大地电磁资料偏移成像

在研究Zhdanov的二维电磁场偏移成像的基础上, 针对它在某些方面的不足, 研究并提出了改进算法. 首先, 用一维反演结果确定初始背景电阻率, 然后在延拓计算过程中, 利用迭代算法进行逐层修正、 延拓计算, 从而获得较准确的电阻率; 第二, 研究了有限差分方程的改进算法, 根据大地电磁场变化的特点, 设计了纵向以等比级数变网格的迭代算法. 通过以上两方面技术的改进, 经理论模型和实际资料试算验证, 取得了较好的结果.      

二维大地电磁资料的偏移

本文采用反射映像（即U/D成像）原理对大地电磁资料进行成像,将地面观测的波场分解成上行波和下行波,并分别向下延拓,利用上、下行波的时间一致性原理确定地下电性界面的位置.对TE和TM模式的MT响应分别处理可以得到两个深度剖面,两剖面的一致性便为实际反射界面的最佳估计.理论和实际资料的计算表明,MT偏移技术是比较有效的,它可获得地下界面直观的映像.和常规MT反演方法相比较,该方法具有算法简单等优点,并且还能获得表示地下地质构造真实映像的MT深度剖面.     

二维大地电磁资料的偏移

本文采用反射映像（即U/D成像）原理对大地电磁资料进行成像,将地面观测的波场分解成上行波和下行波,并分别向下延拓,利用上、下行波的时间一致性原理确定地下电性界面的位置.对TE和TM模式的MT响应分别处理可以得到两个深度剖面,两剖面的一致性便为实际反射界面的最佳估计.理论和实际资料的计算表明,MT偏移技术是比较有效的,它可获得地下界面直观的映像.和常规MT反演方法相比较,该方法具有算法简单等优点,并且还能获得表示地下地质构造真实映像的MT深度剖面.     

玄武岩内非均质性对地震资料的影响:印度德干圈闭的一个研究实例

Hydrocarbon exploration interests have renewed the need for developing new sub basalt imaging techniques. One of the most important problems encountered today is seismic imaging below basalt. In recent years, this problem appears to have been overcome partly by using long offset seismic data. However near offset data are yet to be fully utilised due to the complex waveform caused by the surface as well as internal heterogeneity of the basalts. The near normal incidence data, which influence the sub-basalt imaging, are highly useful to understand the internal structure within a basalt layer. The use of converted waves for such targets has been proposed as an alternative in a rather homogeneous basalt layer. With a few synthetic modelling exercises here we highlight the practical difficulties in dealing with more realistic and heterogeneous basalt flow. Full waveform seismograms are computed to understand the effects of intra-trappean sediments on the seismic data. A case study from the Deccan Traps of India is presented in this paper. First, we discuss the effects of intercalated sediments on the overall seismic image. Later, the sonic log data from the field are used to compute the full wave-field response using the reflectivity method and compared with the field data. The feasibility of using mode converted waves （P to S and vice-versa at the top and bottom basalt interfaces） for sub-basalt imaging in Kutch region is discussed through a series of velocity-depth profiles. By comparing with the field data we demonstrate that the effects of multiple thin layering within the basalt can strongly deteriorate the image we seek to interpret and exploit.     

一种大地电磁成像技术

文中将层析成像技术中广泛采用的广义脉冲谱技术应用了解大地电磁 维反演成像问题,实现了电阻率剖面成像.由于在做有限元二维止演的同时,就完成了Frechet导函数的计算,从而提高了反演的速度。文中给出了模型、实例的计算,均取得了较好的效果.     

一种大地电磁成像技术

文中将层析成像技术中广泛采用的广义脉冲谱技术应用了解大地电磁 维反演成像问题,实现了电阻率剖面成像.由于在做有限元二维止演的同时,就完成了Frechet导函数的计算,从而提高了反演的速度。文中给出了模型、实例的计算,均取得了较好的效果.     

大地电磁资料的灵敏度研究

本文研究了大地电磁资料对地球电性结构模型参数的灵敏度,它在反演解释中具有重要意义.通过研究均匀半空间中视电导率资料的灵敏度函数与计算分析实际观测资料的Fréchet导数和斜率表明,视电导率资料的灵敏度在地表最大,从地表向下呈指数衰减,频率愈高,电导率愈大,衰减也愈快.视电阻率资料和阻抗相位资料对高频、浅层和低阻层具有更高的灵敏度,几何参数的平均灵敏度高于电性参数的平均灵敏度,通常阻抗相位资料比视电阻率资料更灵敏.同时,根据阻抗实部定义的视电阻率与Kunetz定义的时间域大地电磁响应,反映地层电性差异的灵敏度最高. 文章还指出,在实际的反演解释中,如何正确有效地应用阻抗相位资料,提高分辨能力,仍是急待解决的问题.     

