的三维各向异性模型为例,分析各向异性体的规模以及参数变化对阻抗相位的影响.在下部各向异性体规模明显大于上部各向异性体或表现为层状特征的情况下,上部各向异性体在两个水平方向上的尺度差异较大,可以看作准二维体时容易发生相位超象限;当上部各向异性体在两个水平方向上尺度相近表现为规则三维体时,要产生相位超象限的现象则需要各向异性体具有更高的各向异性比.在同等条件下,增加各向异性体的各向异性比更容易发生相位超象限;而各向异性走向方位角的变化将直接影响到发生相位超象限的范围.对于准二维模型引起相位超象限的条件,沿用二维模型的近似解析分析方法,进一步构建了基于各向异性体的电导率、背景电导率以及各向异性走向角的相位超象限指标函数,从而更加直观地解释在二维或准二维条件下发生相位超象限现象的模型参数特征.     

基于交叉梯度耦合的大地电磁与地震走时资料三维联合反演

为了有效解决目前大地电磁和地震走时资料单方法反演结果一致性不好的问题,同时克服基于岩石不同物性参数间关系耦合约束联合反演的局限性,本文研究了基于交叉梯度耦合约束的大地电磁与地震走时资料的三维联合反演算法.以较为成熟的天然地震走时资料三维正反演和大地电磁三维正反演算法为基础,实现了具有共同的反演网格,以交叉梯度结构耦合约束,并能同时获得电阻率和速度模型的三维联合反演算法.分别利用单棱柱体模型和双棱柱体模型合成数据进行了联合反演试算.结果表明:无论是单棱柱体模型还是双棱柱体模型,联合反演结果比单独反演对异常体的空间形态都有更好的恢复,其中单棱柱体模型反演的异常体电阻率更接近于真实电阻率,双棱柱体模型的联合反演结果不仅消除了围岩的部分电阻率假异常,而且增强了对异常体深部速度结构特征的恢复程度.联合反演还能同时改善电阻率和速度反向变化异常体的单独反演结果,进一步证明交叉梯度耦合不依赖于岩石物性关系,而强调地下结构的相似性,具有更普遍的适用性.     

Application of 2D magnetotelluric methods in a geological complex area, Xinjiang, China

Magnetotelluric (MT) data along two profiles were collected in Tulargen Cu-Ni mining area, Xinjiang, China. The MT profiles, crossing the known ore-bearing magmatic body, are interpreted to have further understanding of the ore-bearing magmatic bodies' stretch and shape in depth, and to evaluate the prospect in the mining area. The values of phase sensitivity skewness indicate a contaminated two-dimensional (2D) structure (there exist local three-dimensional (3D) structures at shallow level) over 3D structures. G-B decomposition of the impedance tensors indicates the dominant geoelectrical strike in survey area is in East-West direction. To obtain reliable electrical structures, four different 2D inversion codes as REBOCC, NLCG, OCCAM, and ABIC are adopted, inversions of a synthetic 3D model using these codes indicate TM data are more suitable to invert the electrical structures along the profiles in Tulargen Cu-Ni mining area, while TE and TE + TM inversions can be used to verify the transverse variation of electrical structures at shallow level. The 2D electrical structures show high-resistivity basement covered by relatively low-resistivity strata in the mining area, the conductive anomalies are imbedded in the upper covers. The known ore-bearing magmatic body is coincided with the positions of two conductive anomalies in both profiles at shallow level; its extension at depth is deduced according to the shapes of the anomalies. Other conductive anomalies, isolated and existing only at limited depth as the known ore-bearing magmatic body, are presumed of good prospect.     

大地电磁测深在火山区地热研究中的应用

大地电磁测深(MT)由于勘探深度范围较大,且对温度、流体、岩浆房和岩性等与热储相关的地质条件的敏感度较高,因而成为火山区地热勘探和岩石圈结构研究中常用的一种地球物理勘探手段。地壳和上地幔的电性结构与热结构之间存在着密切的联系,通过解读二者的关系,可以刻画更为精细的岩石圈结构模型,进而掌握火山区的构造特征和热演化过程,了解其岩石圈地球动力学机制。本文着重介绍了MT方法的原理以及从野外数据采集到后期数据处理的过程,综述了MT法的应用特点以及电导率与温度之间的关系,通过实例分析,介绍了国际上这一方法在火山区地热勘探和岩石圈热结构研究中的应用进展,展示了MT法在新西兰Taupo火山区Ngatamariki高温地热田0~3km地热资源勘探中的应用;以埃塞俄比亚Afar省的Tendaho地热田和Badi火山为例,分别讨论了0~20km和0~50km不同深度的电性结构特征及其与温度存在的内在联系,探讨了形成火山的驱动机制;以日本九州岛的Shinmoe-dake火山为例,介绍了大地电磁测深和温度监测在火山监测方面的应用。最后简述了国内MT法在火山区的应用进展以及存在的问题,并利用上地幔电导率与温度的关系以及岩石圈内硅酸盐熔体不同含水量引起的电导率随温度的变化关系,初步估算了长白山天池和阿尔山火山区的莫霍面以下的温度以及长白山天池火山区的高温岩浆房温度。     

