基于改进线性Radon变换的井孔地震上下行波场分离方法

反射波场分离是井孔地震资料处理中极其重要的一个环节,波场分离的质量直接影响成像结果的精度.不管是VSP还是井间地震资料,其反射波时距曲线都近似直线型,根据这一特征,本文提出一种改进的线性Radon变换方法来进行井孔资料的反射波上下行波场分离.该方法基于频率域线性Radon变换,通过引入一个新的变量λ来消除变换算子对频率的依赖性,避免了求取每一频率分量对应的不同变换算子,显著降低了计算成本;文中在求解该方法对应的最小二乘问题时,引入了发展较为成熟的高分辨率Radon变换技术来进一步提高波场分离的精度.采用本文方法进行井孔地震资料的上下行波场分离可以在保证分离精度的前提下有效地提高计算效率.根据上下行波在λ-f域内分布的特殊性,设计简单的滤波算子就可实现上下行波场的分离.最后通过合成数据试算以及实际资料处理（VSP数据和井间地震数据）验证了该方法的可行性和有效性.     

Filter formulation and wavefield separation of cross-well seismic data

Multichannel filtering to obtain wavefield separation has been used in seismic processing for decades and has become an essential component in VSP and cross-well reflection imaging. The need for good multichannel wavefield separation filters is acute in borehole seismic imaging techniques such as VSP and cross-well reflection imaging, where strong interfering arrivals such as tube waves, shear conversions, multiples, direct arrivals and guided waves can overlap temporally with desired arrivals. We investigate the effects of preprocessing (alignment and equalization) on the quality of cross-well reflection imaging wavefield separation and we show that the choice of the multichannel filter and filter parameters is critical to the wavefield separation of cross-well data (median filters, f–k pie-slice filters, eigenvector filters). We show that spatial aliasing creates situations where the application of purely spatial filters (median filters) will create notches in the frequency spectrum of the desired reflection arrival. Eigenvector filters allow us to work past the limits of aliasing, but these kinds of filter are strongly dependent on the ratio of undesired to desired signal amplitude. On the basis of these observations, we developed a new type of multichannel filter that combined the best characteristics of spatial filters and eigenvector filters. We call this filter a ‘constrained eigenvector filter’. We use two real data sets of cross-well seismic experiments with small and large well spacing to evaluate the effects of these factors on the quality of cross-well wavefield separation. We apply median filters, f–k pie-slice filters and constrained eigenvector filters in multiple domains available for these data sets (common-source, common-receiver, common-offset and common-midpoint gathers). We show that the results of applying the constrained eigenvector filter to the entire cross-well data set are superior to both the spatial and standard eigenvector filter results.     

井间地震数据的波动方程偏移成像

针对井间地震数据成像技术的一些缺陷并根据井间地震数据的特点,利用井间上、下行反射波信息,提出了一种基于波动方程的井间地震反射波偏移成像技术,该技术既考虑了井间地震数据的运动学特征(时间信息),又考虑了动力学特征(振幅信息).以PC机集群为计算平台,Linux为操作系统,设计了井间地震炮域波动方程偏移成像并行计算流程,并且在64位联想机群上用井间模型数据和某油田的实际数据对该技术和计算流程进行了测试和试处理,试验结果表明成像方法和并行算法正确、实用且计算效率高.     

Automatic cross-well tomography by semblance and differential semblance optimization: theory and gradient computation

A technique for automatic cross-well tomography based on semblance and differential semblance optimization is presented. Given a background velocity, the recorded seismic data traces are back-propagated towards the source, i.e. shifted towards time zero using the modelled traveltime between the source and the receiver and corrected for the geometrical spreading. Therefore each back-propagated trace should be a pulse, close to time zero. The mismatches between the back-propagated traces indicate an error in the velocity model. This error can be measured by stacking the back-propagated traces (semblance optimization) or by computing the norm of the difference between adjacent traces (differential semblance optimization).
It is known from surface seismic reflection tomography that both the semblance and differential semblance functional have good convexity properties, although the differential semblance functional is believed to have a larger basin of attraction (region of convergence) around the true velocity model. In the case of the cross-well transmission tomography described in this paper, similar properties are found for these functionals.
The implementation of this automatic method for cross-well tomography is based on the high-frequency approximation to wave propagation. The wavefronts are constructed using a ray-tracing algorithm. The gradient of the cost function is computed by the adjoint-state technique, which has the same complexity as the computation of the functional. This provides an efficient algorithm to invert cross-well data. The method is applied to a synthetic data set to demonstrate its efficacy.     

