安徽省铜陵地区是我国著名的矿集区之一,是长江中下游成矿带的重要组成部分。为研究其深部结构,SinoProbe-03项目部署了6条大地电磁剖面。通过数据维性分析,表明该区深部2D/3D构造特征明显,进行3D反演是必要的。通过挑选适当的测点和频域数据,设计合理的参数,利用WSINV3DMT程序,进行了大地电磁3D反演研究,得到了铜陵矿集区3D电性模型;利用模型响应与实测数据的拟合对比对反演结果进行了评价,表明所选反演模型整体可被接受,但部分区域精度低。该结果的剖面切片与2D反演模型相比,整体结构一致,局部存在差异;利用不同深度、不同方向的空间切片,讨论了该3D反演模型的电性结构及其所揭示的铜陵矿集区构造格局,浅部的局部电性差异大于深部;平面的电性分布显示了一定的分区特征;垂向电性分布显示出一定的分层特征;高阻体的分布代表着侵入岩的活动范围;深部的低阻区反映了该段下地壳未受到大规模岩浆的扰动。

塔里木盆地内油气资源非常丰富,但是深层勘探程度低,需要开展深入的地球物理调查。经过在盆地内大地电磁测站的数据采集、资料处理和三维反演,取得盆地三维电阻率模型,从电性结构角度刻划了盆地的三维构造。塔里木盆地的碎屑岩和碳酸盐岩的电阻率变化范围很大,主要受其中的孔隙裂隙发育程度控制。在同一深度的平面上,盆地侏罗纪以前地层的构造隆起显示为相对高阻区,而深层低阻带主要反映古生代碳酸盐岩中的裂隙和流体汇聚,提供了油气储层分布的信息。从6km到10km深度,低电阻率区面积向下缩小,但沉积盆地总面积仍有近20万km^2。由于油气成藏伴随着活跃的流体活动,而孔隙流体呈现低电阻率,可以认为低电阻率区分布范围和产状指示深层流体活动及可能的油气储层的分布范围。深层电阻率平面图显示的低电阻率区包括：满加尔坳陷中北部到塔北隆起西段,塔西南的和田坳陷与莎车坳陷。于田-民丰坳陷以及唐古孜巴斯坳陷在深度6-10km的平面图也显示为较低电阻率,也是深层油气勘探的有利区段。塔里木盆地结晶基底可分为三种类型：正常的克拉通结晶基底呈现高电阻率,玄武岩浆侵位的基底对应高电阻率区,而含裂隙水的变质基底呈现低电阻率。对比大地电磁法和反射地震结果认为,地球物理调查资料不支持塔里木盆地二叠纪＂大火成岩省＂的猜测。

当前,大地电磁测深法一维、二维反演技术是实际资料处理和解释主要应用的技术手段,三维反演技术随着算法的改进和硬件设施的提升也取得重要进展,但三维反演计算还存在计算速度慢、消耗大量内存等问题,导致其在实际应用中并不多见.本文以长江中下游重要成矿区带及邻区大地电磁资料为例,以REBOCC三维反演算法作为计算手段,在具有16G内存的普通计算机工作站上开展了武汉图幅面积约25.68万km2的大面积区域范围的大地电磁三维反演工作,计算耗时约一个半月,取得较为理想的三维反演结果.研究区内共布置完成大地电磁测深点425个,由于测点数目较多,工区范围较大,单台工作站一次性进行全部数据三维反演计算根本无法运行,这就导致必须选择采用分区块、抽稀频点的解决办法.课题组首先将整个工区划分为6个子区块,相邻2个子区块之间保留一定的重叠区域,同时将每个测点抽稀使用20个频点;其次,将每个子区块依次轮流在工作站上进行三维反演计算,分别取得相应子区块三维反演结果;最后,将每个子区块的三维反演结果在重叠区域按照一定的加权规则进行拼接,最终得到长江中下游地区岩石圈三维电性结构模型.长江中下游地区是我国重要的铁铜矿产业基地,揭露岩石圈的三维空间电性结构特征,为下一步找矿突破与预测具有重要指导意义.本文通过对长江中下游重要成矿区带及邻区大地电磁测深资料的三维反演研究表明:三维电磁探测技术可行、优势明显,通过三维反演计算可以得到研究区内纵向和横向上的电性分布特征;在现代观测设备和先进的计算技术的支撑下,深部三维电性结构模型可以为认识地下三维地质结构提供丰富信息;针对大面积区域范围的大地电磁资料在单台工作站上先实行分块反演计算再进行拼接的工作方法同样可以取得较为理想的三维反演结果,其在工作成本和时效性等方面均可以接受.

