基于多次波和自适应预测误差滤波器的地震近炮检距数据重建方法

地震数据采集过程中,受野外施工条件的制约,往往很难获得完整的地震波场,尤其近炮检距数据的缺失尤为严重.当前,很多地震数据处理方法的应用都依赖于近炮检距数据,如何对近炮检距的缺失数据进行重建,是一个重要的研究课题.本文通过多次波和一次波的互相关构建虚拟一次波,利用数据本身的波场信息,对缺失的近炮检距数据进行插值重建.由于通过多次波构建的一次波与真实的一次波存在振幅和相位方面的差异,提出通过基于非平稳自回归过程的自适应预测误差滤波器来表征虚拟一次波的能量谱,利用最小二乘反演方法重建近炮检距缺失数据,自适应预测误差滤波器通过求解正则化约束下的数学欠定问题来实现局部自适应特征.通过对Sigsbee2B模型和实际数据的测试结果表明新方法可以合理地重建复杂的近炮检距缺失数据.     

Kirchhoff型偏移反偏移串联地震数据炮检距转换技术

零炮检距数据在海洋地震资料处理中有很多优势和用途,然而,受海洋水平拖缆地震采集作业方式的限制,接收数据的最小炮检距一般在200 m左右,小于该炮检距的数据是无法直接获得的.通常的做法是通过对较大炮检距数据进行动校,通过外推来变相获得零炮检距（包括小炮检距）数据,其外推的精度会受到地震资料信噪比、动校正速度的精度等因素影响,并且保幅性较差.本文通过一种基于Kirchhoff真振幅偏移和反偏移串联的技术,在反偏移过程中改变观测系统,有效实现了大炮检距反射地震数据向零炮检距（包括小炮检距）数据之间的转换,且很好地保持了零炮检距（包括小炮检距）数据的振幅特性.同时,经偏移-反偏移串联处理后,有效压制了地震数据中的随机噪声,地震资料信噪比和成像精度均得到显著提高.     

基于高密度速度分析的天然气水合物地震资料各向异性参数分析

在天然气水合物地震资料处理中,经过基于双曲线的高密度速度分析后,远炮检距数据仍然存在较大的剩余动校正量,分析认为:火成岩各向异性和垂向速度变化引起非双曲线时差,利用近炮检距地震数据得到均方根动校正速度,同时采用扫描法和旅行时法分别得到可以相互验证的各向异性参数,用提取的各向异性参数进行各向异性速度分析,剩余动校正得到消除,成像效果有显著改善。     

基于实际数据驱动的非纵波场模拟分析

在黄土塬、山地等低信噪比地区,单炮近道受面波、激发噪声等影响,地震资料品质极差.为此提出了非纵观测,使目的层反射波避开近道干扰,来提高地震资料的信噪比.如何选择合适的非纵距进行观测系统设计?常规方法是采用建立三维地质模型,利用射线追踪或波动方程正演模拟地震记录选择观测系统的非纵距,但由于地质现象的复杂性、建模软件的应用性、建模方法适用条件的局限性等等,使得在实际生产应用中,模拟出的地震记录还存在一些不足之处.为此融合探区以往的地震资料,建立了如下流程确定合理的非纵距:首先搜集探区以往的炮集数据,在炮集数据上拾取地震波(初至波、反射波和面波)和目的层倾角,获取地震波的走时、均方根振幅、主频、相位等信息,建立炮检距与分析地震波属性值(走时、均方根振幅、主频、相位等)的关系曲线;然后根据地质任务设计单炮点的非纵观测系统,根据炮检距选择对应地震波的关系曲线相应的属性值,模拟地震记录;最后根据目的层反射波能否有效避开面波、初至波、近道干扰,确定观测系统非纵离.通过实例验证,对于地质构造复杂、资料信噪比低的地区,模拟的地震记录更符合探区地质构造特点,能有效地指导观测系统非纵距的设计,达到合理压制噪声和提高地震资料信噪比的目的,克服了传统宽线压噪能力低和三维采集投资过大的矛盾.     