大地电磁资料的灵敏度研究

本文研究了大地电磁资料对地球电性结构模型参数的灵敏度,它在反演解释中具有重要意义.通过研究均匀半空间中视电导率资料的灵敏度函数与计算分析实际观测资料的Fréchet导数和斜率表明,视电导率资料的灵敏度在地表最大,从地表向下呈指数衰减,频率愈高,电导率愈大,衰减也愈快.视电阻率资料和阻抗相位资料对高频、浅层和低阻层具有更高的灵敏度,几何参数的平均灵敏度高于电性参数的平均灵敏度,通常阻抗相位资料比视电阻率资料更灵敏.同时,根据阻抗实部定义的视电阻率与Kunetz定义的时间域大地电磁响应,反映地层电性差异的灵敏度最高. 文章还指出,在实际的反演解释中,如何正确有效地应用阻抗相位资料,提高分辨能力,仍是急待解决的问题.     

玄武岩内非均质性对地震资料的影响:印度德干圈闭的一个研究实例(英文)

油气勘探需要重新开发玄武岩下目的层的成像技术。我们今天遇到的最重要的问题之一是玄武岩下地震成像。近年来,由于利用长炮检距这个问题部分已经得到解决。然而,由于地表以及玄武岩的内部非均质性引起的复杂波形,近炮检距的资料仍然不能充分地被利用。影响玄武岩下成像的近垂直入射资料对于了解玄武岩层内部结构是十分有用的。对比较均匀的玄武岩目标层可选用转换波。这里我们利用几个合成模型例子重点描述了更接近现实的非均质玄武岩流引起的实际困难。模拟计算了全波地震记录以有助于了解玄武岩内沉积物对地震资料的影响。本文介绍了印度德干圈闭的一个研究实例。首先探讨了夹层沉积物对整个地震成像的影响。其次利用该区声测井资料以反射系数法计算全波场响应与实际地震资料对比, 通过一系列速度-深度剖面探讨了利用模式转换波(顶底玄武岩界面上的P波转化为S波或相反)对印度库奇地区玄武岩下成像的可行性。通过与野外资料相比较证明玄武岩中多个薄层的效应严重恶化我们所要解释和利用的图象的质量。     

大地电磁场成像方法综述与新进展

对电磁场成像技术进行了系统总结，分析阐述了大地电磁场成像技术的研究现状与存在问题，并对各种方法的特点进行了比较和归纳，进而在Zhdanov等对偏移电磁场的研究和成像技术基础上提出了改进的有限差分法大地电磁场偏移成像技术，该方法提高了差分方程的精度和成像分辨率，并对客观选取背景电导率、两种极化模式联合成像、多参数和再次成像以及全频段成像和成像结合反演技术进行了研究。     

Ichthyofauna (Chondrichthyes,Osteichthyes) from the Upper Cretaceous intertrappean beds of Piplanarayanwar,Chhindwara District,Madhya Pradesh,India

A new fossiliferous intertrappean section is found 92 km southwest of the extensively studied intertrappean beds of Mohgaonkalan and Jhilmili in Chhindwara District, Central India. Application of the bulk screen‐washing method led to the recovery of a rich microvertebrate fauna represented by fish, amphibians, crocodiles, snakes and dinosaurs. In this paper, the ichthyofauna recovered from the new intertrappean section consisting of Igdabatis indicus Prasad and Cappetta, 1993, Lepisosteus indicus Woodward, 1908, Osteoglossidae gen. et sp. indet., Pycnodontidae gen. et sp. indet. and Siluriformes indet. is described. The ichthyofauna dominantly represented by the teeth of Igdabatis indicus is suggestive of a nearshore, deltaic or estuarine palaeoenvironment and Late Cretaceous (Maastrichtian) age for the intertrappean beds. The remarkable similarity of the new intertrappean fauna to that of Asifabad and the infratrappean beds of Marepalli also is suggestive of coastal‐plain conditions all along the Godavari lineament in the Late Cretaceous. Although the presence of planktonic foraminifera in the intertrappean beds of Jhilmili in the Mandla Lobe of Deccan volcanic province has been inferred in terms of a short term marine incursion from the west coast along the Narmada lineament in the Early Palaeocene, currently there is no definitive evidence for the prevalence of marine or coastal‐plain conditions along the Narmada lineament at least in the latest Cretaceous.     