Exploration of geothermal structure in Puga geothermal field, Ladakh Himalayas, India by magnetotelluric studies

To understand the crustal electric structure of the Puga geothermal field located in the Ladakh Himalayas, wide band (1000 Hz–0.001 Hz) magnetotelluric (MT) study have been carried out in the Puga area. Thirty-five MT sites were occupied with site spacing varying from 0.4 to 1 km. The measurements were carried out along three profiles oriented in east–west direction. After the preliminary analysis, the MT data were subjected to decomposition techniques. The one-dimensional inversion of the effective impedance data and the two-dimensional inversion of the TE (transverse electric) and TM (transverse magnetic) data confirm the presence of low resistive (5–25 Ω m) near surface region of 200–300 m thick in the anomalous geothermal part of the area related to the shallow geothermal reservoir. Additionally, the present study delineated an anomalous conductive zone (resistivity less than 10 Ω m) at a depth of about 2 km which is possibly related to the geothermal source in the area. A highly resistive basement layer separates the surface low resistive region and anomalous conductive part. The estimated minimum temperature at the top of conductive part is about 250 °C. The significance of the deeper conductive zone and its relation to the geothermal anomaly in the area is discussed.     

Volcanic structure of the southern sector of Mt. Etna after the 2001 and 2002 eruptions defined by magnetotelluric measurements

Two magnetotelluric (MT) surveys were carried out on the Mt. Etna volcano after two of the most intense eruptions of the last 30 years which took place in summer 2001 and winter 2002–2003. Surveying was pursued for two main reasons. First, we sought to contribute to the definition of the first-order structure and physico-chemical state (temperature, fluids, melts) of a volcano that has been extensively explored and monitored by means of various geophysical methods, but where only few electrical and electromagnetic surveys have been performed. Secondly, we acquired MT data in the same sites in the two different surveys with the aim of monitoring the possible changes of the first-order structure, since conditions are expected to vary on an active volcano such as Etna, and are supposed to be linked to the eruptive events. Soundings have been acquired in an E-W 10 km-long profile across the southern flank of Mt. Etna, at a distance of almost 6 km south from the Central Crater. The first survey was carried out three months after the 2001 eruption. Inverse models define a pronounced (4 km thickness) low resistivity section at a depth of about 1 km b.s.l. to the west. To the east, a low resistivity section is still present, but appears deeper, thinner and more resistive, and a shallow low resistivity anomaly also exists. The shallow anomaly to the east is tentatively correlated with altered and clayey volcanic units and/or temporary groundwater storage. The deep anomalies are interpreted as being due to melt storage at shallow depths which was not exhausted during the eruption. This would be confirmed by the abundance of lava erupted within one year from the end of the survey. The few good sites retrieved in the second survey, carried out a few weeks after the eruption of 2002–2003, confirm the picture defined in the first survey, and provide a better definition of the bottom of the deep anomaly located in the sedimentary basement.     

Geophysical images of the creeping segment of the San Andreas fault: implications for the role of crustal fluids in the earthquake process

High-resolution magnetotelluric (MT) studies of the San Andreas fault (SAF) near Hollister, CA have imaged a zone of high fluid content flanking the San Andreas fault and extending to midcrustal depths. This zone, extending northeastward to the Calaveras fault, is imaged as several focused regions of high conductivity, believed to be the expression of tectonically bound fluid pockets separated by northeast dipping, impermeable fault seals. Furthermore, the spatial relationship between this zone and local seismicity suggests that where present, fluids inhibit seismicity within the upper crust (0–4 km). The correlation of coincident seismic and electromagnetic tomography models is used to sharply delineate geologic and tectonic boundaries. These studies show that the San Andreas fault plane is vertical below 2 km depth, bounding the southwest edge of the imaged fault-zone conductor (FZC). Thus, in the region of study, the San Andreas fault acts both as a conduit for along-strike fluid flow and a barrier for fluid flow across the fault. Combined with previous work, these results suggest that the geologic setting of the San Andreas fault gives rise to the observed distribution of fluids in and surrounding the fault, as well as the observed along-strike variation in seismicity.     

大地电磁法三维交错采样有限差分数值模拟

系统地论述了大地电磁三维交错采样有限差分数值模拟算法实现过程中交错网格剖分、积分公式离散化、边界条件、方程组求解、三维张量阻抗的计算等内容. 由于提出了简洁的边界条件，采用了解大型系数矩阵方程组的双共轭梯度稳定解法，所实现的三维交错采样有限差分数值模拟算法具有迭代收敛稳定、计算精度高、速度快等特点. 通过两个理论模型的计算结果检验了算法的正确性和计算精度. 所实现的三维交错采样有限差分数值模拟算法为研究三维反演问题奠定了基础.