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.     

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.     

Quantitative imaging of complex structures from dense wide-aperture seismic data by multiscale traveltime and waveform inversions: a case study

An integrated multiscale seismic imaging flow is applied to dense onshore wide‐aperture seismic data recorded in a complex geological setting (thrust belt). An initial P‐wave velocity macromodel is first developed by first‐arrival traveltime tomography. This model is used as an initial guess for subsequent full‐waveform tomography, which leads to greatly improved spatial resolution of the P‐wave velocity model. However, the application of full‐waveform tomography to the high‐frequency part of the source bandwidth is difficult, due to the non‐linearity of this kind of method. Moreover, it is computationally expensive at high frequencies since a finite‐difference method is used to model the wave propagation. Hence, full‐waveform tomography was complemented by asymptotic prestack depth migration to process the full‐source bandwidth and develop a sharp image of the short wavelengths. The final traveltime tomography model and two smoothed versions of the final full‐waveform tomography model were used as a macromodel for the prestack depth migration. In this study, wide‐aperture multifold seismic data are used. After specific preprocessing of the data, 16 frequency components ranging from 5.4 Hz to 20 Hz were inverted in cascade by the full‐waveform tomography algorithm. The full‐waveform tomography successfully imaged SW‐dipping structures previously identified as high‐resistivity bodies. The relevance of the full‐waveform tomography models is demonstrated locally by comparison with a coincident vertical seismic profiling (VSP) log available on the profile. The prestack depth‐migrated images, inferred from the traveltime, and the smoothed full‐waveform tomography macromodels are shown to be, on the whole, consistent with the final full‐waveform tomography model. A more detailed analysis, based on common‐image gather computations, and local comparison with the VSP log revealed that the most accurate migrated sections are those obtained from the full‐waveform tomography macromodels. A resolution analysis suggests that the asymptotic prestack depth migration successfully migrated the wide‐aperture components of the data, allowing medium wavelengths in addition to the short wavelengths of the structure to be imaged. The processing flow that we applied to dense wide‐aperture seismic data is shown to provide a promising approach, complementary to more classical seismic reflection data processing, to quantitative imaging of complex geological structures.     

Seismotectonics of the Himalaya and its vicinity from centroid-moment tensor (CMT) solution of earthquakes

The centroid-moment tensor solutions of more than 300 earthquakes that occurred in the Himalayas and its vicinity regions during the period of 1977–1996 are examined. The resultant seismic moment tensor components of these earthquakes are estimated. The Burmese arc region shows prominent east–west compression and north–south extension with very little vertical extension. Northeast India and Pamir–Hindu Kush regions show prominent vertical extension and east–west compression. The Indian plate is subducting eastward beneath the northeast India and Burmese arc regions. The overriding Burmese arc has overthrust horizontally with the underthrusting Indian plate at a depth of 20–80 km and below 80 km depth, it has merged with the Indian plate making “Y” shape structure and as a result the aseismic zone has been formed in the region lying between 26°N–28°N and 91.5°E–94°E at a depth of 10–50 km. Similarly, the Indian plate is underthrusting in the western side beneath the Pamir–Hindu Kush region and the overriding Eurasian plate has overthrust it to form a “Y” shape structure at a depth of 10–40 km and below 60 km depth, it has merged with the Indian plate and both the plates are subducting below 60–260 km depth. Further south, the overriding Eurasian plate has come in contact with the Indian plate at a depth of 20–60 km beneath northwest India and Pakistan regions with left lateral strike slip motion.     

天山中部地壳及上地幔三维速度层析成像

应用新疆区域台网记录的地震资料,结合前人对新疆速度结构研究的结果作为初始模型,利用地震层析成像的方法反演了天山中部地壳及上地幔三维速度结构。三维图像结果显示：天山中部地区波速呈现出不均匀性,45km深度以上各层波速表现出高速性质,45km以下的区域波速却表现出低速性质;纬向剖面显示出0～40km深度范围内波速起伏较大,而40km深度以下速度相对稳定。     