Precursory signatures in the visibility graph analysis of seismicity: An application to the Kachchh (Western India) seismicity

The Visibility Graph (VG) method maps time series into networks or graphs, converting dynamical properties of time series in topological properties of networks. The VG method was applied to the aftershock depleted catalogue of the Kachchh Gujarat (Western India) seismicity from 2003 to 2012, in order to identify possible precursory signatures in the pattern of the VG parameters. The k–M slope (the slope of the line fitting the relationship between the magnitude of the events and their connectivity degrees) seems to sharply increase significantly before the occurrence of the largest shocks (M ⩾ 4.5) of the sequence.     

Cretaceous volcanic rocks of the South Tethyan suture zone, Pakistan: implications for the Réunion hotspot and Deccan Traps

The location, ages, and geochemical characteristics of marine volcanic rocks preserved in the South Tethyan suture zone of Pakistan suggest that the Réunion hotspot was active off northwestern Greater India well before the emplacement, far to the south, of the Deccan flood basalts, the great bulk of which were erupted at 65-66 Ma and are widely believed to be associated with the hotspot’s plume-head phase. Most of the suture zone samples have Nd-Pb-Sr isotopic ratios (e.g. age-corrected ?_Nd(t)=+3.0 to +4.6) close to those expected for modern-type Réunion source mantle in the Late Cretaceous, and their incompatible element patterns resemble those of recent Réunion shield lavas. ⁴⁰Ar-³⁹Ar incremental heating yields ages of 73.4-72.0 Ma. Nevertheless, unless even older ages are discovered among the suture zone rocks, a pre-Deccan marine phase of Réunion hotspot activity on the Tethyan side of Greater India can be accommodated within the framework of the plume-head model.     

Gravity and magnetic signatures of volcanic plugs related to Deccan volcanism in Saurashtra, India and their physical and geochemical properties

The Bouguer anomaly and the total intensity magnetic maps of Saurashtra have delineated six circular gravity highs and magnetic anomalies of 40-60 mGal (10⁻⁵m/s²) and 800-1000 nT, respectively. Three of them in western Saurashtra coincide with known volcanic plugs associated with Deccan Volcanic Province (DVP), while the other three in SE Saurashtra coincide with rather concealed plugs exposed partially. The DVP represents different phases of eruption during 65.5±2.5 Ma from the Reunion plume. The geochemical data of the exposed rock samples from these plugs exhibit a wide variation in source composition, which varies from ultramafic/mafic to felsic composition of volcanic plugs in western Saurashtra and an alkaline composition for those in SE Saurashtra. Detailed studies of granophyres and alkaline rocks from these volcanic plugs reveal a calc-alkaline differentiation trend and a continental tectonic setting of emplacement. The alkaline plugs of SE Saurashtra are associated with NE-SW oriented structural trends, related to the Gulf of Cambay and the Cambay rift basin along the track of the Reunion plume. This indicates a deeper source for these plugs compared to those in the western part and may represent the primary source magma. The Junagadh plug with well differentiated ring complexes in western Saurashtra shows well defined centers of magnetic anomaly while the magnetic anomalies due to other plugs are diffused though of the same amplitude. This implies that other plugs are also associated with mafic/ultramafic components, which may not be differentiated and may be present at subsurface levels. Paleomagnetic measurements on surface rock samples from DVP in Saurashtra suggest a susceptibility of 5.5×10⁻² SI units with an average Koenigsberger ratio (Q_n) of almost one and average direction of remanent magnetization of D=147.4° and I=+56.1°. The virtual geomagnetic pole (VGP) position computed from the mean direction of magnetization for the volcanic plugs and Deccan basalt of Saurashtra is 30°N and 74°W, which is close to the VGP position corresponding to the early phases of Deccan eruption. Modeling of gravity and magnetic anomalies along two representative profiles across Junagadh and Barda volcanic plugs suggest a bulk density of 2900 and 2880 kg/m³, respectively and susceptibility of 3.14×10⁻² SI units with a Q_n ratio of 0.56 which are within the range of their values obtained from laboratory measurements on exposed rock samples. The same order of gravity and magnetic anomalies observed over the volcanic plugs of Saurashtra indicates almost similar bulk physical properties for them. The inferred directions of magnetization from magnetic anomalies, however, are D=337° and 340° and I=−38° and −50° which represent the bulk direction of magnetization and also indicate a reversal of the magnetic field during the eruption of these plugs. Some of these plugs are associated with seismic activities of magnitude ≤4 at their contacts. Based on this analysis, other circular/semi-circular gravity highs of NW India can be qualitatively attributed to similar subsurface volcanic plugs.     

