Anisotropy of magnetic susceptibility of rocks in the Rayleigh Law region: Modelling errors arising from linear fit to non-linear data

The anisotropy of magnetic susceptibility (AMS) within the Rayleigh Law range was investigated theoretically, using mathematical modelling. It was revealed that the orientations of the principal susceptibilities and the shape parameter vary with field so weakly that these variations can be regarded as negligible from the practical point of view. The degree of AMS increases with field according to the degree of anisotropy of the initial susceptibility used and according to the intensity of susceptibility change with field of the mineral considered. The degree of AMS calculated using linear theory is very near to the degree of AMS following from the analysis of AMS within the Rayleigh Law range. If it is desirable to correct the field-dependent degree of AMS, a simple technique is suggested based on measurement of the AMS in two fields. fhrouda@agico.cz     

中强地震前菏泽、郯城地震地电阻率各向异性值变化特征

对山东菏泽和郯城两台站地震地电阻率观测资料进行处理的基础上,研究了地震地电阻率各向异性的前兆异常特征,得出两台站周围300 km内的5级以上地震发生前均存在明显的地震地电阻率各向异性前兆异常,在中强地震发生时其各向异性值基本处于高值附近。对其物理机理进行了初步的探讨。     

Experimentally Determined P-Wave Velocity Anisotropy for Rocks Related to the Western Bohemia Seismoactive Region

The elastic properties of granites from Western Bohemia which we measured and published earlier have been supplemented with measurements of olivine nephelinite from the same region and with data on lherzolitic xenolith from the vicinity of elezný Brod. The set of velocities measured under laboratory conditions has been compared with depth profiles suggested for the purpose of locating seismic swarm events which occur in this region. P-wave velocities were measured under pressures of up to 400 MPa. This pressure corresponds to depths of about 15 km for this region. The data were extrapolated for larger depths. Comparing laboratory measurements and seismic profiles, we can conclude that the rocks under study may be constituents of crust structures.     

永登地震剪切波分裂及电性各向异性的变化特征

依据永登地震前后小震三分量数字记录和中法合作松山电磁台的观测资料,基于EDA裂隙模型,从各向异性的角度,将地震剪切波(S波)分裂方法和大地电磁测深(MT)方法共同应用于地震孕育过程中前兆的各向异性特征的研究.结果表明,永登地震前,快S波偏振方向由北偏东逐渐转为北偏西,发震后又急转为北偏东,快、慢S波的时间延迟在震前也是急剧上升的.另外,MT电性主轴的计算表明,各频点的电性主轴方向基本一致,原与断层近于垂直的电性主轴(ρxy),从震前为北偏东17°左右逐渐转为北偏西15°并发震,震后恢复到北偏东.相应的视电阻率变化表明160～226秒周期的变化最大,其对应的深度与该次地震的震源深度相当.而震前平行于断层的电性主轴其视电阻率ρyx变化相对小些.这次地震的震源机制解表明其发震应力场的P轴是北偏西15°.结论是(1)从各向异性的方向特征看,不同方法得到的3个方向在总体上是一致的,时间演化过程是协调的,其机理就是应力所致的EDA裂隙定向排列.(2)不同频率电性主轴变化的一致性表明,震前应力所致裂隙定向排列而导致的各向异性在地壳一定范围内是广泛存在的.(3)视电阻率量值的变化也存在各向异性并有其自己的特点.视电阻率静态的各向异性特征是由台站所处构造条件决定的,但动态各向异性变化受地震应力场作用的影响;与P轴一致的电性主轴视电阻率ρxy变化最大;震前同一方向、不同频率视电阻率变化的差异表明其变化强度还取决于介质的分层性质.     

