大地电磁、重力、磁法和地震初至波走时的交叉梯度二维联合反演研究

为了准确的探测和描绘地下复杂的地质结构,同时克服地球物理单一方法反演的多解性和单一参数反演模型的不一致性等问题,近年来基于交叉梯度联合反演的综合地球物理解释已经得到了广泛的关注和应用.本文首先研究了两种地球物理方法的交叉梯度联合反演算法,在此基础上,推导并实现了多种地球物理方法（大地电磁,重力,磁法,地震初至波走时）的多交叉梯度约束的二维联合反演算法;其次,我们设计了结构不一致模型和复杂模型,针对多物性联合反演算法的准确性和有效性进行了模拟试算,并对复杂模型的单独反演结果和联合反演结果进行了交叉梯度值和物性交会图的对比;最后,本文将成熟的卫星资料多光谱综合分析技术应用到联合反演中,将多物性参数反演模型结果图通过RGB（红-绿-蓝）模式进行合成,得到融合的RGB合成图.结果表明：通过对结构不一致模型和复杂模型的联合反演结果和单独反演结果的对比分析,可以得出联合反演得到的结果更接近真实模型,并从得到的交叉梯度值进一步证明了联合反演模型相似度高,也从物性交会图中得到联合反演的物性相关性更好的结论,反向证明了算法的正确性.最终从得到的RGB合成图像,我们可以更直观的分析反演结果,更有利于准确划分地下模型结构.

     

电导率随深度分层线性变化的大地电磁响应

本文在电导率随深度分层线性变化的地球模型上,研究了大地电磁的正演问题.由Airy微分方程导出了电场和磁场水平分量的解、地面阻抗的递推表达式和视电阻率的计算公式;并证明了分层线性模型与均匀层状模型一样,也存在高阻薄层的H等值性和低阻薄层的S等值性;总结了电导率随深度的变化率α,对大地电磁测深曲线的影响规律.     

大地电磁有限差分数值解对比

基于网格精度、形成系统方程的方式、边界条件以及预条件线性算子等,文中对大地电磁（MT）有限差分数值解作了对比.对不同网格剖分方式下的三个均匀半空间模型的一维MT响应对比显示,在降低首层厚度的同时保持层间厚度变化在合理范围可以同时提高主场和辅助场的精度.在利用正常中心网格法（主场和辅助场都定义在单元顶面的中心）计算二维（2-D）TM模式响应时,应该从Maxwell一次差分方程开始组建二次差分方程,这样可以更充分考虑模型电阻率的变化.在对边界值如何影响数值解的测试表明,仅仅提高一维（1-D）边界值的精度对提高2-D MT有限差分数值解的精度是有限的.线性算子对提高MT解的效率十分重要,简单的对比进一步表明合适的预条件再配合好的线性算子（如文中求解2-D MT时所采用的DILU-BICGSTAB方法）不仅可以加速收敛,而且可以降低迭代次数.     

大别山超高压变质带深部电性结构及其动力学意义初步研究

通过在大别造山带东部横穿超高压变质带的一条NNE向剖面大地电磁测深资料的分析解释,获得了关于沿剖面的地壳上地幔二维电性结构,显示北淮阳与大别地块是电性差异显著的构造单元,它们之间的界面与晓天—磨子潭断裂对应;晓天—磨子潭断裂倾向北,在中上地壳层位出现错动解耦现象;从地表向深处可划分出4个主要电性层：地表风化层、中上地壳高阻层、壳内相对高导层以及上地幔层;大别地块内中、上地壳层位以高阻层为主,与高压-超高压变质岩分布区对应,高阻层最厚处在岳西—英山之间;在大别地块内,推测存在燕山期花岗质岩浆活动的通道,它们造成了超高压变质岩的进一步抬升,同时影响了大别地块内存在的壳内相对高导层的分布,壳内相对高导层在层位上相差较大.     

A magnetotelluric study of the Damara Belt in Namibia: 2. MT phases over 90° reveal the internal structure of the Waterberg Fault/Omaruru Lineament

Magnetotelluric (MT) observations at some sites in the vicinity of the Waterberg Fault/Omaruru Lineament (WF/OL), a major tectono-stratigraphic zone boundary in the Central Zone of the Damara Belt, show evidence for strong three-dimensional (3D) effects. We observe very high skew values, phases over 90°, and a strong correlation of parallel components of the electric and magnetic fields at long periods. Because of the dense site spacing and good spatial coverage, we can positively attribute these effects to local geology and are able to resolve structural detail within the WF/OL. Mapping LaTorraca’s electric characteristic vectors in form of ellipses proved particularly useful in identifying key elements of the conductivity structure for subsequent modelling. 3D and 2D anisotropic modelling can reproduce most of the observed 3D effects. The conductivity anomalies revealed in the area are: (i) a conductive ring structure in the shallow crust along the northern part of the profile; (ii) an anisotropic region in the upper crust with high conductivity parallel to the WF/OL; (iii) anisotropy in the lower crust with a different but undetermined strike direction; and (iv) a shallow elongated conductor sub-normal to the WF/OL. Modelling studies further suggest that the (anisotropic) fault zone is approximately 10 km wide and may reach down to a depth of 14 km or more.     