A modelling study of open-hole single-well seismic imaging

Single-well, or uni-well, imaging uses an acoustic source and an array of receivers located in the same borehole to image local geological structure. Due to the intrinsic attenuation of the formation it is likely that a source emitting frequencies in the typical cross-well range would be necessary to illuminate structure at distances above 100 m from the borehole. At these frequencies a significant proportion of the source energy is converted into tube-waves which are, for the purpose of these surveys, noise. This paper reports the results of a modelling study designed to assess the feasibility of using existing cross-well hardware, i.e. a piezo-electric source and hydrophone array, modified to run in a single borehole, to perform single-well surveys. In particular we study the case of an open borehole in a gas-filled, low-permeability sandstone reservoir. Our results suggest that the amplitudes of the tube-wave reverberations generated by calliper variations are such that reflections of interest arriving in the time window after the first tube-wave arrival will not be visible. However, reflections may be visible in the time window preceding the first tube-wave arrival provided tube waves from previous shots are not still present and the long source–receiver offsets required to make observations in this window can be incorporated into the tool design.     

Design considerations for using Distributed Acoustic Sensing for cross-well seismics: A case study for CO2 storage

Downhole monitoring with fibre-optic Distributed Acoustic Sensing (DAS) systems offers unprecedented spatial resolution. At the same time, costs are reduced since repeated wireline surveys can be replaced by the permanent installation of comparatively cheap fibre cables. However, the single component nature of fibre data requires novel approaches when designing a monitoring project such as cross-well seismics. At the example of the shallow CO₂ injection test site in Svelvik, Norway, we model the evolution of velocity changes during CO₂ injection based on rock physics theory. Different cross-well seismic design scenarios are then considered to evaluate the best design and the limits of this method to detect containment breach. We present a series of evaluation tools to compare the effect of different well spacings for cross-well seismic tomography. In addition to travel-times, we also consider characteristic amplitude changes along the fibre unique to DAS strain measurements, which might add a constraint to the inversion. We also compare the effect of using helical fibres instead of classical straight fibres. We thus present a toolbox to evaluate and compare different monitoring design options for fibre optic downhole installations for cross-well monitoring.     

Imaging of offset VSP data acquired in complex areas with modified reverse-time migration

A modified reverse-time migration algorithm for offset vertical seismic profiling data is proposed. This algorithm performs depth imaging of target areas in the borehole vicinity without taking into account the overburden. Originally recorded seismograms are used; reliable results can be obtained using only the velocity profile obtained along the well. The downgoing wavefield emitted from a surface source is approximated in the target area using the transmitted P-wave, recorded by the receivers deployed in the well. This is achieved through a reverse-time extrapolation of the direct transmitted P-wave into the target area after its separation in offset vertical seismic profiling seismograms generated using a finite-difference scheme for the solution of the scalar wave equation.
The proposed approach produces 'kinematically' reliable images from reflected PP- and PS-waves and, furthermore, can be applied as a salt proximity tool for salt body flank imaging based on the transmitted PS-waves. Our experiments on synthetic data demonstrate that the modified reverse-time migration provides reliable depth images based on offset vertical seismic profiling data even if only the velocity profile obtained along the borehole is used.     