厚层河道砂体储层非均质性研究——以葡萄花油层组PⅠ1-PⅠ4小层为例

储层非均质性对油气田的勘探和开发效果影响很大。结合测井资料二次解释的孔隙度（Φ）、泥质含量（Vsh）、粒度中值（Md）、束缚水饱和度（Swi）等资料,借助BP网络对渗透率进行预测。对萨尔图油田中部葡萄花油层组的非均质性进行了定量半定量的研究,认为萨尔图油田中部葡萄花油层组PⅠ2小层层内非均质性最严重,与沉积微相密切相关。单砂层垂向上渗透率的变化以正韵律和复合式韵律为主,局部发育反韵律模式。葡萄花油层组PⅠ1—PⅠ4小层层内非均质性强弱依次为PⅠ2a,PⅠ3,PⅠ2b,PⅠ1,PⅠ4;层间非均质性在PⅠ3和PⅠ4间表现得最强,其他相对要弱;各小层平面非均质性相差无几,整体表现为较强的非均质性。     

Analysis and comparison of mixed finite element and multipoint flux approximation methods for homogeneous media

We consider discretization on quadrilateral grids of an elliptic operator occurring, for example, in the pressure equation for porous-media flow. In a realistic setting – with non-orthogonal grid, and anisotropic, heterogeneous permeability – special discretization techniques are required. Mixed finite element (MFE) and multipoint flux approximation (MPFA) are two methods that can handle such situations. Previously, a framework for analytical comparison of MFE and MPFA in special cases has been suggested. A comparison of MFE and MPFA-O (one of two main variants of MPFA) for isotropic, homogeneous permeability on a uniformly distorted grid was also performed. In the current paper, we utilize the suggested framework in a slightly different manner to analyze and compare MFE, MPFA-O and MPFA-U (the second main variant of MPFA). We reconsider the case previously analyzed. We also consider the case of generally anisotropic, homogeneous permeability on an orthogonal grid.     

甘肃定西原状Q_3黄土各向异性试验研究

通过室内三轴试验,研究了甘肃定西原状Q3黄土的各向异性对于黄土抗剪强度和变形参数的影响。研究结果表明,此种黄土的各向异性对土体力学性质的影响显著。垂直向原状黄土抗剪强度明显高于水平向,随着围压的增大,二者之差明显下降,最终稳定在10%的差值范围内。围压小于100 k Pa时,水平向破坏应变明显大于垂直向,且随着围压逐渐增大二者差值逐渐减小,并在围压高于200 k Pa时,破坏应变达到一致。利用邓肯-张模型对偏差应力实测值与计算值进行差值分析表明,低应变处垂直向与水平向的差值均较大;随着围压增大,差值逐渐减小并最终稳定在±5%差值范围内,且最大偏差应力均符合误差为±5%的范围。无论是垂直样还是水平样,大约在2%应变之前的应力应变曲线都近似为回归系数很高的线性关系,可近似估算不同方向黄土的变形模量,其值分别为130 MPa和85 MPa。     

页岩储层岩石物理模型及地震AVO响应模拟

通过模拟并分析各向异性页岩岩石物理模型,研究页岩中干酪根含量变化对页岩弹性各向异性的影响,其结果为应用AVO方法定量解释页岩储层厚度和干酪根含量提供依据。岩石物理模型预测了页岩速度和各向异性参数随干酪根含量的变化规律。应用传播矩阵方法计算薄层页岩的AVO响应,得到反射系数关于页岩层厚度、入射波频率、干酪根含量和入射角的函数。数值模拟结果表明:在干酪根含量不变且在同一入射角下的反射系数在频率域呈周期变化,该周期与薄层厚度成反比;对同一频率入射波和相同入射角,反射系数幅值随干酪根含量增加而变大。     

无侧限压缩条件下黏性土磁各向异性研究

近年来基坑、隧洞等开挖工程活动中的卸载导致黏性土体发生过大侧向变形,引起的工程事故越来越多。目前多采用测斜仪对基坑周围土体变形进行水平位移监测,然而由于软土的特殊工程性质,在监测中软土无法带动测斜仪一起变形,往往监测效果不尽人意,不能准确给出实际软土的真实变形。因此,需要加强开挖卸荷工程施工中土体的侧向变形监测技术研究工作。本文基于磁性矿物受荷载变形具有定向性特征,运用磁组构试验对无侧限压缩后的黏性土试样进行磁各向异性研究,同时与有侧限压缩试验变形测试结果进行比较,分析试验轴向应力、土样含水率对无侧限压缩条件下磁各向异性的影响规律,以及主磁化率值与应变的数学关系,探讨了由黏性土磁各向异性推求其受力变形规律的可行性。研究成果表明,受力作用的黏性土变形与其磁性矿物定向性具有较好的一致性。这一认识对于有效监测软土地区卸荷工程周围软土变形情况具有重要的理论意义和实际应用价值。