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.     

3D magnetotelluric characterization of the geothermal anomaly in the Llucmajor aquifer system (Majorca, Spain)

In the Llucmajor aquifer system (Majorca Island, Spain) some geothermal evidences have appeared. This phenomenon is not isolated to Majorca and it is present in other areas, where it can be associated with structural conditions, especially to the extensional event suffered by the island after the Alpine Orogeny. However, the origin of this anomaly in Llucmajor is not well known, and there is no surface geological evidence of these structural conditions. With the aim of delineating the geoelectrical structure of the zone and identifying the geological structure that allows the presence of this anomaly, an audiomagnetotelluric (AMT) survey was carried out. The AMT data was processed using a Wavelet Transform-based scheme. Dimensionality analysis indicates that the geoelectrical structure is mainly 3D. The 3D model was obtained by trial and error forward modeling, taking accounting of the responses from the determinant of the impedance tensor. The model shows a vertical resistivity distribution with three horizons associated with different units: on the top, a shallow high resistive media related to an unconfined shallow aquifer; in the middle, a conductive layer related to the aquitard, and below it, another resistive media related to the confined deeper aquifer. The intermediate horizon shows a sudden thinning beneath the thermal anomalous zone that can be identified as a weakness zone (fault or fracture) connecting both aquifers. An exploratory well was drilled after the AMT survey and reached almost 700 m in depth. This allowed correlating the resistivity distribution of the 3D model with data logging and lithology obtained from the well, showing a proper agreement between them.     

山西阳高～河北容城剖面大地电磁资料的初步解释──阻抗张量分解技术及其应用

介绍了阻抗张量分解技术及其在阳高～容城剖面的大地电磁资料解释中的应用。在某些测点局部电流畸变的影响较严重，用张量分解技术对资料分析后表明，沿剖面的区域异常总体上具二维性质，分解技术处理后的视电阻率和相位资料与地质构造有较好的对应性。事实表明，阻抗张量分解技术是对大地电磁资料进行分析解释的有力工具     

Study of the Structure of the Shenzhen Fault Zone with the Magnetotelluric Method

The magnetotelluric sounding method was used to study the active fault in Shenzhen city.Four magnetotelluric profiles with a dense station interval were laid out across the Shenzhen fault zone.The remote reference technique was used in both data observation and processing to eliminate the electromagnetic noise near the survey sites,and relatively smooth apparent resistivity curves were obtained.TM mode data and two-dimensional inversion method as NLCG were used to obtain the electrical structures underground.According to the surficial geology survey results on regional strata and distribution of magmatic bodies and faults,the electrical structures at depths less than 2000m of each profile were interpreted.Two regional faults,the Henggang-Luohu fault and the Liantang fault,and seven local faults consistent with the electrical boundaries were verified or discovered.The strata in the survey area were then related with the change of resistivity.Electrical horizontal slices of depths less than 5000m indicate that the Henggang-Luohu fault and the Liantang fault bifurcate at the shallow part but tend to merge in the deep part.     

青藏高原东缘川滇构造区深部电性结构特征

本文对位于青藏高原东缘川滇构造区的贡山一绥江大地电磁测深(MT)剖面数据进行反演,获得沿剖面的深部电性结构,为研究喜马拉雅东构造结、川滇菱形地块与华南地块的构造变形特征、壳幔耦合关系、地块间接触关系以及相互作用等问题,提供电性结构的依据.研究发现:(1)电性结构揭示澜沧江断裂带和小金河断裂带为深大断裂带,控制着研究区的深部结构特征和形变机制;(2)澜沧江断裂带和金沙江断裂带之间的高阻体,可能是扬子古地块的残留部分;小金河断裂带和安宁河断裂带之间的高阻体,则是峨眉山大火山省喷发形成的冕宁一越西杂岩带;(3)在滇西地块、川滇地块和大凉山地块均存在低阻层,它们的介质属性有所不同,滇西地块下的低阻层"疑似"高热状态的岩浆囊,主要由缅甸弧向东俯冲运动引起的,中上地壳的高热状态使地块的活动性增强;川滇地块内部的壳内低阻层的成因为:理塘断裂带和小金河断裂带之间的地表低阻层由破碎带充水所致,而金沙江断裂带和理塘断裂带之间的中地壳低阻层可能是由局部熔融物质或含盐流体导致的,其为壳内物质运移的通道.从而在地下物质发生大规模走滑运动的过程中起到引导作用;川滇地块东部和大凉山地块西部的壳内低阻层可能与地慢物质的上涌有关;马边断裂带附近的低阻体可能与破碎带变宽和破碎带内的流体有关.