Seismic interferometry experiment in a shallow cased borehole using a seismic vibrator source‡

We present the results of a seismic interferometry experiment in a shallow cased borehole. The experiment is an initial study for subsequent borehole seismic surveys in an instrumented well site, where we plan to test other surface/borehole seismic techniques. The purpose of this application is to improve the knowledge of the reflectivity sequence and to verify the potential of the seismic interferometry approach to retrieve high‐frequency signals in the single well geometry, overcoming the loss and attenuation effects introduced by the overburden. We used a walkaway vertical seismic profile (VSP) geometry with a seismic vibrator to generate polarized vertical and horizontal components along a surface seismic line and an array of 3C geophones cemented outside the casing. The recorded traces are processed to obtain virtual sources in the borehole and to simulate single‐well gathers with a variable source‐receiver offset in the vertical array. We compare the results obtained by processing the field data with synthetic signals calculated by numerical simulation and analyse the signal bandwidth and amplitude versus offset to evaluate near‐field effects in the virtual signals. The application provides direct and reflected signals with improved bandwidth after vibrator signal deconvolution. Clear reflections are detected in the virtual seismic sections in agreement with the geology and other surface and borehole seismic data recorded with conventional seismic exploration techniques.     

Imaging pre‐existing natural fractures using microseismic data

In hydraulic fracturing treatments, locating not only hydraulic fractures but also any pre‐existing natural fractures and faults in a subsurface reservoir is very important. Hydraulic fractures can be tracked by locating microseismic events, but to identify the locations of natural fractures, an additional technique is required. In this paper, we present a method to image pre‐existing fractures and faults near a borehole with virtual reverse vertical seismic profiling data or virtual single‐well profiling data (limited to seismic reflection data) created from microseismic monitoring using seismic interferometry. The virtual source data contain reflections from natural fractures and faults, and these features can be imaged by applying migration to the virtual source data. However, the imaging zone of fractures in the proposed method is strongly dependent on the geographic extent of the microseismic events and the location and direction of the fracture. To verify our method, we produced virtual reverse vertical seismic profiling and single‐well profiling data from synthetic microseismic data and compared them with data from real sources in the same relative position as the virtual sources. The results show that the reflection travel times from the fractures in the virtual source data agree well with travel times in the real‐source data. By applying pre‐stack depth migration to the virtual source data, images of the natural fractures were obtained with accurate locations. However, the migrated section of the single‐well profiling data with both real and virtual sources contained spurious fracture images on the opposite side of the borehole. In the case of virtual single‐well profiling data, we could produce correct migration images of fractures by adopting directional redatuming for which the occurrence region of microseismic events is divided into several subdivisions, and fractures located only on the opposite side of the borehole are imaged for each subdivision.     

陆上纵波源VSP资料中的纯横波

常规陆上VSP(Vertical Seismic Profiling)勘探普遍采用纵波震源激发,三分量检波器接收,主要利用的是纵波和转换横波信息。已有的研究表明,炸药震源在井下激发、可控震源在地面垂向振动,均会产生较强的纯纵波和一定强度的纯横波;泊松比差别较大的分界面有利于形成较强的透射转换横波。本文通过对激发形成的纯横波和下行转换形成的横波进行对比分析,认为纯横波的主频往往低于纯纵波的主频,而下行转换横波的主频通常接近纵波的主频。本文分别对两个陆上纵波源零偏和非零偏VSP资料进行分析,结果表明这些资料中普遍存在纯横波,只是横波的强弱存在不同程度的变化。利用纵波源零偏VSP资料,可以获得横波速度信。最后对VSP纵波和横波联合应用前景进行了分析,应该充分利用纵波源VSP资料中的横波信息。     

SEISMIC MIGRATION IN ELLIPTICALLY ANISOTROPIC MEDIA1

The study of wave propagation in media with elliptical velocity anisotropy shows that seismic energy is focused according to the horizontal component of the velocity field while the vertical component controls the time-to-depth relation. This implies that the vertical component cannot be determined from surface seismic velocity analysis but must be obtained using borehole or regional geological information. Both components of the velocity field are required to produce a correctly focused depth image. A paraxial wave equation is developed for elliptical anisotropic wave propagation which can be used for modelling or migration. This equation is then transformed by a change of variable to a second paraxial equation which only depends on one effective velocity field. A complete anisotropic depth migration using this transformed equation involves an imaging step followed by a depth stretching operation. This allows an approximate separation or splitting of the focusing and depth conversion steps of depth migration allowing a different velocity model to be used for each step. This split anisotropic depth migration produces a more accurate result than that obtained by a time migration using the horizontal velocity field followed by an image-ray depth conversion using the vertical velocity field. The results are also more accurate than isotropic depth migration and yield accurate imaging in depth as long as the lateral variations in the anisotropy are slow.     