Sedimentological and petrochemical studies of Jurassic clastic rocks,Habo Dome Basin,Kachchh Mainland,Northwest India: Implications for depositional environment,provenance, and tectonic setting

Mesozoic rocks are extensively and excellently preserved in the western Indian shield in several basins. The Kachchh Mainland Basin (KMB), comprising six small sub‐basins, is the main repository of these sediments. Habo Dome Basin, situated in the easternmost part of KMB and largest among the six basins, hosts clastics of the Chari Formation of Jurassic age. The fluctuating transgressive–regressive facies cycle, developed during the Callovian and Late Early Oxfordian in the Habo Dome Basin, was mainly controlled by local tectonics and not by global eustatic fluctuations. Near magmatic relationships are displayed by various elements of the clastic rocks of Habo Dome Basin. Two litho‐chemical groups have been identified in Habo Dome Basin, which are cyclically repeated over entire lithostratigraphic sequence, indicating alternate pulses of sediment inputs from two different sources under palpitating tectonic conditions. Provenance indicator elements and their ratios coupled with source modeling indicate predominantly felsic source with basic and alkalic components. Integrated analysis of petrograhic and geochemical characteristics suggests two source terranes for these rocks: a granitoid source with significant basic volcanics (Banded Gneissic Complex) and a granite–gneissic source with minor alkaline volcanics (Nagarparkar Massif) lying to northeast and NNW respectively. The petrochemistry of Habo Dome clastics suggests their deposition in a fault controlled sink which was influenced by sea level changes. Drifting of the Indian plate resulted in the opening of series of rifted basins in the Kachchh Mainland during Late Triassic/Early Jurassic, which were closed later during collision of Indian plate with Eurasia at early Eocene. The Habo Dome Basin which opened up as a half graben in response to the initial stress regime, remained tectonically unstable until the cessation of pre and post collisional stress regimes.     

Late Quaternary drainage reorganization assisted by surface faulting: The example of the Katrol Hill Fault zone,Kachchh, western India

Drainage reorganization on restricted temporal and spatial scales is poorly-documented. We attempt to decode the relatively complicated mechanism of drainage realignment involving two small rivers that show structurally controlled, highly anomalous channel networks. We provide geomorphic and shallow subsurface evidence using ground-penetrating radar (GPR) for the presence of a buried paleo-valley flowing northward through the wind gap and surface faulting along the range bounding Katrol Hill Fault (KHF) which correlates with the previously known three surface faulting events in last ~30 ka bp . Most of the present river channels and the KHF zone are occupied by aeolian miliolite (local name) which is stratigraphic and lithologic equivalent of the Late Quaternary carbonate rich aeolianite deposits occurring in several parts of the globe. The history of drainage evolution in the study area comprises pre-miliolite, syn-miliolite and post-miliolite phases. Geomorphic evidences show that the paleo-Gangeshwar River flowed north through the wind gap and paleo-valley, while the short paleo-Gunawari occupied the saddle zone to the east of Ler dome prior to and during the phase of miliolite deposition which ended by ~40 ka bp . Southward tilting of the Katrol Hill Range (KHR) due to surface faulting cut off the catchment of the paleo-Gangeshwar River. The abandoned catchment stream extended its channel eastward along the strike through top-down process while the paleo-Gunawari River extended its course westward by headward erosion (bottom-up process). As the channels advanced towards each other they joined to produce the “S”-shaped bend which formed the capture point. We conclude that multiple surface faulting events along the KHF in the last ~30 ka bp , resulted in uplift and tilting of the KHR which caused drainage realignment by river diversion, beheading and river capture. Our study shows that the complexity of drainage reorganization processes is more explicit on shorter rather than longer timescales.     

网格剖分方式对大地电磁数据反演的影响

随着计算机技术的飞速发展,大地电磁测深数据的各种反演方法得到空前发展。对于反演过程中的网格剖分问题,前人研究有限。本文就此进行研究,确定影响网格剖分方式的因素,找到一种合适的网格剖分方法。为此,设计一个复杂模型,对正演模拟得到的数据加入随机噪声作为反演的输入数据,选择不同的网格剖分方式,使用DASOCC与NLCG反演方法,以相同参数分别进行反演,对所得结果进行对比分析。结果显示,在设定的模型及参数条件下,针对DASOCC反演方法,纵向及横向网格均不加密,反演效果比较理想,即横向采用一个测点对应一个网格的剖分方式,纵向采用表层加密,100 m以下后一网格为前一网格厚度1.1倍的剖分方式;针对NLCG反演方法,纵向与DASOCC反演方法相同,采用不加密网格的剖分方式得到的结果较好,而横向则建议采用一个测点对应一个网格,并在2个测点之间插入一个网格的剖分方式。