Design,modelling and imaging of marine seismic swarm surveys

Autonomous marine vehicles instrumented with seismic sensors allow for new efficient seismic survey designs. One such design is the swarm survey, where a group, or swarm, of slow moving autonomous marine vehicles record seismic data from shots fired by a source vessel sailing around circles within the swarm. The size of the swarm is dictated by the maximum offset requirement of the survey, and it can be shaped to acquire wide‐ and full‐azimuth data. The swarm survey design equation describes the relationship between the source and receiver positions of the survey and the subsurface coverage or fold. It is used to adapt the swarm to the seismic survey requirements and to calculate survey duration time estimates as function of available equipment. It is shown that a survey conducted by a slowly moving swarm requires six times fewer shots than an equivalent seabed node survey conducted over 85.5 km². Swarm surveys can also be adapted to efficiently conduct infill surveys and replace multi‐vessel undershoots. The efficiency of the survey can further be increased when the autonomous marine vehicles are towing short streamers with multiple receivers. Synthetic tests show that the seismic images for swarm surveys are comparable to those from streamer surveys, while little variation in image quality is found when reducing the number of autonomous marine vehicles but equipping them with a short streamer with multiple receivers.     

Seafloor seismic acquisition using autonomous underwater vehicles

Although narrow-azimuth towed-streamer data provide good image quality for structural interpretation, it is generally accepted that for wide-azimuth marine surveys seabed receivers deliver superior seismic reflection measurements and seismically derived reservoir attributes. However, seabed surveys are not widely used due to the higher acquisition costs when compared to streamer acquisition. In recent years, there have been significant engineering efforts to automate receiver deployment and retrieval in order to minimize the cost differential and conduct cost-efficient seabed receiver seismic surveys. These engineering efforts include industrially engineered nodes, nodes on a rope deployment schemes and even robotic nodes, which swim to and from the deployment location. This move to automation is inevitable, leading to robotization of seismic data acquisition for exploration and development activities in the oil and gas industry. We are developing a robotic-based technology, which utilizes autonomous underwater vehicles as seismic sensors without the need of using a remotely operated vehicle for deployment and retrieval. In this paper, we describe the autonomous underwater vehicle evolution throughout the project years from initial heavy and bulky nodes to fully autonomous light and flexible underwater receivers. Results obtained from two field pilot tests using different generations of autonomous underwater vehicles indicate that the seismic coupling, and navigation based on underwater acoustics are very reliable and robust.     

频率慢度域自适应迭代反演算法压制海上倾斜缆鬼波方法及其应用

海上倾斜缆采集技术具有多样的陷波特征,通过去鬼波处理可获得宽频数据.针对海水面波浪起伏及缆深误差引起的鬼波延迟时间估计误差以及崎岖海底和目的层深度变化使得鬼波和一次反射波的振幅差异系数随偏移距的变化而难以给定一个固定值的问题,本文推导出频率慢度域中鬼波滤波算子以及自适应迭代反演求解上行波算法,该鬼波滤波算子与不同水平慢度对应的鬼波和一次反射波的振幅差异系数以及鬼波延迟时间有关.并基于计算出的理论下行波与实际下行波之间的平方误差最小理论实现自适应反演迭代最优计算该振幅差异系数和鬼波延迟时间.合成的及某海上采集的倾斜缆数据去鬼波处理结果表明,频率慢度域自适应迭代反演算法能较好地去除海上变深度缆鬼波,能达到拓宽地震记录频带目的.     

海底电缆多次波压制方法研究

多次波一直是影响地震处理与解释的主要因素之一.本文基于表层相关多次波衰减(Surface Related Multiple Elimination,SRME)理论,对其进行一定的改造,使其可以预测出海底电缆(Ocean Bottom Cable,OBC)数据中所有与表层相关的多次波,而后将预测出的多次波自适应减去.本文...     

基于波动方程的上下缆地震数据鬼波压制方法研究

本文发展基于波动方程的上下缆鬼波压制方法,推导了上下缆地震波场频率波数域波动方程延拓合并公式.基于Fourier变换的波场解析延拓确保上下缆资料振幅相位的一致性,消除了长拖缆远偏移距信号的计算误差,同时具有较高的计算效率;上下缆地震波场的波动方程法合并有效解偶鬼波干涉,实现综合利用上下缆地震数据压制鬼波.理论模型数据和实际采集地震数据的测试表明了方法的有效性.     