库车坳陷复杂高陡构造地震成像研究

复杂构造地震成像主要取决于叠前地震数据品质、偏移速度可靠性和偏移算子成像精度. 库车坳陷异常复杂的近地表条件导致极低信噪比的地震采集数据. 该区逆冲推覆高陡构造刺穿盐体大面积分布, 盐层厚度变化大、顶底面形态复杂, 盐下断裂带破碎、小断块发育, 形成异常复杂的地震成像问题. 本文重点研究三个关键环节:(1)精细的叠前地震预处理研究: 根据该区地震地质复杂性和地震资料特征, 采用一些新的方法技术和技术组合从振幅与时移的大、中、小尺度变化三个层次来解决资料信噪比问题, 重建深部反射信号; (2)三级偏移速度分析研究:利用库车坳陷盐刺穿逆冲推覆构造建模理论及变速成图配套技术解决叠前时间偏移速度场时深转换问题,利用井约束低频速度地震迭代反演技术解决连井层速度场与偏移速度场的融合问题,实现从DMO速度分析、叠前时间偏移速度分析到叠前深度偏移速度分析的有机衔接,建立拓扑结构相对保持的叠前深度偏移速度模型;(3)基于退化Fourier偏移算子的半解析波动方程叠前时间和深度偏移研究, 极大地改善了地震偏移过程中高波数波的成像问题. 通过对库车坳陷大北、博孜、却勒、西秋4和西秋10等复杂高陡构造的叠前时间和深度偏移地震成像处理,取得了较好的应用效果.     

Field and synthetic experiments for virtual source crosswell tomography in vertical wells: Perth Basin,Western Australia

It is common for at least one monitoring well to be located proximally to a production well. This presents the possibility of applying crosswell technologies to resolve a range of earth properties between the wells. We present both field and synthetic examples of dual well walk-away vertical seismic profiling in vertical wells and show how the direct arrivals from a virtual source may be used to create velocity images between the wells. The synthetic experiments highlight the potential of virtual source crosswell tomography where large numbers of closely spaced receivers can be deployed in multiple wells. The field experiment is completed in two monitoring wells at an aquifer storage and recovery site near Perth, Western Australia. For this site, the crosswell velocity distribution recovered from inversion of travel times between in-hole virtual sources and receivers is highly consistent with what is expected from sonic logging and detailed zero-offset vertical seismic profiling. When compared to conventional walkaway vertical seismic profiling, the only additional effort required to complete dual-well walkaway vertical seismic profiling is the deployment of seismic sensors in the second well. The significant advantage of virtual source crosswell tomography is realised where strong near surface heterogeneity results in large travel time statics.     

南海中北部地壳深部结构探测新进展

2006年8～10月间．国家海洋局第二海洋研究所使用德国SEDIS型三分量海底地震仪以气枪为震源在南海中北部开展了广角地震反射／折射勘测．这是我国大陆科研单位第一次自主开展长距离多剖面的海底地震仪人工地震探测。勘测线共三条。总长达1176．8km．工区最大水深4137m．共投放48台次,回收成功率95-8％。所获得的数据质量良好,深部信息丰富．可直观地分辨出照、Pn、PmP等震相,为南海中北部深部地壳结构、洋壳与陆壳分界和西北、西南次海盆的形成演化研究提供了重要的依据。     

Earthquake Source Zones in Northeast India: Seismic Tomography, Fractal Dimension and b Value Mapping

We have imaged earthquake source zones beneath the northeast India region by seismic tomography, fractal dimension and b value mapping. 3D P-wave velocity (Vp) structure is imaged by the Local Earthquake Tomography (LET) method. High precision P-wave (3,494) and S-wave (3,064) travel times of 980 selected earthquakes, m _d ≥ 2.5, are used. The events were recorded by 77 temporary/permanent seismic stations in the region during 1993–1999. By the LET method simultaneous inversion is made for precise location of the events as well as for 3D seismic imaging of the velocity structure. Fractal dimension and seismic b value has been estimated using the 980 LET relocated epicenters. A prominent northwest–southeast low Vp structure is imaged between the Shillong Plateau and Mikir hills; that reflects the Kopili fault. At the fault end, a high-Vp structure is imaged at a depth of 40 km; this is inferred to be the source zone for high seismic activity along this fault. A similar high Vp seismic source zone is imaged beneath the Shillong Plateau at 30 km depth. Both of the source zones have high fractal dimension, from 1.80 to 1.90, indicating that most of the earthquake associated fractures are approaching a 2D space. The spatial fractal dimension variation map has revealed the seismogenic structures and the crustal heterogeneities in the region. The seismic b value in northeast India is found to vary from 0.6 to 1.0. Higher b value contours are obtained along the Kopili fault (~1.0), and in the Shillong Plateau (~0.9) The correlation coefficient between the fractal dimension and b value is found to be 0.79, indicating that the correlation is positive and significant. To the south of Shillong Plateau, a low Vp structure is interpreted as thick (~20 km) sediments in the Bengal basin, with almost no seismic activity in the basin.     