基于表层多次波数据的近道地震数据插值方法研究(英文)

本文提出基于原始含表层多次波数据实现叠前共炮集地震数据插值。相对于利用相邻道的信息变换或外推插值用于缺失的地震数据重建,本文方法利用表层多次波数据互相关构建准一次波,将蕴含在表层多次波数据中的,而在采集记录中表现为缺失的近炮检距信息提取出来,并在滑动时间空间窗内采用最小二乘匹配滤波和均方根振幅校正方法进行准一次波校正而后用于数据插值重建。本文方法适用于表层多次波比较发育,同时又存在数据缺失尤其是近炮检距数据缺失情况。方法易于实现,不需多次波和一次波的提取,利用多次波中蕴含的信息实现缺失的地震数据弥补,为含有表层多次波的数据进行近炮检距地震信息的插值重建提供了一个很好的思路。     

地震勘探中广义弹性阻抗的正反演

常规的地震道反演方法建立在反射P波垂直入射假设 的基础上,而实际地震资料采集时多数是非零炮检距的，反射振幅是共中心点道集叠加的结 果 . 因此，利用常规地震道反演方法就不能得到可靠的波阻抗或其他岩性信息. 本文利用Patr ick Connolly弹性阻抗的思想，通过对Zoeppritz方程的进一步简化，推导出适合常规叠后 资料的、非零炮检距条件下纵波反射系数递推公式，提出了广义弹性阻抗的概念，解决了非 零炮检距条件下，常规叠后地震道正反演的关键问题. 广义弹性阻抗不仅包含波阻抗，还包 含了纵横波速度等岩性信息，具有很好的实用价值. 进行广义弹性阻抗的反演，能较常规地 震道反演获得更多、更可靠的流体、孔隙度、砂泥含量等信息，有助于解释常规地震道反演 和道积分剖面中的假象，降低反演的多解性，提高储层预测的精度.     

振幅保真的单程波方程偏移理论

本报告综述了近年来发展起来的真振幅单程波方程偏移理论,对各种基于单程波方程的偏移振幅作出了系统的分析介绍.为了得到正确的偏移振幅,叠后偏移之前必须进行球面扩散校正.但是,作为叠后相位移偏移算法的推广,Dubrulle氏的共炮检距偏移除了零炮检距和平层两种特例外,它不能给出正确的偏移振幅.通过分析,我们发现传统的单程波单炮偏移不是保振幅算法.利用真振幅单程波方程分解和校正地表的初值条件,我们可以将常规偏移方法改造为真振幅算法并且证明在高频渐近的意义下,新方法的偏移振幅等价于Kirchhoff反演的结果.进一步研究发现可以利用单平方根算子和双平方根算子输出真振幅共反射角剖面.我们的分析指出,这时正确的成像条件为乘积pUp*D.与真振幅共炮偏移所需的商型成像条件pU/pD相比,共反射角偏移的计算稳定性大为改善并且在实际地震资料处理中有重要的应用.     

几种反射波时距方程的比较

在地震资料处理中，速度分析和成像技术极为重要.常规地震资料处理方法是利用双曲线方程来描述反射波时距曲线规律，此方程随着地层非均质性、各向异性和排列长度的增加，其误差变大.目前发展的反射波非双曲时距方程，主要有基于层状各向同性模型的非双曲时距方程、基于均匀弱各向异性模型的时距方程、基于速度随炮检距变化模型的时距方程、基于线性连续速度模型的时距方程.本文针对三个典型的模型：层状均匀各向同性模型、层状弱各向异性模型和层状非均匀模型，对这几种时距方程进行了精度比较与分析，得出了一些有益的结论.最后，从不同角度说明了应用这几种方程的合理性.     