井下地震计方位角检测技术应用研究

本文利用波形相关分析法对井下地震计方位角检测精度及一致性进行分析研究.研究中在地表布设地震计进行不同距离、不同频带多套地震计方位角检测对比测试,并在四川泸州、宁夏灵武、陶乐、河南安阳、清丰、陕西定边等6个分别安装井下甚宽频带、宽频带、短周期地震计的台站,进行地表同台址不同频带多套地震计与井下地震计方位角检测及一致性对比测试.根据全球噪声模型1~10s之间存在明显的噪声峰值的特点,对测试数据进行0.2~0.3Hz带通滤波和仿真处理,通过分析得出,对于不同地震计组合,测试地震计方位角检测结果一致性较好,根据地表不同距离、不同频带地震计组合的检测精度和地脉动记录的特点,认为井下地震计方位角检测精度优于4°.     

地震相干偏移与数据自参照偏移的关系

地震相干偏移和数据自参照偏移是最近几年发展起来的地震成像新方法,并且已在地震勘探研究领域开始应用.该方法在VSP（垂直地震剖面）中的应用尤其值得关注,因为它部分地解决了一些在地面反射地震方法中难以解决的问题.地震相干偏移是用地震数据的互相关处理后的数据进行偏移,而数据自参照偏移则不必在偏移前对数据进行互相关处理,而是通过记录数据中的两个不同部分波场的延拓来互相参照进行偏移.二者的优点都是在偏移运算中可以不考虑原始震源位置、激发时间以及地震波初始传播路径.本文从偏移成像的理论出发分析了这两类算法,发现二者在成像原理上是一致的.本文还通过一个二维VSP多次波偏移的数值模拟实验,比较和归纳了这两类算法在实际应用中的特点.     

高采样四分量钻孔应变仪记录应变地震波的可靠性分析

自2017年开始,陆续对中国地球物理台网10个钻孔应变测点的数据采集器进行升级,产出了大量的高采样钻孔应变观测数据。利用相关系数和自检精度2个检验四分量观测数据自洽性能的指标,定量分析了台站记录的2020年2月3日成都5.1级和2020年3月25日千岛群岛7.5级地震的应变地震波,结果显示高采样观测数据信度较高。对比分析了10个台站记录的2次地震的震相,证明高采样四分量钻孔应变仪具备记录清晰应变地震波的能力,且其可靠性较高。综合结果证明,高采样四分量钻孔应变观测可进一步为地震学研究提供新的数据资源。     

