Applications of hybrid genetic algorithms in seismic tomography     

Pantelis Soupios  Irfan Akca  Petros Mpogiatzis  Ahmet T. Basokur  Constantinos Papazachos 《Journal of Applied Geophysics》2011,75(3):479-489

Almost all earth sciences inverse problems are nonlinear and involve a large number of unknown parameters, making the application of analytical inversion methods quite restrictive. In practice, most analytical methods are local in nature and rely on a linearized form of the problem equations, adopting an iterative procedure which typically employs partial derivatives in order to optimize the starting (initial) model by minimizing a misfit (penalty) function. Unfortunately, especially for highly non-linear cases, the final model strongly depends on the initial model, hence it is prone to solution-entrapment in local minima of the misfit function, while the derivative calculation is often computationally inefficient and creates instabilities when numerical approximations are used. An alternative is to employ global techniques which do not rely on partial derivatives, are independent of the misfit form and are computationally robust. Such methods employ pseudo-randomly generated models (sampling an appropriately selected section of the model space) which are assessed in terms of their data-fit. A typical example is the class of methods known as genetic algorithms (GA), which achieves the aforementioned approximation through model representation and manipulations, and has attracted the attention of the earth sciences community during the last decade, with several applications already presented for several geophysical problems.In this paper, we examine the efficiency of the combination of the typical regularized least-squares and genetic methods for a typical seismic tomography problem. The proposed approach combines a local (LOM) and a global (GOM) optimization method, in an attempt to overcome the limitations of each individual approach, such as local minima and slow convergence, respectively. The potential of both optimization methods is tested and compared, both independently and jointly, using the several test models and synthetic refraction travel-time date sets that employ the same experimental geometry, wavelength and geometrical characteristics of the model anomalies. Moreover, real data from a crosswell tomographic project for the subsurface mapping of an ancient wall foundation are used for testing the efficiency of the proposed algorithm. The results show that the combined use of both methods can exploit the benefits of each approach, leading to improved final models and producing realistic velocity models, without significantly increasing the required computation time.  相似文献   

利用多种地球物理观测资料直接反演地幔对流模型   总被引：4，自引：3，他引：4       下载免费PDF全文

傅容珊  黄建华 《地球物理学报》1993,36(3):297-307

假定地幔为一个均匀的、粘滞系数为常数、同时均匀分布放射性热源的流体球层,其内部存在的对流则由流体力学3个基本方程:运动方程、能量方程和连续性方程确定.如果假定地幔处于低瑞利数的状态（临界瑞利数1.5倍左右）,那么上述方程中的非线性项可以忽略不计.作为一类可能的模型,本文计算一组用6个边界条件确定6个未知数的线性方程组.这些条件包括板块绝对运动极型场、地球大地水准面异常和地震层析结果提供的地幔密度分布横向不均匀相应的“刚性地球”水准面异常等.模型计算表明:1.地幔中流体运动格局不仅受地幔热动力学参数（瑞利数）控制,而且强烈地受边界条件的影响.2.若不限定下边界为等温边界,则上、下地幔之间并不呈现出活动性明显差异;但是在模型瑞利数加大到一定值时,核-幔边界附近将出现一些局部的小尺度对流环.3.当模型瑞利数从很小增加时,对流格局将发生变化,这些格局可能反应由地幔热动力学参数决定的地幔固有特性.4.当瑞利数为50000和80000时,核-幔边界形变与PcP波得到的结果吻合较好.  相似文献   

井间电磁波CT中的非块体分段反演算法   总被引：1，自引：0，他引：1  

吴建平  冯锐 《CT理论与应用研究》1993,(3)

本文给出了用网格节点表示的介质中直射线追踪的算法。在实际工作中,我们常发现当井间CT 应用于强不均匀体存在的区域时,反演结果往往会严重失真,为了解决这一问题,我们采用对测量数据分段反演的新算法。首先利用强信号(即弱衰减)确定衰减量较小的背景介质的衰减系数,固定背景介质的衰减系数(或对其加入强约束),然后逐渐用较弱的信号确定衰减系数较大的介质在井间的分布,直至反演出整个井间的吸收系数分布。实践表明,当强不均匀体存在时,运用这一方法得到的结果明显优于不作分段时得到的结果。  相似文献   

大别─苏鲁碰撞造山带的地震层析成像研究──岩石圈三维速度结构   总被引：56，自引：6，他引：50       下载免费PDF全文

徐佩芬  刘福田  王清晨  从柏林  陈辉  孙若昧 《地球物理学报》2000,43(3):377-385

对包含大别－苏鲁碰撞造山带在内的东经 １１２°－１２４°,北纬２８°－３９°区域进行地 震层析成像研究,重建其地壳及上部地幔的三维速度图像．结果表明：造山带岩石圈速度横 向不均匀性显著;大别造山带以商城－麻城断裂为界,东侧的大别地块与西侧红安地块在地 壳速度上是两个不同的速度块体;中地壳 １５-２５ｋｍ深度范围内存在低速带,与伸展滑脱构造 有关;南、北大别构造单元之下,莫霍面下凹,地壳内发育了速度为６．５－６．６ｋｍ／ｓ、向北倾斜的 相对高速体,与超高压变质岩体相对应;在大别－苏鲁造山带下方的上部地幔中存在向北倾 斜的板片状高速体,结合已有地质、地球化学证据推测,它是三叠纪俯冲的扬子地块的残留 体;俯冲板片在深部发生了断离．本文利用地震层析成像方法揭示的造山带岩石圈速度结构 细节,对研究与地表地质有关的地球动力学无疑是十分重要的．  相似文献   