层状各向异性介质转换波克希霍夫叠前时间偏移

在克希霍夫叠前时间偏移处理中,地震波走时的计算方法是决定大偏移距地震资料成像品质的重要因素.在常规的三维转换波各向异性叠前时间偏移公式中,走时的计算是基于等效单层各向异性介质的非双曲线方法.用这种方法处理的成像道集,在偏移/深度比超过一定阈值后,成像道集中的反射同相轴将出现过偏现象,这种偏移不平的同相轴将影响偏移叠加的最佳响应,使得偏移成像波组呈低频化特征,最终降低三维转换波偏移成像质量.我们采用层状介质的走时计算方法代替常规算法,并且利用了常规方法的转换波各向异性偏移速度模型.基于层状介质的算法能够提高大偏移距转换波走时计算精度,克服中浅地层大偏移距远道成像道集中反射同相轴逐渐上翘的问题.两个地区的三维转换波资料处理结果证实,基于层状各向异性介质的转换波克希霍夫叠前时间偏移方法,明显改善了反射成像剖面的连续性和分辨率,提高成像剖面构造的可解释性.     

应用低频信号提高高速玄武岩下的成像质量

针对高速玄武岩屏蔽层下深层成像困难的实际问题,采用波动方程波场数值模拟技术,根据玄武岩地层的特点,设计三个相应的简单高速玄武岩模型,通过对深层反射地震信号能量的分析,说明了低频地震信号既具有较强的穿透薄高速玄武岩屏蔽层的能力,也具有减弱因粗糙表面所产生的绕射噪音的能力。一个完整的2D玄武岩模型的模拟试验证明了利用低频信号可以提高高速玄武岩屏蔽层下深层成像的质量,实际资料的低通滤波处理也取得了预期的效果。     

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.     

Sub-basalt imaging problems and the application of Artificial Neural Networks

In many areas of the world, the presence of shallow high velocity, highly heterogeneous layers complicate seismic imaging of deeper reflectors. Of particular economic interest are areas where potentially hydrocarbon-bearing strata are obscured by layers of basalt. Basalt layers are highly reflective and heterogeneous. Using reflection seismic, top basalt is typified by a high-amplitude, coherent reflector with poor resolution of reflectors below the basalt, and even bottom basalt. Here, we present a new approach to the imaging problem using the pattern recognition abilities of a back-propagation Artificial Neural Network (ANN). ANNs are computational systems that attempt to mimic natural biological neural networks. They have the ability to recognize patterns and develop their own generalizations about a given data set. Back-propagation neural networks are trained on data sets for which the solution is known and tested on the data that are not previously presented to the ANN in order to validate the network result. We show that Artificial Neural Networks, due to their pattern recognition capabilities, can invert the medium statistics based on the seismic character. We produce statistically defined models involving a basalt analogous layer, and calculate full wavefield finite difference synthetic seismograms. We vary basalt layer thickness and source frequency to generate a synthetic model that produces seismic that is similar to real sub-basalt seismic, i.e. high amplitude top basalt reflector and the absence of base basalt and sub-basalt events. Using synthetic shot gathers, generated in a synthetic representation of the sub-basalt case, we can invert the velocity medium standard deviation by using an ANN. By inverting the velocity medium standard deviation, we successfully identified the transition from basalt to sub-basalt on the synthetic shot gathers. We also show that ANNs are capable of identifying the basalt to sub-basalt transition in the presence of incoherent noise. This is important for any future applications of this technique to the real-world seismic data, as this data is never completely noise-free. There is always a certain level of residual (noise remaining after initial noise filtering) environmental/ambient noise present on the recorded seismics, hence, neural network training with noise-free synthetic seismic is less than optimal.     

