Fresnel zone inversion for lateral heterogeneities in the earth

We propose a different kind of seismic inversion from travel-time or waveform inversion for lateral heterogeneities in the earth: Fresnel zone inversion. Amplitude and phase delay of data in several frequency ranges are inverted for model space around ray paths with a width corresponding to the considered frequency so that primary effect of finiteness of wavelength be included. For vertically heterogeneous media, Fréchet derivatives for inversion are obtained very efficiently using the paraxial ray approximation, with nearly similar amounts of computation compared to travel-time inversion. As an example, Fréchet derivatives are computed for a teleseismic observation system for a three-dimensional structure in the lithosphere beneath an array of seismic stations. Even if the used frequency is around 2 Hz, the width of Fréchet derivatives cannot be neglected, particularly near the bottom of the lithosphere. Sensitivity of model parameters to observations is, moreover, different in our approach from conventional travel-time inversion: it is zero along ray paths but large slightly away from them. Some model calculations show that travel-time inversion, particularly with models divided into very fine meshes or blocks, might give misleading results. An example of inversion for a simple Camembert model, in the event that travel-time inversion gives no reliable results, shows how this technique works with much smaller data sets and computation than waveform inversions.     

重磁联合广义反演磁、密度界面

本文利用广义线性反演技术,讨论了综合重磁两种地球物理资料集,联合反演磁、密度界面的方法及效果,所做的模型实验证明了重磁联合反演方法在削减解的非唯一性,提高解的可信度方面,是一种有前途的综合反演方法。     

时间域航空电磁数据的反演

本文用广义逆矩阵理论讨论了时间域航空电磁数据的反演方法.以水平二层大地和球体模型为例进行了计算,结果表明这种方法对时间域电磁数据的反演是行之有效的.对于理论数据,一般仅需迭代数次即可稳定地收敛到真值.通过奇异值分析,可以确定一个模型中的相对重要参数和无关紧要参数;对于求解某一特定参数,可确定哪些数据起决定性的作用.由奇异值分解而得出的信息密度矩阵、模型分辨矩阵,分别给出了模型响应拟合数据和模型参数分辨率的度量.这些信息对于时间域航空电磁系统的设计、野外测量和资料解释都具有重要的参考价值.最后给出了实例.     

Magsat studies over the Indian region

Data collected by Magsat have been extensively used by Indian scientists in studies of the crust beneath India. Results obtained by various workers have been summarized and the reasons for differences in findings have been discussed. It is concluded that methods that work well for higher latitudes do not give the best estimates of crustal field and magnetization in equatorial regions. A better estimate of the crustal component is obtained when the external current contribution is estimated using the symmetry properties of associatedX and Z-fields. Inversion technique that provides stable crustal magnetization in midlatitudes, becomes unstable near the equator. Why such an instability arises and how it can be circumvented are discussed. That the Peninsular shield, the Ganga basin and the Himalayas are three different geotectonic blocks is clearly reflected in the magnetization distribution. A thick magnetic crust under Aravalli, Singhbum and Dharwar suggest these areas to be comparatively stable. In general, seismic, gravity and heat flow data agree characteristically well with the magnetization estimates.     

Magnetics in geoexploration

The magnetic method is the oldest and one of the most widely used geophysical techniques for exploring the earth’s subsurface. It is a relatively easy and inexpensive tool to employ, being applicable to a wide variety of subsurface exploration problems involving horizontal magnetic property variations occurring from near the base of the crust to within the uppermost meter of soil. Successful applications of the magnetic method require an in-depth understanding of its basic principles and careful field work, data reduction, and interpretation. Commonly, interpretations are limited to qualitative approaches which simply map the spatial location of anomalous subsurface conditions, but under favourable circumstances the technological status of the method will permit more quantitative interpretations involving specification of the nature of the anomalous sources. No other geophysical method provides critical input to such a wide variety of problems. However, seldom does the magnetic method provide the complete answer to an investigation problem. As a result, it is generally used in concert with other geophysical and geological data to limit its interpretational ambiguities.     