Resistivity tomography study on samples with water-bearing structure     

郝锦绮  冯锐  周建国  钱书清  高金田 《地震学报(英文版)》2000,13(3):325-330

IntroductionThestudyonthevariationoftheeIectricaIpropertiesofsimulatingsamplesatcertainphysi-calconditionsinthelaboratoryisanimportantmethodtounderstandthecomposition,movementandevolutionofthematerialsintheseismicsourceregionduringseismogenicprocessandisaIsoanimPortantsupplementtothefieldexperimentofeathquakemonitoringbymeansofelectricaImethod.ThepreviouslymadesimuIationexperimentsindicatethatthevalueofapparentresistivityofwatCrsaturatedintactrocksamplesmaydecreaseoneorderofmagnitudebeforefr…  相似文献   

Signature of remnant slabs in the North Pacific from P-wave tomography   总被引：1，自引：0，他引：1  

A. Gorbatov  S. Widiyantoro  Y. Fukao  & E. Gordeev 《Geophysical Journal International》2000,142(1):27-36

A 3-D ray-tracing technique was used in a global tomographic inversion in order to obtain tomographic images of the North Pacific. The data reported by the Geophysical Survey of Russia (1955–1997) were used together with the catalogues of the International Seismological Center (1964–1991) and the US Geological Survey National Earthquake Information Center (1991–1998), and the recompiled catalogue was reprocessed. The final data set, used for following the inversion, contained 523 430 summary ray paths. The whole of the Earth's mantle was parametrized by cells of 2° × 2° and 19 layers. The large and sparse system of observation equations was solved using an iterative LSQR algorithm.
A subhorizontal high-velocity anomaly is revealed just above the 660 km discontinuity beneath the Aleutian subduction zone. This high-velocity feature is observed at latitudes of up to ~70°N and is interpreted as a remnant of the subducted Kula plate, which disappeared through ridge subduction at about 48 Ma. A further positive velocity perturbation feature can be identified beneath the Chukotka peninsula and Okhotsk Sea, extending from ~300 to ~660 km depth and then either extending further down to ~800 km (Chukotka) or deflecting along the 660 km discontinuity (Okhotsk Sea). This high-velocity anomaly is interpreted as a remnant slab of the Okhotsk plate accreted to Siberia at ~55 Ma.  相似文献   

成像区外异常体对电阻率层析成像结果的影响     

毛先进  鲍光淑 《物探化探计算技术》2000,22(1):18-20,72

作者研究了成像区外不均匀体对电阻率层析成像结果的影响、其结果表明：成像区外非均匀体的存在对成像结果的影响是明显的;在用地面和钻孔中采集的数据成像时这种影响表现为产生一个相对于钻孔呈大致对称关系的假像;这种影响着区外非均匀体远离成像区而减小,。当区外非均匀体与距其较近的一口井的距离大于井的一半时,这种影响即可忽略。  相似文献   

中国天然地震探测深部构造的研究现状与发展   总被引：3，自引：2，他引：1  

钱辉  姜枚  宿和平  董英君 《物探与化探》2000,24(1):1-6,11

在青藏高原等地区为研究下地壳上地幔的构造特征,最近10a投入了较大工作量的天然地震探测。它可以获得较大深度上的地震速度图像、上地幔的各向异性,划分出低速体低速层,为研究地球动力学特征提供了新的资料。随着仪器的发展完善,这种方法在我国深部研究中成为重要而且经济的手段。地震层析、接收函数的研究、以及剪切波分裂的分析都取得了很大的成绩。在这些方面还有许多工作要深入研究,继续深入发展具有重要意义。  相似文献   

颅内钙化性转移的CT诊断   总被引：1，自引：1，他引：0  

李晓阳  张立君  钟洪波  马巨文  李会菊  胡文检 《CT理论与应用研究》2004,13(3):19-23

目的总结钙化性转移的CT表现及其与原发肿瘤的关系.材料与方法复习经过病理及手术证实身体各部位存在原发恶性肿瘤并伴有身体其他部位钙化性转移的病例25例,病人年龄34-83岁,其中肺癌20例,乳腺胶质癌3例,结肠粘液腺癌3例,成骨性骨肉瘤1例,卵巢粘液腺癌2例,采用CT平扫加强化扫描.结果 20例患者CT扫描显示,颅内钙化性转移表现为脑实质内结节状或不规则斑片状高密度影,边缘欠清晰,增强扫描结节状及斑片状高密度影轻微强化,边缘变清楚,外形轻微增大.结论钙化性转移是一种少见的转移方式,当某一患者身体某一部为患有原发癌,而其它部位发现钙化性病变,不应忽视钙化性转移的可能.  相似文献   

胆囊癌CT误诊分析   总被引：1，自引：0，他引：1  

伊东升  何玉梅  王洪兵 《CT理论与应用研究》2004,13(2):51-53

目的分析胆囊癌CT误诊原因,以提高诊断正确率.方法搜集经手术病理证实的胆囊癌病例43例,均行CT扫描,其中误诊23例.与手术结果对照,分析误诊原因.结果 23例误诊中,诊断为急慢性胆囊炎5例,胆囊息肉1例.肝门区胆管癌2例,胆管癌1例,胆总管下端.壶腹部占位3例,胰头癌5例,胆总管炎症1例,梗阻性黄疸2例,胆囊肿大2例,结肠癌1例.结论明确胆囊癌的CT征象及其转移途径,可以避免误诊,提高诊断正确率.  相似文献