Use of marine controlled-source electromagnetic sounding for sub-basalt exploration

The problem of imaging sedimentary structure obscured by high-velocity layers, such as carbonate, basalt or salt, using conventional seismic techniques is well known. When this problem is encountered in offshore areas, marine electromagnetic data can provide valuable, alternative and complementary constraints on the structure. We concentrate on the use of marine controlled-source electromagnetic (CSEM) sounding in the frequency domain. The feasibility of mapping sub-basalt sedimentary structure using this technique is explored by means of modelling studies which mimic a type of survey which could readily be employed in practice. As a starting point the problem is addressed in terms of 1D resistivity structure. We show that sub-basalt sediments can be detected and their depth of burial quantified to within 200 m in the examples shown. The presence of small-scale inhomogeneities in the basalt (which cause much of the scattering in seismic data) is shown to have no appreciable effect on the ability of the CSEM data to detect the sediments. The modelling is then extended to 2.5 dimensions. Again the presence of sub-basalt sediments can be readily detected and their properties and geometry quantified, especially if the electromagnetic data are combined with constraints from complementary geophysical techniques.     

利用大地电磁资料对玄武岩成像的可行性

The magnetotelluric （MT） method has been among the favorite supporting tools for seismic imaging of sub-salt and sub-basalt targets. In this paper we present an example from Kachchh, India （where basaltic rocks overlie Mesozoic sedimentary rocks）, and discuss the feasibility of using MT method as an exploration tool in this geological setting. Our results highlight the difference in magnetotelluric response caused by the thin intrabasalt layering. The key issue addressed in this paper is what MT can and cannot provide in such geological settings. First, we compute apparent resistivity and phase response curves using representative resistivity-depth models and borehole data from the study area. Later, we compare these results to assess the plausibility of using MT to image the sub-volcanic sediments at Kachchh. Finally, we substantiate our discussion through one-dimensional inversion of the field observed MT data from this region that exhibits poor sensitivity of MT for thin basalt layers.     

A quantitative study on the use of converted waves for sub-basalt imaging

The idea of imaging beneath a high-velocity layer using converted waves has been popular since 1990. Because these wave types have their maximum amplitudes at mid- to far-offsets, the search for pure P-waves at the highly multiple-contaminated near-offsets can be avoided. For the Atlantic Margin, with buried thin-layered basalts, our quantitative study shows that the initial single-layered approach is not viable. Even in an unrealistic ideal geological setting, the amplitude of the symmetrical PSP-mode is far too weak to be recognized on towed streamer data. Furthermore, in the far-offset window, where locally converted waves have their strongest amplitudes, there is a multitude of other reflections, refractions and interbedded multiples, which have similar moveouts and, often, higher amplitudes. Without the removal of these events, a reliable image of the subsurface cannot be produced. We show that even if this problem were solved, it would be far easier to use the P-wave reflection from beneath the basalt at near-offsets. Our study shows that this wave type is by far the strongest response. A borehole-derived model using a thin-layered basalt sequence reveals that the strongest locally converted wave has an asymmetrical path and is 10 times weaker. All our results indicate that the pure P-modes provide the best chance of imaging sub-basalt sedimentary interfaces.     

Phase behaviour of P-SV converted waves recorded at the sea-bed

Shear waves recorded at or near the sea-bed, i.e. a water–sediment interface, may suffer from unwanted phase change, which is detrimental to velocity analysis and processing and degrades the quality of the final stacked or migrated image. In this study, this phenomenon is analysed for P-SV converted waves recorded at the sea-bed. Theoretical analysis shows that phase change does not occur if the converted shear waves always maintain raypaths that lie within the critical angle, provided the subsurface layering is horizontal. A phase change that is asymmetric with offset can readily be explained as being due to dipping layers at targets or the dipping sea-bed. This analysis is extended to multiple layers and anisotropic media and shows that the same conclusions hold. The analysis performed on two sets of ocean-bottom-cable seismic data shows that the majority of observations show little evidence of phase change, and occasionally display the asymmetric phase change with offset. This finding underlines the robustness of converted shear waves for imaging the horizontal subsurfaces and sea-bed as all of the offset information may be used.     