超浅层地震勘探在青岛王哥庄断裂探测中的应用

目前国内外对目标层埋深仅有几十米（甚至十几米）的超浅层地震勘探经验不多,理论研究也不够深入。本文从城市活断层探测的角度出发,利用地质学、地球物理学及数学等学科中的相关理论和方法,探讨超浅层地震勘探在青岛复杂地质构造背景下取得有效探测结果的前提条件,并对青岛市主要活断层的典型剖面进行重点研究,力求在城市活断层超浅层地震勘探数据采集技术、数据处理等方面有所进展,为青岛及类似地质构造背景的地区开展活动断层超浅层地震探测提供参考。研究表明超浅层地震反射波法可以获取深度仅有十几米的地层反射信号,且大部分反射剖面都可较清楚地揭示出超浅部断层位置和断层特征。     

A few case histories of subsurface imaging with EMAP as an aid to seismic processing and interpretation1

The electromagnetic array profiling (EMAP) exploration method can be combined with a direct one-dimensional inversion process for conversion to depth to produce a subsurface resistivity cross-section. This cross-section may then be interpreted in parallel with a seismic cross-section to enhance the prediction of rock type and structure. In complex thrust environments and areas of shallow carbonate rocks, the EMAP method is often used to provide additional data either to help the seismic processor and/or to aid the seismic interpretation. In particular, the electromagnetic (EM) data can be used to build an independent seismic velocity file for depth migration. Three EMAP test areas in the western United States are used to demonstrate such a use of EMAP as an expioration tool. The first shows how a velocity file is estimated from resistivity data for seismic depth migration processing in a complex thrust environment. In the second example, the method is applied in layer-cake geology with high seismic velocity rocks at the earth's surface. The third example is another complex thrust environment, but in this case the velocity file derived from the resistivity data is used for stacking the seismic data.     

用接收函数方法研究汶川和芦山地震之间未破裂段的地壳结构

芦山与汶川地震之间存在约40 km的地震空区.震源区和地震空区的深部构造背景的研究对深入了解中强地震的深部孕育环境及地震空区的地震活动性具有重要科学意义.利用本小组布设的15个临时观测地震台以及21个芦山科考台站和21个四川省地震局固定台站记录的远震数据,用H-K叠加方法得到各个台站的地壳厚度和平均泊松比,并构建了接收函数共转换点(CCP)偏移叠加图像以及反演得到台站下方的S波速度模型.我们的结果揭示了震源区和地震空区地壳结构特征差异:(1)汶川震源区的地壳平均泊松比为~0.28;芦山震源区为~0.29;而地震空区处于泊松比变化剧烈的区域;(2)汶川地震与芦山地震的震源区以西下方的Moho面呈现深度上的突变(这与前人的研究成果基本一致),分别从~44 km突变到~59 km,~40 km突变到~50 km,而地震空区地壳平均厚度呈现渐变性变化;(3)地震空区Moho面下凹且具有低速的上地壳.综合一维S波速度结构和H-k以及CCP的初步结果,这可能显示汶川地震的发震断裂在深部方向上向西倾斜并形成切割整个地壳的大型断裂;芦山地震则可能是由于上、下地壳解耦引起的;而地震空区处于两种地震形成机制控制区域的过渡带中.     

High-resolution estimates of lithospheric thickness from Missouri to Massachusetts, USA

This paper presents a new three-dimensional (3-D) model, NA00, of the S-velocity of the upper mantle beneath North America. The model differs from its predecessor NA95 in that it exploits seismograms recorded by a recent dense, broadband array, MOMA, and from independent measurements of North American crustal thickness. Model NA00 is derived by fitting the waveforms of broadband seismic S and surface waves recorded by the MOMA array and inverting them together with the database of waveform fits used for NA95 and the crustal thickness estimates. It is demonstrated that including data from the dense, broadband MOMA array yields a resolving power beneath the array that is of unprecedented quality and relatively constant over a large depth range. This improved resolution provides a unique opportunity for quantifying the structure of the upper mantle in and below the lower, thick Precambrian lithosphere. The high-resolution seismic structure of the imaged high-velocity lithosphere is compared with the thermal structure (estimated from heat flow), compositional structure (estimated from xenoliths and electrical conductivity) and the elastic structure (estimated from gravity and topography). There is a remarkable agreement between the seismic, thermal, and compositional estimates. The seismic lithosphere is 180 km thick below Missouri and Illinois, 200 km thick below Indiana, Ohio and Pennsylvania, practically undefined below New York, and 80 km below Massachusetts and the Atlantic continental shelf. The thick lithosphere is underlain by a layer with lower S-velocities that could represent a relatively low-viscosity channel. However, the S-velocities in this layer are much higher than those of typical oceanic asthenosphere.     

