大地电磁法三维快速松弛反演

实现大地电磁法快速三维反演的关键在于找到快速计算灵敏度矩阵的方案. 本文在对大地电磁三维张量阻抗表达式进行深入分析的基础上，获得了三维快速松弛反演算法的灵敏度函数表达式，解决了三维快速松弛反演的核心问题；为了减小反演解的多解性，针对三维问题定义了最小构造函数，实现了求最小构造的三维快速松弛反演算法. 对二维棱柱体、三维棱柱体理论模型的大地电磁合成数据进行了反演试算，反演结果与理论模型相吻合. 对日本Kayabe地区实测资料进行了XY模式反演，反演得到的地电模型较好地反映了地热资源的分布. 反演试算结果表明大地电磁三维快速松弛反演具有计算速度快、结果稳定可靠等特点；使三维快速松弛反演在普通微机上得以实现.     

Area selection for diamond exploration using deep-probing electromagnetic surveying

Previously proposed methods of area selection for diamond-prospective regions have predominantly relied on till geochemistry, airborne geophysics, and/or an appraisal of tectonic setting. Herein we suggest that a novel, deep-probing geophysical technique—electromagnetic studies using the natural-source magnetotelluric (MT) method—can contribute to such an activity. Essentially, diamondiferous regions must have (1) old lithosphere, (2) thick lithosphere, and (3) lithosphere that contains high concentrations of carbon. Deep-probing MT studies are able to address all three of these. The second and the third of these can be accomplished independently using MT, but for the first the geometries produced from modelling the MT observations must be interpreted with appropriate interaction with geologists, geochemists and other geophysicists. Examples are given from the Slave and Superior cratons in North America, with a brief mention of an area of the Rae craton, and general speculations about possible diamondiferous regions.     

Proterozoic tectonothermal processes imaged with magnetotellurics and seismic reflection in southern Australia

Over the last two decades,co-located seismic and magnetotelluric(MT) profiles provided fundamental geophysical data sets to image the Australian crust.Despite their complimentary nature,the data are processed and often interpreted separately without common processes in mind.We here qualitatively compare 2 D resistivity inversion models derived from MT and seismic reflection profiles across a region of Archean-Proterozoic Australia to address the causes of variations in seismic response and anomalous conductivity in the crust.We find that there exists a spatial association between regions of low reflectivity in seismic sections and low resistivity in co-located2 D MT modelled sections.These relationships elucidate possible signatures of past magmatic and fluid-related events.Depending on their diffuse or discrete character,we hypothesize these signatures signify fossil melting of the crust due to mafic underplating,magma movement or hydrothermal fluid flow through the crust.The approach discussed herein is a process-oriented approach to interpretation of geophysical images and a significant extension to traditional geophysical methods which are primarily sensitive to a singular bulk rock property or state.     

利用接收函数和大地电磁数据联合反演南迦巴瓦构造结中部地区壳幔结构

利用2010年布设在西藏南迦巴瓦构造结的郎嘎、崩嘎、直白和拉格四个宽频地震台所观测到的近5个月的地震记录,采用时间域迭代反褶积技术处理得到接收函数,通过筛选多条相近震中距和反方位角的高质量接收函数求取其叠加平均.对大地电磁数据做Rhoplus分析处理得到视电阻率和相位曲线.利用单台接收函数和相同位置的大地电磁视电阻率和相位联合反演地下一维壳幔结构.联合反演采用遗传算法,并通过权衡图分析大地电磁和地震数据的兼容性.理论值和实测值的对比显示两种数据能同时得到较好拟合.联合反演结果表明:(1)中上地壳为9 km至14 km厚的高阻高速层覆盖于低阻低速层之上的结构,中地壳低阻低速层可能与深部流体和局部熔融共同作用有关.(2)下地壳存在最厚达20 km的高导的壳幔过渡层,波速在4 km/s左右;上地幔约130 km至150 km以下存在软流圈.(3)上地壳的高阻高速层解释为多雄拉组混合岩化角闪岩相变质岩,而直白台所显示的低阻低速层与高压麻粒岩的少量部分熔融有关,可能源于壳幔过渡带镁铁质岩石的相变或更深处幔源岩浆底侵作用的产物.     

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.     

木里—盐源地区深部电性结构及构造意义

木里—盐源地区地处青藏高原东南缘,属于古特提斯洋构造域,是松潘—甘孜地块及扬子地块的交接地带,是研究青藏高原东南缘构造演化过程的重要区域.本文介绍的是横穿木里—盐源地区的大地电磁剖面,自北西向南东依次跨越锦屏山断裂、木里弧形构造区、丽江—小金河断裂、盐源盆地、金河—箐河断裂等构造.维性分析表明木里弧形构造区和金河—箐河断裂都表现为较强的三维性,因此本文采用大地电磁三维反演技术,获得了木里—盐源地区的精细电性结构.电性模型显示,沿剖面可以划分为4个主要的电性构造单元.锦屏山断裂以北的川西北次级地块下方10~20 km处,发育北西向低阻体,推断是古老的义敦岛弧区残留的物质;锦屏山断裂以南至丽江—小金河断裂为高阻体,可能是锦屏山山根;丽江—小金河断裂下方~10 km处发育北东向的低阻体,与龙门山—锦屏山构造带走向一致,结合剖面附近表现为张性的震源机制解特点,推测该低阻体很可能是北部的塑性物质受阻后一部分往西南沿着丽江—小金河断裂缝隙挤入的结果;盐源盆地下方在3~7 km发育厚度约5 km、长度达40 km的低阻层,电性主轴方向为北西向,与盐源断裂走向一致,解释为盐岩层,尤其是南段低阻体表现为延伸至地表的特征,与地表盐泉对应,为在盐源地区开展深部找钾盐矿提供了电磁方面的证据.