Separation of PP- and PS-wave reflected seismic data using two-dimensional finite offset common-reflection-surface traveltime approximation

Recently, the interest in PS-converted waves has increased for several applications, such as sub-basalt layer imaging, impedance estimates and amplitude-versus-offset analysis. In this study, we consider the problem of separation of PP- and PS-waves from pre-stacked multicomponent seismic data in two-dimensional isotropic medium. We aim to demonstrate that the finite-offset common-reflection-surface traveltime approximation is a good alternative for separating PP- and PS-converted waves in common-offset and common shot configurations by considering a two-dimensional isotropic medium. The five parameters of the finite-offset common-reflection-surface are firstly estimated through the inversion methodology called very fast simulated annealing, which estimates all parameters simultaneously. Next, the emergence angle, one of the inverted parameters, is used to build an analytical separation function of PP and PS reflection separation based on the wave polarization equations. Once the PP- and PS-converted waves were separated, the sections are stacked to increase the signal-to-noise ratio using the special curves derived from finite-offset common-reflection-surface approximation. We applied this methodology to a synthetic dataset from simple-layered to complex-structured media. The numerical results showed that the inverted parameters of the finite offset common-reflection-surface and the separation function yield good results for separating PP- and PS-converted waves in noisy common-offset and common shot gathers.     

角度域弹性波Kirchhoff叠前深度偏移速度分析方法

为提高地震成像结果的准确性并真实反映实际地震波场在介质中的传播特性,应该充分利用多分量地震数据的矢量特征进行弹性波成像,其中,最为棘手的问题是纵横波偏移速度场的确定,为此,本文提出了直接利用多分量地震数据进行弹性波角度域偏移速度分析的方法.基于空移成像条件的弹性波Kirchhoff偏移方程提取了弹性波局部偏移距域共成像...     

Borehole study of compressional and shear attenuation of basalt flows penetrated by the Brugdan and William wells on the Faroes shelf

We investigated the seismic attenuation of compressional (P‐) and converted shear (S‐) waves through stacked basalt flows using short‐offset vertical seismic profile (VSP) recordings from the Brugdan (6104/21–1) and William (6005/13–1A) wells in the Faroe‐Shetland Trough. The seismic quality factors (Q) were evaluated with the classical spectral ratio method and a root‐mean‐square time‐domain amplitude technique. We found the latter method showed more robust results when analysing signals within the basalt sequence. For the Brugdan well we calculated effective Q estimates of 22–26 and 13–17 for P‐ and S‐waves, respectively, and 25–33 for P‐waves in the William well. An effective Q_S/Q_P ratio of 0.50–0.77 was found from a depth interval in the basalt flow sequence where we expect fully saturated rocks. P‐wave quality factor estimates are consistent with results from other VSP experiments in the North Atlantic Margin, while the S‐wave quality factor is one of the first estimates from a stacked basalt formation using VSP data. Synthetic modelling demonstrates that seismic attenuation for P‐ and S‐waves in the stacked basalt flow sequence is mainly caused by one‐dimensional scattering, while intrinsic absorption is small.     

Sub-basalt decoupled walkaway VSP imaging

Walkaway vertical seismic profile (VSP) data acquired in basalt‐covered areas can be used to improve knowledge of the sub‐basalt structure. A synthetic example and a case study from the North Atlantic (UK) show that elastic two‐way downward‐continuation migration combined with the stationary‐phase principle is well suited to the processing of VSP data. Vector data are processed using decoupled elastic migration algorithms in both isotropic and anisotropic media. To illustrate the value of decoupled imaging equations, conventional PP imaging is carried out on the enhanced VSP data and compared with the decoupled scheme. Decoupled vector migration operates directly on the displacement vector, and uses various wave modes. Downgoing waves are migrated to image basalt lava flows and measure their anisotropy. Upgoing waves are used for high‐resolution sub‐basalt imaging.