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大地电磁研究和应用中,面临的实际地质结构多数是三维的,但是当前三维反演技术还不成熟,仍需要利用二维模型进行近似反演解释。在进行二维反演解释时,如何分析认识三维局部畸变的性质,最大限度地克服三维局部畸变的影响,获得尽可能可信的地质解释结果,如何认识三维模型的二维近似程度, 相似文献
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大地电磁法一直以来基本都遵循场源为平面电磁波的假设,这种假设普遍适用于中高频的大地电磁研究,但在具有显著非平面波场特性的区域或开展长周期大地电磁研究时,则对场源平面波的理论基础带来了挑战,若不满足理论假设,将会导致深部探测的可靠性降低,勘探目标定位风险增高.本文通过选择线状、片状及非规则场源模型来模拟极光电集流、赤道环电流和横向波动的波数场源,开展了大地电磁场源效应的模拟研究,对比分析了非平面波模式和平面波模式的响应差别.模拟表明大地电磁场源效应强度与模型平均电阻率正相关,与频率和测点与源水平距离负相关,且与场源的高度、宽度和横向波长等参数均有关系,非平面波模式和平面波模式正演响应在一些情况下具有超过10％的差异.基于场源效应特征,归纳并提出了五种场源效应校正方法,一是计算极限模型来截断场源效应影响频段的频域截断法,二是对多个同步观测的邻近测点进行平均的测点平均法,三是延长测点观测时间的时间域延长法,四是剔除较强垂直磁场分量所对应场源效应较强段数据的时间域剔除法,五是直接考虑场源的反演,五种方法均能一定程度上降低大地电磁场源效应的影响.对于受场源效应影响的大地电磁数据,若直接采用平面波方式进行反演则可能在深部产生较大畸变,若在获知场源相关参数的基础上,可开展考虑场源的大地电磁反演来消除场源效应影响. 相似文献
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提出了一种简单的大地电磁阻抗张量畸变分解方法，引入3d/2d模型，使得全畸变的大地电磁阻抗张量被生意人经，通过对比Swift旋转和Bahr分解后的资料，用联系二的畸变矩阵表示浅层三维电性异常体对区域二维构造的局部畸变影响，通过研究畸变张量矩阵，并对其进行分解，可将局部三维电性不均匀体产生的畸变与区域构造分离开来，从而了解区域构造的维数特征和局部畸变类型及其对区域构造的影响，最后，应用3个测点的实际资料，对这种简单的畸变分解方法进行了检验，结果表明，该方法简单且应用方便，解释效果较好。 相似文献
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大地电磁测深的有效信号为天然电磁场,容易受到人文电磁噪声干扰,如何提高大地电磁资料的信噪比,已经成为制约大地电磁应用和发展的一个重要因素.本文利用地磁台站数据,首先将地磁台站数据转化为长周期大地电磁测深所采集的磁场数据格式,利用远参考方法估算长周期大地电磁测点与地磁台站之间的站间传递函数,包括长周期大地电磁测点电场与地磁台站水平磁场之间的拟阻抗,以及长周期大地电磁测点水平磁场与地磁台站水平磁场之间的水平磁场传递函数,将拟阻抗与水平磁场传递函数相乘便可以得到长周期大地电磁测点本地阻抗的无偏估计.实测数据处理结果表明,该方法可以提高本地阻抗估算的数据质量,减小阻抗估算误差. 相似文献
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局部畸变问题曾经困扰大地电磁资料反演解释几十年,大地电磁三维数值模拟技术的发展为剖析局部畸变特点和得到可靠的反演成像结果提供了技术基础。本文采用三维数值模拟成像方法对典型三维局部畸变模型进行模拟分析。三维数值模拟结果显示:电场分量垂直电性分界面的极化模式视电阻率曲线(对应二维情况下TM模式)在穿越低阻异常体界面时,曲线会先上移后下移,而在穿越高阻异常体界面时,曲线会先下移后上移,这与电性分界面处积累面电荷产生的二次电场有关。三维模型中XY模式、YX模式视电阻率和相位在三维异常体附近的水平变化是呈现近似垂向对称的,该现象与电场垂直跨越电性界面时视电阻率的变化规律是吻合的,当测线分别沿X方向和Y方向展布时,三维情况下的XY和YX模式分别对应二维情况下的TM模式。低阻小异常体对区域构造响应的畸变影响比高阻小异常体要严重。低阻小异常体对二维区域响应的两种极化模式视电阻率和相位都有非常明显的畸变影响,相比较而言对TE模式的畸变要大于TM模式,因此我们在做二维反演解释时,可优先考虑拟合TM模式数据。位于小异常体中心上方测点的三维畸变响应虽然与对应真实二维区域响应的差异比较大,但可以等效于某种二维模型响应,这种由局部畸变造成的假二维响应在实际野外数据的解释中是需要注意的。 相似文献
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在对张量阻抗数据、倾子数据和共轭梯度算法深人分析的基础上,我们实现了大地电磁全信息资料三维共轭梯度反演算法。基于全信息资料的三维共轭梯度反演研究,探讨了同时利用五个电磁场分量整理得到的大地电磁资料进行三维反演定量解释的方法以及全信息数据在三维反演中的作用。理论模型合成数据的反演结果表明,在三维反演中使用张量阻抗和倾子数据结合的全信息数据的反演结果优于只使用张量阻抗数据(或只使用倾子数据)的反演结果,提高了反演结果的分辨率和可信度。合成数据的反演算例也验证了所实现的大地电磁全信息资料三维共轭梯度反演算法的正确性和稳定性。 相似文献
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《地球物理学进展》2016,(4)
在大地电磁实测数据中阻抗张量次对角元素的相位值超出正常象限(一、三或二、四象限)的现象称为相位超象限.这种现象无法利用各向同性介质的一、二维反演来解释,而与特定形态的三维各向同性介质有关的电流槽效应有可能是引起相位超象限现象的原因之一.本文首先重建了四个能引起相位超象限的三维各向同性介质理论模型,对模型结构和响应特征进行了总结.随后结合麦克斯韦方程组介绍了电流槽效应产生的原理,并通过近似解析公式分析了电流槽效应对电、磁场的影响.最后,我们针对最近几年来在青藏高原北缘观测到的相位超象限大地电磁测深数据,重点对河西走廊北侧的三个连续相位异常数据进行了三维反演研究,反演模型能够很好地拟合实测的相位超象限数据.通过对模型中存在于上地壳的不同规模高导异常结构进行三维正演检验,最终确定位于测点北东方向的巴丹吉林沙漠构成了区域高导体,它与紧邻测点东侧的局部高导体相连,在平面上形成了"L"型的高导构造,是引起河西走廊北侧相位超象限大地电磁观测数据的主要原因.除此之外,紧邻测点西侧的孤立高导体也会对相位异常的分布范围产生影响. 相似文献
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MT data from a detailed 84-site grid array (28 × 12 km) in the Paraná Basin (Brazil) reveal a high degree of frequency-independent and parallel amplitude distortion. The Palaeozoic sediments across the survey area are covered by 1.2 km of flood basalts. A deep well-log provides some control regarding the emplacement of thin diabase sills but information regarding vertical feeder dikes is non-existent. The degree of parallel behaviour is identified using anisotropy ratios which quantify the extent and bandwidth of the distortion characteristics. A 2D modelling study is carried out using the concept of a horizontally layered (1D) basin with superimposed, small-scale inhomogeneities. Two likely geological distortion structures are considered. The first represents at- or near-surface (0 to 100 m) inhomogeneities. The second represents thin but vertically elongate dikes. The two distortion effects are found to produce different and characteristic behaviour in the H-polarization (TM) mode model data. The modelled E-polarization (TE) mode data are undistorted and provide control. The model developed to represent vertical dikes gives rise to a characteristic frequency dependence which is distinct from that of the undistorted response. This model also generates subparallel distortion effects which give rise to a form of variance in the response estimates as a function of location. The behaviour occurs, importantly, in both the amplitude and phase of the distorted data. A comparison of the normalized amplitude data, observed and modelled, indicates a high degree of correspondence down to a low frequency limit (< 0.1 Hz) where additional and deep contributions to the observed response become apparent. The modelling results and data characteristics indicate that the survey area is underlain by a series of closely parallel, thin intrusive dikes (a dike swarm). 相似文献
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本文将Ｓ．Ｐａｒｋ１９８４年提出的解决三维大地电磁测深问题的薄层近似方法，用于甘肃省河西地区的大地电磁测深定点复测结果的资料解释，展示了这种方法应用于复杂介质结构下的大地电磁测深工作的良好前景。依据该方法理论而编制的程序有较高的计算精度。 相似文献
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某些大震前本溪井水位会出现固体潮畸变现象,其出现时间与地震前驱波十分相似。多在震前一个星期以内,而其形态和到时也较为清晰。本文分析探讨了水位固体潮畸变的形态与特点、与震前水位趋势性异常及同震阶跃现象的关系,引起水位固体潮畸变的其它干扰因素排除方法,并对进一步利用多井震前固体潮畸变资料进行震前定位的可行性做了初步的分析和探讨。 相似文献
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Topographic irregularities cause some distortions of magnetotelluric (MT) fields. In the vicinity of a topographic feature, the TM-mode distortion increases with the height and inclination of the slope. It is well-known that TM-mode ( E ⊥ ) topographic effects are much greater than TE-mode ( E ı ) distortions. We have made a study of MT anomalies in TM-mode due to two-dimensional topography. In order to reduce these effects, the distortion tensor stripping technique was used. After corrections, the resulting data can be interpreted as if they were obtained over a flat surface and depend only on the subsurface structure. However, this technique sometimes causes some geometrical distortions of the real subsurface structure. One of our aims is to overcome this failure. We have modified the correction coefficients by considering the actual one-dimensional geology. Model studies showed that our approach is especially useful in removing the terrain effects on complex 2D subsurface structures. The other purpose of this study is to emphasize the importance of a proper terrain correction for data from sites having mountainous topography over complex geology, e.g. strike-slip faults, suture zones and rift valleys. Some examples of MT data sets collected from the North Anatolian Fault Zone and from the thrust regions of the Western Taurides will be presented. 相似文献
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M. H. SAFAR 《Geophysical Prospecting》1978,26(3):538-549
It is well known that the application of the “bright spot’ technique has been more successful in marine prospecting than in land prospecting. This is due partly to the problem of distortion of the seismic signal caused by the geophone-ground coupling, especially when carrying out high resolution, shallow seismic surveys in swampy terrain. The effect of geophone-ground coupling on the response of a single geophone to the incident compressional waves has been treated by several authors. However, they have always neglected the influence of mutual interaction between an array of geophones on the response of each geophone forming the array. We show that mutual interaction, which results from the re-radiation of the incident compressional waves by the geophones forming the array, can have considerable effect on the response of each geophone. The effect of the geophone-ground coupling on the response of a seismic channel is considered in the absence and presence of mutual interaction between a group of geophones for the case when the shear wave velocity of the soil varies by a factor of three. 相似文献