INFLUENCE OF PUMPING ON FOUR-COMPONENT BOREHOLE STRAIN AT TAI'AN SEISMIC STATION AND ITS INTERFERENCE MECHANISM

Four-component borehole strainmeter (FCBS) is one kind of high-precision borehole strain observation instruments invented in China. As a kind of near-surface deformation observation instrument, FCBS is also easily disturbed by the external environment factors. As a common factor, pumping has significant influence on FCBS observation. Existing studies mostly identify the pumping interference from the perspective of observation curve morphology, relatively few studies focus on its interference mechanism. In order to truly capture earthquake precursor information, it is necessary to study the interference mechanism. In recent years, RZB-3 type FCBS at Tai'an seismic station has been seriously affected by pumping, so it is necessary and also feasible to study the interference mechanism of pumping. Since the influence of pumping interference on borehole strainmeter is common, this work would be very practical and be used for reference by other borehole strain observation stations. We find that the original observation curves and observed surface strain, shear strain from RZB-3 type FCBS at Tai'an seismic station have the characteristics of synchronous change with the borehole water level, in which the linear correlation coefficient between the two observed shear strain curves and borehole water level reached 0.70 and 0.82 respectively. We further find that the principal strain direction of borehole and borehole water level after normalization meet the nonlinear function as y=1.217arctan(x)^0.224-0.284. The above phenomenon indicates that the observation of RZB-3 type FCBS at Tai'an seismic station is significantly affected by the borehole water level, and the influence is more obvious and the gradient is larger at the stage of low water level. Pumping interference often appears in low water level stage and changes the rock pore pressure state. Statistics show that pumping interference affects the borehole strain state. To investigate the interference mechanism of pumping to RZB-3 type FCBS at Tai'an seismic station, we take a known pumping as an example, in which we study the principal strain state of the borehole in three periods of normal pumping, interruption of pumping and resuming pumping respectively. During each period, we solve 3 parameters of the principal plane strain state, i.e. the maximum principal strain rate, the minimum principal strain rate and the maximum principal strain direction from four observation equations of FCBS by nonlinear iterative least squares algorithm. On the other hand, concentrated load model (CLM) is used to simulate the mechanical mechanism of pumping. Firstly, the depth of FCBS relative to pumping source and the concentrated load at pumping source are inversed, then, the strain state surrounding the pumping well, including the state at RZB-3 borehole, is simulated by forward modeling. By comparing these results, we find that: (1)The concentrated load at pumping source inversed by CLM during periods of normal pumping and resuming pumping are both located at or near the bottom of the pumping well, which is consistent with the actual situation, indicating that mechanism and degree of the influence of pumping on borehole strain are well simulated by CLM. (2)The observed strain state is consistent with the simulation result of pumping interference by forward modeling, indicating that the principal strain state of borehole calculated based on observation of FCBS reflects the true strain state of borehole under different pumping states.(3)The inversed concentrated load at pumping source during pumping periods is significant greater than the load of the pumped water, indicating that the pumping process has more significant influence on the pore pressure of rocks than the load of the pumped water. Even though CLM is an approximate simulation since it's based on some elastic assumptions, the interference mechanism of pumping on RZB-3 type FCBS at Tai'an seismic station is well explained, which is maybe very helpful for studying the influence of pumping interference on other deformation instruments, locating the unknown pumping source and studying the characteristics of pore pressure of rocks.     

异常地层压力声波衰减分析及地层压力地震数据外推反演

澳大利亚西北大陆架卡拉汶盆地含有丰富的油气资源, 但其地质条件复杂, 普遍存在异常地层高压分布,特别是该区区域盖层-中深层巨厚的Muderong页岩层内存在局部异常高压带,给石油钻井带来巨大困难.本文根据卡拉汶盆地北部地区10余口井的实测地层压力资料、声波测井数据和联井地震剖面, 研究该页岩层内异常压力千赫兹尺度声波响应的衰减特征,及其与地层有效压力之间的关联计算模型.通过联井地震数据的井-震相关性分析进行井-震过渡,研究井旁地震数据在异常高压带的衰减特征,及其与地层有效压力之间的关联计算模型,从而实现井震资料联合进行地层压力地震外推反演.研究表明,Muderong页岩层地层有效压力与井中声波速度/阻抗品质因子和井旁地震阻抗品质因子具有良好的正向相关性,根据二者交汇数据拟合经验公式反算得到的各井有效压力结果基本反映出实测有效压力曲线的变化特征.卡拉汶盆地北部地区地震数据具有良好的井-震相关性,地震资料品质相对可靠,确保了声-压相关分析的井-震过渡.最后,通过地震阻抗品质因子与地层有效压力之间的关联计算模型,进行该区异常地层压力地震外推反演,预测结果与钻井证实的异常压力分布特征基本符合.本研究为卡拉汶盆地中深部区域盖层局部异常高压带的地层压力外推反演提供了一种有效途径.     