     

阿尔金走滑断裂带昌马段的电性结构样式及构造意义

阿尔金断裂带东段走滑速率沿断裂走向方向存在明显的流失现象,有关阿尔金断裂带的影响范围及走滑速率变化的机制需要有更多的深部结构证据来提供支撑.本文以阿尔金断裂带昌马段为窗口,获取了4条横穿阿尔金断裂带及相邻地区的大地电磁测深剖面.二维电性剖面显示在阿尔金断裂带北侧中上地壳以连续的高阻体为主,而南侧祁连山内部的深部电性结构在横向上有较为复杂的变化.这一点与区域构造背景相对应,即北侧的塔里木盆地东缘依然具有较好的整体性,南侧的祁连山是青藏高原北缘生长的最前端,变形强烈.在断裂带的结构特征上,阿尔金断裂带沿走向方向的切割深度在昌马盆地西侧发生了显著的降低,与阿尔金断裂带相对应的电性边界在这里向南偏移了约15 km,对应F18断裂,并与昌马盆地相接.祁连山北部的断裂带,包括昌马断裂、旱峡—大黄沟断裂总体呈现出低角度南倾的样式,切过高阻异常体的顶部.虽然昌马盆地可以起到连接断裂带的阶区的作用,将部分阿尔金断裂的走滑分量转移到盆地南侧的昌马断裂上,但是昌马断裂的走滑速率从西向东是增加的,东侧的走滑速率甚至大于阿尔金断裂沿走向方向的流失分量.我们认为在青藏高原北部主要断裂带的活动还是受印度—欧亚板块碰撞引起的远程挤压效应的影响,包括阿尔金断裂以及祁连山内部系列断层都处于斜向挤压应力环境.在这种基本构造模式下,阿尔金断裂、断裂F18、昌马盆地、昌马断裂构成了一个局部的走滑速率分解-转换-吸收体系,对局部应力状态产生影响.     

大地电磁测深二维反演方法求解复杂电性结构问题的适应性研究

为探讨二维反演方法在三维电性结构中的适应性问题,本研究中设计了一系列的二维/三维正演模型进行实验计算,分析了三维高阻/低阻异常体对模型响应的畸变作用,并从反演模式选择和数据旋转方向两个方面进行模型二维反演的对比分析,与三维反演的结果进行了比较,最后采用了实测数据进一步进行了二维和三维反演的比较实验.实验和研究结果表明,在剖面选择方面,在剖面方向与垂直主构造方向相差不大的情况下,截取剖面方向,将电性主轴旋转到垂直剖面方向的二维反演结果与垂直主构造方向的反演结果都可以较好地还原正演模型,在大的构造的反映上并无太大差异.在地下为二维或近三维条件时,正演模型的主要结构都可以较好地被二维和三维反演解析出来.二维的反演结果可能甚至会比三维的反演结果的边界更清晰,更精确.然而,对于具有较强的三维结构的模型而言,其二维反演结果与原始模型可能仍然存在较大差异,其中TM+TP或TM模式的二维反演结果相对更接近原始模型,而TE模式的结果往往会有较大误差,需要在解释时特别注意以免得出错误结论.     

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.

     

青藏高原东缘及四川盆地的壳幔导电性结构研究

自从2008年MS8.0级汶川大地震发生以来,青藏高原东缘便成为地质与地球物理研究的热点区域.该区域的龙门山断裂带标志着青藏高原东缘与四川盆地的边界.汶川地震即发生于龙门山断裂带内的映秀—北川断裂上.该地区现有的研究工作多集中于青藏高原东缘及四川盆地的西部,对四川盆地东部构造情况的研究目前较少.在SinoProbe项目的资助下,完成了一条跨越青藏高原东缘及整个四川盆地的大地电磁测深剖面.该剖面自西北始于青藏高原内部的松潘—甘孜地块,向东南延伸穿过龙门山断裂带、四川盆地内部及四川盆地东部的华蓥山断裂,最终止于重庆东南的川东滑脱褶皱带附近.维性分析表明剖面数据整体二维性较好,通过二维反演得到了最终的电性结构模型.该模型表明,从电性结构上看,沿剖面可分为三个主要的电性结构单元,分别为:浅部高阻、中下地壳低阻的松潘—甘孜地块,浅部低阻、中下地壳相对高阻的四川盆地,以及华蓥山以东整体为高阻特征的扬子克拉通地块.龙门山断裂带在电性结构上表现为倾角较缓、北西倾向的逆冲低阻体,反映了青藏高原东缘相对四川盆地的推覆作用.其在地下向青藏高原内部延伸,深度约为20 km左右.在标志逆冲推覆滑脱面的低阻层下存在—电性梯度带,表征着低阻的青藏高原中下地壳与高阻的扬子地壳之间的电性转换.位于四川盆地东边界的华蓥山断裂在电性结构上表现为—倾向为南东向的低阻体插入高阻的扬子克拉通结晶基底,切割深度约为30 km左右.这一结构反映出华蓥山向西的推覆作用.在电性结构模型的基础上,进一步讨论了青藏高原东缘的壳内物质流、青藏块体与扬子块体的深部关系以及青藏高原东部的隆升机制等构造问题.