黏声介质平面波有限差分叠前深度偏移及在莺歌海盆地的应用

莺琼盆地诸探区中存在底劈现象,深层气源产生的气体沿底劈产生的裂隙通道向上漫溢.漫溢过程中,一是充填在遇到的砂体中形成气藏;二是弥漫在上溢通道中,使得通道中的纵波速度发生变化,进而纵波波阻抗差异变小,反射变弱.另外,通道中气体的存在,会加强地层的吸收衰减,使得地震波振幅变弱、高频成分损失导致同相轴分辨率降低.利用OBC数据进行多波地震勘探和利用黏声介质的叠前深度偏移都是改善模糊区成像质量的重要方法技术.为此,本文提出用黏声介质平面波有限差分法叠前深度偏移成像方法改善气体充填区域的成像质量.黏声介质成像目的是补偿地震波的吸收衰减;平面波偏移成像目的是适应海上单炮数量巨大,提高波动方程叠前深度偏移成像的效率;有限差分法叠前偏移的目的是适应该区浅层气分布局域性极强、Q值的空间变化大的情况.在莺歌海某探区的实际数据上的黏声介质平面波有限差分叠前深度偏移试验证明,本方法是改善模糊区成像质量的较为有效的途径.     

Sea surface shape derivation above the seismic streamer

The rough sea surface causes perturbations in the seismic data that can be significant for time‐lapse studies. The perturbations arise because the reflection response of the non‐flat sea perturbs the seismic wavelet. In order to remove these perturbations from the received seismic data, special deconvolution methods can be used, but these methods require, as input, the time varying wave elevation above each hydrophone in the streamer. In addition, the vertical displacement of the streamer itself must also be known at the position of each hydrophone and at all times. This information is not available in conventional seismic acquisition. However, it can be obtained from the hydrophone measurements provided that the hydrophones are recorded individually (not grouped), that the recording bandwidth is extended down to 0.05 Hz and that data are recorded without gaps between the shot records. The sea surface elevation, and also the wave‐induced vertical displacement of the streamer, can be determined from the time‐varying pressure that the sea waves cause in the hydrophone measurements. When this was done experimentally, using a single sensor seismic streamer without a conventional low cut filter, the wave induced pressure variations were easily detected. The inversion of these experimental data gives results for the sea surface elevation that are consistent with the weather and sea state at the time of acquisition. A high tension approximation allows a simplified solution of the equations that does not demand a knowledge of the streamer tension. However, best results at the tail end of the streamer are obtained using the general equation.     

A HIGH RESOLUTION MARINE SEISMIC SURVEY *

Sub-bottom profilers achieve high resolution but limited penetration beneath the seabed. Marine seismic surveys with long streamers and conventional sources suffer from an incomplete CDP stack and an unnecessarily low signal frequency in the early part of the section. Consequently there is a depth interval over which the optimum resolution of the seismic reflection method is not achieved. This paper describes the design, conduct, and results of an experimental survey made to obtain high resolution sections (predominant frequency about 200 Hz) with reflection times up to one second in the northern North Sea. The prime motivation for the experiment was to assess the possibility of mapping a shallow gas sand which had been encountered by the drillers. The design concept of the survey was to keep all the benefits of conventional marine seismic data gathering and processing, but to reduce their dimensions and time scales by a factor of five to ten. The survey in 1972 recorded a total of twenty-four lines using twenty-four fold cover with 5 m spacing between the reflector points, and a separation of 50 m between adjacent lines. Multi-electrode sparkers were used with a shallow purpose-built twenty-four channel 240 m hydrophone streamer. The recording sample interval was 1 ms. The final sections, obtained after conventional data processing, were of good quality. The results of velocity analysis helped to distinguish primary and multiple energy. Reflections with large negative reflection coefficients were seen in some parts of the survey area, and these are thought to indicate thin gas sands.     

最小二乘反演迭代算法在压制海上变深度缆采集数据虚反射中的应用

海水面的虚反射(鬼波)引起海上拖缆采集数据陷波,导致地震记录频带变窄,而近年发展的变深度缆采集技术,具有多样的陷波特征,通过专门的去虚反射处理方法可获得宽频数据.本文基于已有研究成果,将最小二乘反演迭代压制虚反射算法应用于某海上变深度缆宽频处理.基于频率波数域镜像记录生成方法获得镜像炮集记录,并采用最小二乘解从变深度缆原始和镜像炮集记录中提取上行波.针对镜像炮集记录生成受初始速度模型精度的影响,使得某深度缆接收的上行波和下行波之间的实际延迟时间存在误差,采用最小二乘反演迭代算法最优化计算下行波与上行波之间的平均延迟时间和上行波记录,并采用时空数据窗口滑动克服延迟时间随炮检距和目的层深度变化问题.合成数据及某海上实际变深度缆数据处理测试结果表明,该方法能较好地压制变深度缆由海水面产生的虚反射,能达到拓宽地震记录频带目的.     