When 4D seismic is not applicable: Alternative monitoring scenarios for the Arab-D reservoir in the Ghawar Field

Ghawar, the largest oilfield in the world, produces oil from the Upper Jurassic Arab‐D carbonate reservoir. The high rigidity of the limestone–dolomite reservoir rock matrix and the small contrast between the elastic properties of the pore fluids, i.e. oil and water, are responsible for the weak 4D seismic effect due to oil production. A feasibility study was recently completed to quantify the 4D seismic response of reservoir saturation changes as brine replaced oil. The study consisted of analysing reservoir rock physics, petro‐acoustic data and seismic modelling. A seismic model of flow simulation using fluid substitution concluded that time‐lapse surface seismic or conventional 4D seismic is unlikely to detect the floodfront within the repeatability of surface seismic measurements. Thus, an alternative approach to 4D seismic for reservoir fluid monitoring is proposed. Permanent seismic sensors could be installed in a borehole and on the surface for passive monitoring of microseismic activity from reservoir pore‐pressure perturbations. Reservoir production and injection operations create these pressure or stress perturbations. Reservoir heterogeneities affecting the fluid flow could be mapped by recording the distribution of epicentre locations of these microseisms or small earthquakes. The permanent borehole sensors could also record repeated offset vertical seismic profiling surveys using a surface source at a fixed location to ensure repeatability. The repeated vertical seismic profiling could image the change in reservoir properties with production.     

天然地震频率范围内首都圈地区近地表S波速度结构c

近地表沉积层的S速度结构是强地面震动模拟和地震灾害估计的重要参数,尤其是浅部的S波速度结构在工程上具有重要的应用意义．目前大部分资料来源于工程钻孔或工程地震探测,很少有地震波频率范围内的S波速度结构,或者深度达数百米的S波速度结构．通过对天然地震的井下摆波形记录的分析,提供了一种测量地震波频率范围深达数百米的S波速度的有效方法．收集了首都圈地区44个井下摆的近震记录,利用广义射线方法确认了直达S波及其在地表的反射波震相,并通过测量不同台站上两个震相的到时差,获得了首都圈地区浅层100——500m 深度范围的S波速度结构．研究发现,浅部100m 的平均S波速度低于300m/s．当深度增加到500m 时S波速增加到800m/s,平均速度梯度为0.8 （m/s）/m．研究结果表明,井下摆地震记录波形是研究沉积盆地浅层S波结构的重要资料,将为沉积盆地的强地面震动模拟提供重要基础参数．      

Impact of processing on the amplitude versus offset response of a marine seismic data set1

The analysis of prestack reflections may have a high diagnostic potential in the evaluation of the petrophysical characteristics of subsurface targets. However, the recovery of reliable seismic responses, especially when examining amplitude versus offset (AVO) variations, is of the utmost importance and is strictly dependent on the acquisition and processing steps. In order to evaluate the impact of different processing sequences, we examine the AVO responses of three seismic events from a marine data set. Borehole data indicate that these events are related to a lignitic sand, a gas sand and a cineritic bed. The AVO analysis is focused mainly on the reflections from the gas sand. In particular, we compare the results of a standard processing sequence with results from a surface-consistent approach and with results from a processing sequence tailored to this specific case. A decreasing AVO trend of the gas-sand reflection results from the analyses of data that have undergone the standard and the surface-consistent processing sequences. This contrasts with both theory and borehole information, which both predict an energy increase with offset. A detailed study shows that the receiver array attenuation, neglected in previous Studies, plays a major role in attenuating the far-offset reflections. Other propagation factors, such as offset-dependent geometrical spreading and Q absorption, produce only minor effects. Taking into account the above factors, we apply a third processing sequence whose impact on the AVO trend is evaluated step by step and whose results are compared with the previously applied sequences. This new sequence leads to better agreement between the AVOs predicted from borehole data and those measured on surface seismic data.     

湖南保靖X区页岩气储层“甜点”区地震资料非线性反演预测方法

针对湖南保靖地区地表覆盖层复杂、地下高陡构造多变、地震勘探资料信噪比低,以及页岩气储层与周围泥岩层波阻抗对比度小等特点,研究敏感测井曲线重构与优化处理、地震资料相控非线性反演和叠后地震属性流体检测的综合储层评价方法。通过对多条测线的地震资料反演和流体检测并与实钻井结果对比,获得了良好的地质效果,初步验证了所研究方法的有效性和实用性。