A seismic profile in southern Norway

Data from a reversed seismic crustal profile crossing the Caledonides of southern Norway suggest a continuously increasing P-wave velocity followed by a low-velocity zone extending from a depth of about 14 km downwards. Interpretation of observed travel time and amplitude characteristics leads to a thickness of 4 km and velocity of 6.0 km/s for this layer, the velocity above and below it being about 6.6 km/s. The role of such a velocity inversion is discussed in the tectonic framework of the Caledonian orogeny and other relevant geophysical information, and it is suggested that the low-velocity zone materializes deep-seated sediments under Precambrian crystalline rocks. The structure of the lower crust is more tentatively established, but still in reasonable agreement with other seismic investigations in the general profiling area.     

影响松潘地区深反射地震数据品质的几种因素

深反射地震剖面技术是揭示岩石圈精细结构的有效手段,获得高质量的原始资料是揭示岩石圈精细结构探讨地球动力学过程的前提和基础.松潘地块地表条件和地下地质构造复杂,资料信噪比低.本文针对松潘地区地表地质条件、激发接收条件及环境等因素,通过实例数据对比分析影响该工区原始单炮记录品质的主要原因,为反射地震勘探数据采集工作提供几点参考和建议.     

The structure of the crust in TESZ area by kriging interpolation

A precise 3D model of the crust is necessary to start any tectonic or geodynamic interpretation. It is also essential for seismic interpretations of structures lying below as well as for correct analysis of shallow structures using reflection seismics. During the last decades, a number of wide-angle refraction experiments were performed on the territory of central and eastern Europe (POLONAISE’97, CELEBRATION 2000, SUDETES 2003), resulting in many high quality 2D models. It is an interesting and complicated transition zone between Precambrian and Palaeozoic Platforms. This paper presents 3D model of the velocity distribution in the crust and upper mantle interpolated from 2D models of the structure along 33 profiles. The obtained model extends to a depth of 50 km and accurately describes the main features of the crustal structures of Poland and surrounding areas. Different interpolation techniques (Kriging, linear) are compared to assure maximum precision. The final model with estimated uncertainty is an interesting reference of the area for other studies.     

Experience from the Ecors program in regions of complex geology

The French Ecors program was launched in 1983 by a cooperation agreement between universities and petroleum companies. Crustal surveys have tried to find explanations for the formation of geological features, such as rifts, mountains ranges or subsidence in sedimentary basins. Several seismic surveys were carried out, some across areas with complex geological structures. The seismic techniques and equipment used were those developed by petroleum geophysicists, adapted to the depth aimed at (30–50 km) and to various physical constraints encountered in the field. In France, Ecors has recorded 850 km of deep seismic lines onshore across plains and mountains, on various kinds of geological formations. Different variations of the seismic method (reflection, refraction, long-offset seismic) were used, often simultaneously. Multiple coverage profiling constitutes the essential part of this data acquisition. Vibrators and dynamite shots were employed with a spread generally 15 km long, but sometimes 100 km long.Some typical seismic examples show that obtaining crustal reflections essentialy depends on two factors: (1) the type and structure of shallow formations, and (2) the sources used. Thus, when seismic energy is strongly absorbed across the first kilometers in shallow formations, or when these formations are highly structured, standard multiple-coverage profiling is not able to provide results beyond a few seconds. In this case, it is recommended to simultaneously carry out long-offset seismic in low multiple coverage.Other more methodological examples show: how the impact on the crust of a surface fault may be evaluated according to the seismic method implemented (vibroseis 96-fold coverage or single dynamite shot); that vibrators make it possible to implement wide-angle seismic surveying with an offset 80 km long; how to implement the seismic reflection method on complex formations in high mountains.All data were processed using industrial seismic software, which was not always appropriate for records at least 20 s long. Therefore, a specific procedure adapted to deep seismic surveys was developed for several processing steps. The long duration of the vibroseis sweeps often makes it impossible to perform correlation and stack in the recording truck in the field. Such field records were first preprocessed, in order to be later correlated and stacked in the processing center. Because of the long duration of the recordings and the great length of the spread, several types of final sections were replayed, such as: (1) detailed surface sections (0–5 s), (2) entire sections (0–20 s) after data compression, (3) near-trace sections and far-trace sections, which often yield complementary information.Standard methods of reflection migration gave unsatisfactory results. Velocities in depth are inaccurate, the many diffractions do not all come from the vertical plane of the line, and the migration software is poorly adapted to deep crustal reflections. Therefore, migration is often performed graphically from arrivals picked in the time section. Some line-drawings of various onshore lines, especially those across the Alps and the Pyrenees, enable to judge the results obtained by Ecors.     