地热资源的埋藏性和地下空间的复杂性,使得地热调查技术的选取显得尤为重要.介绍了电磁测深在地热调查中的应用现状,针对MT法在深部平原埋藏型地热探寻中的缺口,以银川盆地地热调查为例,利用电磁测深MT技术解译银川盆地热储范围,并结合盖层温度、控热断裂给予分析,结果显示,利用电磁测深MT法可以较好地圈定银川平原深部热储范围.说明了电磁测深MT技术在平原区深部埋藏型地热调查中是适用的.

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.

本文用有限的篇幅,重点介绍近几年地球电磁法在深部结构探测、能源和资源勘探、环境和工程探查、地震等灾害监测以及新技术和软件等方面的最新发展状况,介绍几个有发展和应用潜力的新电磁技术.

In defining apparent resistivity and impedance phase for transverse magnetic (TM) modeling of 3-D bodies, a coordinate system derived using tipper-strike, calculated at the frequency for which tipper magnitude due to the structure of interest is large relative to that due to any nearby geological noise, is recommended.

In recent years, the number of publications dealing with the mathematical and physical 3-D aspects of the magnetotelluric method has increased drastically. However, field experiments on a grid are often impractical and surveys are frequently restricted to single or widely separated profiles. So, in many cases we find ourselves with the following question: is the applicability of the 2-D hypothesis valid to extract geoelectric and geological information from real 3-D environments? The aim of this paper is to explore a few instructive but general situations to understand the basics of a 2-D interpretation of 3-D magnetotelluric data and to determine which data subset (TE-mode or TM-mode) is best for obtaining the electrical conductivity distribution of the subsurface using 2-D techniques. A review of the mathematical and physical fundamentals of the electromagnetic fields generated by a simple 3-D structure allows us to prioritise the choice of modes in a 2-D interpretation of responses influenced by 3-D structures. This analysis is corroborated by numerical results from synthetic models and by real data acquired by other authors. One important result of this analysis is that the mode most unaffected by 3-D effects depends on the position of the 3-D structure with respect to the regional 2-D strike direction. When the 3-D body is normal to the regional strike, the TE-mode is affected mainly by galvanic effects, while the TM-mode is affected by galvanic and inductive effects. In this case, a 2-D interpretation of the TM-mode is prone to error. When the 3-D body is parallel to the regional 2-D strike the TE-mode is affected by galvanic and inductive effects and the TM-mode is affected mainly by galvanic effects, making it more suitable for 2-D interpretation. In general, a wise 2-D interpretation of 3-D magnetotelluric data can be a guide to a reasonable geological interpretation.

In this paper, some recent topics on the modeling of magnetotelluric data are introduced. The focus is on the handling of real field data for two-dimensional resistivity modeling. First, the removal of the effects of near surface heterogeneity is reviewed. It covers telluric distortions (phase mixing and static shift) and magnetic distortions using conventional Groom-Bailey type 3D/2D model (three-dimensional local anomaly underlain by regional two-dimensional structure). The extension of a 3D/2D distortion model for multi-site, multi-frequency is a new development. Magnetic distortion seems to be less significant for land observations, but significant for sea floor data, where the regional magnetic field is weak due to seawater. In special cases involving for example, distortion due to topography and bathymetry, explicit removal is possible. There are some schemes proposed for a 3D/3D model (three-dimensional local anomaly underlain by regional three-dimensional structure). Along with the removal of the distortion, it is important to recognize the dimensionality of the dataset prior to modeling. A property using strike estimates for each site is an indicator for dimensionality which is unaffected by local distortion. Mapping the local strike or a rose diagram is an effective visualization of the dimensionality.Two-dimensional inversion is becoming routine. For the fast calculation of derivatives, approximate calculation, reciprocity or conjugate gradient methods are used. In order to incorporate a priori information and to overcome the intrinsic ill-posed nature of the inversion problem, imposing constraints on the model structure is important. A proper tradeoff between the data fit and constraints should be optimized to obtain minimum structures that are required by the field data. However, the choice of constraints is rather subjective and depends on the geological situations. For field data, two-dimensional inversion has limits on modes, area, and period range. Special care must be taken for the structure outside the profile. Two-dimensional inversion incorporating anisotropy is interesting and becoming popular, but the structure may not be unique. Future development in three-dimensional inversion for real datasets should take the above points into consideration.