Source characteristics of the 1992 Nicaragua tsunami earthquake inferred from teleseismic body waves

We analyzed the broadband body waves of the 1992 Nicaragua earthquake to determine the nature of rupture. The rupture propagation was represented by the distribution of point sources with moment-rate functions at 9 grid points with uniform spacing of 20 km along the fault strike. The moment-rate functions were then parameterized, and the parameters were determined with the least squares method with some constraints. The centroid times of the individual moment-rate functions indicate slow and smooth rupture propagation at a velocity of 1.5 km/s toward NW and 1.0 km/s toward SE. Including a small initial break which precedes the main rupture by about 10 s, we obtained a total source duration of 110 s. The total seismic moment isM _o =3.4×10²⁰ Nm, which is consistent with the value determined from long-period surface waves,M _o =3.7×10²⁰ Nm. The average rise time of dislocation is determined to be 10 s. The major moment release occurred along a fault length of 160 km. With the assumption of a fault widthW=50 km, we obtained the dislocationD=1.3 m. From andD the dislocation velocity isD=D/0.1 m/s, significantly smaller than the typical value for ordinary earthquakes. The stress drop =1.1 MPa is also less than the typical value for subduction zone earthquakes by a factor of 2–3. On the other hand, the apparent stress defined by 2E _s /M _o , where andE _s are respectively the rigidity and the seismic wave energy, is 0.037 MPa, more than an order of magnitude smaller than . The Nicaragua tsunami earthquake is characterized by the following three properties: 1) slow rupture propagation; 2) smooth rupture; 3) slow dislocation motion.     

多属性反演技术在储层预测研究中的应用——以珠江口盆地陆丰A区中新统珠江组储层为例

珠江口盆地陆丰A区中新统珠江组中-下段2370层是该地区主力油层之一, 但储层厚度差异大、分布不清, 严重制约了区域勘探部署.本文综合地震、测井和地质资料, 分析得出纵波阻抗和V_P/V_S为研究区识别岩性的敏感参数, 并通过叠前同步反演技术刻画出研究区2370层砂岩分布特征, 整体呈北厚南薄的展布.以地震振幅、分频属性及叠前反演属性为基础, 利用多属性反演技术分析统计了各属性与孔隙度之间的关系, 优选出密度、V_P/V_S、地震导数、LR、V_P、25~30Hz地震分频6种属性为储层物性识别参数, 综合预测研究区2370层储层滨岸砂坝砂体孔隙度发育特征.多属性反演结果表明, 在研究区东北部、西部, 厚层的滩坝储集砂体的孔隙度发育, 普遍在18%以上; 而在中部和东部, 受滩间、坝间泥岩与滩坝砂的叠合区影响呈条带状分布, 孔隙度为10%~18%.预测结果与同期钻井的钻后结果基本一致, 预测成功率在75%以上.多属性反演技术在研究区的储层预测中表现出良好的有效性和实用性, 对类似地区的储层预测也具有借鉴意义.     

台州低空逆温层特征分析

利用浙江省洪家气象站L波段雷达逐日探空资料,统计分析了2007—2011年台州接地逆温和悬浮逆温的出现频率、逆温厚度、逆温强度特征,以及与风向风速的关系。结果表明:台州四季均存在低空逆温层,低空逆温出现频率年变化趋势呈单谷型特征,冬春季节逆温出现频率较高。秋冬季节低空逆温厚度增大,春季厚度变薄,夏季最薄;各时次的悬浮逆温厚度大于接地逆温厚度。07:00接地逆温月变化和季节变化强度都弱于19:00接地逆温的;07:00接地逆温,冬季强度最大(1.4℃/100m);19:00接地逆温在秋冬季节强度(2.2和2.0℃/100m)显著大于春夏季节强度(1.3和1.5℃/100m)。两个时次悬浮逆温强度月变化(0.8~1.2℃/100m)和季节变化(0.9~1.1℃/100m)较小,平均底高和顶高的最低值都在7月份,19:00悬浮逆温平均底高、平均顶高普遍大于07:00悬浮逆温观测值。出现接地逆温层时,低空风速一般≤2 m/s,层底风向集中在W到NW之间和静风,天空多为无云或少云;出现悬浮逆温层时,风速小于6 m/s的比例超过50%,层底风向集中在NW到ENE之间和SSW到SW之间,总云量不小于6的比例达70%以上。四季均有霾出现,霾日出现频率年变化趋势与逆温出现频率都是呈单谷型特征。霾污染权重系数季节变化与两个时次的逆温厚度和07:00逆温强度的变化趋势基本一致:冬春季节的大于夏秋季节的;冬春季节霾的污染程度较为严重,污染权重系数分别为49.0和38.1。     

低高度角地基GPS联合反演大气折射率廓线

为充分利用电磁波在大气中传播所携带的信息反演大气折射率廓线,提高低高度角地基GPS反演折射率廓线的精度,本文通过多目标优化反演算法NSGA-II,基本实现了GPS电磁波传播损耗和相位延迟观测量的最优联合反演。数值试验表明,联合反演效果明显优于传统的GA(Genetic Algorithm)单目标反演算法,加噪试验进一步证明了最优联合反演算法在反演折射率廓线方面的可行性及优越性。