鄂尔多斯盆地北缘南北向剖面上地幔远震P波层析成像

为深入理解华北克拉通破坏机理,本研究通过鄂尔多斯盆地北缘南北向宽频带线性高密度流动地震台观测记录的远震波形数据,获得了1985条高质量到时资料,进而利用FMTT(Fast Marching Teleseismic Tomography)快速行进层析成像方法获得了华北克拉通西部鄂尔多斯盆地北缘深至300km范围的P波速度模型.结果显示,鄂尔多斯盆地下方呈现出深至150~200km的高波速异常,说明华北克拉通其西部岩石圈保存完好尚未遭到明显破坏.河套地堑下方存在向鄂尔多斯盆地下方延伸的明显低波速异常,其深度可达300km深度,而阴山造山带下方上地幔存在深至100km左右的弱高波速异常,说明鄂尔多斯盆地周边地区的岩石圈均遭到一定程度破坏并减薄,可能与新生代时期太平洋板块的俯冲引起深部热物质上涌等作用密切相关.这些研究结果说明,华北克拉通在构造演化过程中不同块体经历了不同的破坏演化历史,这对于认识克拉通破坏减薄机制具有重要意义.     

2010年以来四川地区中强地震震源机制反演及深度确定

2008年5月12日四川龙门山断裂带发生了汶川8.0级地震,之后四川境内发生了两次7.0级地震(其中一个是芦山地震),为了研究汶川地震之后龙门山断裂带及周边区域的地震活动性,本研究收集了国家地震台网和四川区域地震台网2010年1月1日—2017年12月31日四川地区发生的17次M≥5.0地震以及120多次5.0>M≥4.0地震的波形资料,利用波形拟合法反演了震源机制解及区域应力场.反演结果显示,位于龙门山断裂带上的地震,震源机制以逆冲型为主,鲜水河断裂带地震震源机制以走滑型为主,而川滇块体西南部的理塘断裂、金沙江断裂附近,震源机制解以正断层为主.根据震源机制解反演得到的龙门山地区、鲜水河地区的主压应力场方向为WNW、近EW向.川滇块体的巴塘、理塘等地区,其主压应力轴方向为12°左右,接近SN向,且仰角接近40°左右.本研究利用面波振幅谱特征对震源深度进行了精确定位,定位结果与中国地震台网中心(CENC),美国地震调查局(USGS),国际地震中心(ISC)等机构地震目录进行了对比.结果显示,四川地区强震震源深度主要分布在20km以上的中上地壳.龙门山地区震源优势分布在10～20km,鲜水河断裂地震震源深度在10km左右,川滇块体西南部的理塘断裂,巴塘断裂,金沙江断裂等地区,震源深度一般在5～10km范围.     

Removal of overburden velocity anomaly effects for depth conversion

A method of compensating for the presence of discrete overburden velocity anomalies during depth conversion of time horizons interpreted on conventional, post-stack time-migrated seismic data is presented. Positive and negative time delays are estimated either from the push-down or pull-up of reflectors directly beneath the anomalies or from interpreted time thickness of the anomalous body and interval velocities estimated from well data. The critical steps are pre‐stack simulation of seismic acquisition across the velocity anomalies, incorporating the effects of a Fresnel volume which changes its width as a function of depth, and simulation of common-midpoint (CMP) stacking using a linear regression of time delay, Δ t , versus offset-squared, X ². The time-correction method predicts the time distortion for any target horizon and the distortion is removed as a correction in time. Depth conversion is then performed using a background velocity function. The final average velocity map is calculated from the resulting depth structure and the raw times at the target horizon. The method is implemented by manipulating time grids within an industry-standard mapping package. The final average velocity map shows steep lateral velocity gradients which are constrained by the interpreted boundaries of the velocity anomalies.