Three‐dimensional inversion of towed streamer electromagnetic data

A towed streamer electromagnetic system capable of simultaneous seismic and electromagnetic data acquisition has recently been developed and tested in the North Sea. We introduce a 3D inversion methodology for towed streamer electromagnetic data that includes a moving sensitivity domain. Our implementation is based on the 3D integral equation method for computing responses and Fréchet derivatives and uses the re‐weighted regularized conjugate gradient method for minimizing the objective functional with focusing regularization. We present two model studies relevant to hydrocarbon exploration in the North Sea. First, we demonstrate the ability of a towed electromagnetic system to detect and characterize the Harding field, a medium‐sized North Sea hydrocarbon target. We compare our 3D inversion of towed streamer electromagnetic data with 3D inversion of conventional marine controlled‐source electromagnetic data and observe few differences between the recovered models. Second, we demonstrate the ability of a towed streamer electromagnetic system to detect and characterize the Peon discovery, which is representative of an infrastructure‐led shallow gas play in the North Sea. We also present an actual case study for the 3D inversion of towed streamer electromagnetic data from the Troll field in the North Sea and demonstrate our ability to image all the Troll West Oil and Gas Provinces and the Troll East Gas Province. We conclude that 3D inversion of data from the current generation of towed streamer electromagnetic systems can adequately recover hydrocarbon‐bearing formations to depths of approximately 2 km. We note that by obviating the need for ocean‐bottom receivers, the towed streamer electromagnetic system enables electromagnetic data to be acquired over very large areas in frontier and mature basins for higher acquisition rates and relatively lower cost than conventional marine controlled‐source electromagnetic methods.     

由宽角反射地震资料重建壳幔反射结构的相似性剖面

针对人工源宽角地震测深中炮点与接收点稀疏的特点,利用几何散射理论进行壳幔散射体的相似性剖面成像,利用互易性原理计算散射体相似性成像的时间条件,研究基于宽角反射地震资料重建壳幔反射结构的相似性成图方法. 其中,成像点的时间采用快速、高精度的程函方程求解技术. 利用该方法处理东南陆缘区宽角反射剖面实际资料,结果表明,较之壳幔反射结构的CDP成图技术,相似性成图方法具有提高反射同相轴横向连续性的能力.     

小波域叠前地震资料衰减参数定性估计方法

叠前地震资料中,高频分量和低频分量随传播距离的衰减特性不同.本文给出了一种在小波域定性估计叠前地震资料衰减参数的方法.该方法利用连续小波变换提取共反射点道集的高、低频分量,以低频分量和高频分量之差定性反映地震波的衰减.通过累加不同偏移距的衰减,提高了估计的稳定性;采用幅度归一化方法,降低了信号幅值对衰减参数估计的影响.将本文提出方法与常用的基于叠后地震资料衰减估计方法用于某油田的地震资料处理,结果表明,本文方法得到的衰减估计结果能够更好地反映油气的空间展布.     

中海油多分量地震处理技术的发展与应用实例

20世纪末中海油在南海油气勘探中成功试验了二维多分量地震技术.其后十多年,中海油进一步在不同近海油气盆地采集了三维多分量OBC地震资料.但由于针对三维转换波的处理未能形成有效的关键技术,除双检叠加利用了水压分量和陆检垂直分量外,大量水平分量数据未被使用.鉴于此,中海油在十二五期间开展三维多分量地震数据处理的关键技术攻关：针对弹性波场的矢量特征、转换波射线路径不对称以及双程旅行时多时间尺度等问题,成功研发了矢量化的信号处理技术、弹性波速度建模以及叠前时间偏移成像等技术;集成了相对完善的三维弹性波成像EWI软件系统;完成了多个试验区的三维四分量OBC地震转换波的处理,取得了好于以往技术的效果;建立了海上多分量OBC地震资料处理流程.但相对于成熟的纵波处理技术,海洋多分量地震处理技术仍需要不断完善与发展.     

基于流体动力学数值模拟的海水层反射地震研究

由于多种海水运动同时存在以及地形的影响,海水层结构非常复杂,解释海水层地震相,分析海水运动过程是地震海洋学研究的新方向.本文提出结合流体动力学数值模拟与反射地震正演分析海水层地震相的方法.首先,对地形和流体建模,得到特定条件下流体运动状态;然后用反射地震正演将模拟获得的海水层温盐剖面转换为反射地震数据;进一步和实际测量得到的地震海洋学剖面进行对比,分析地形、海水运动对海水层地震相的影响.以内孤立波浅化过程为例,通过流体动力学数值模拟,获得其浅化过程中出现的下沉型、分裂、转换型三个阶段的海水层剖面;对温盐剖面进行反射地震正演,分析浅化不同阶段海水层反射几何形态、反射结构等特征.这种新方法有望解释复杂地震海洋学图像,深化海底地形对海水运动影响的认识.     

Volcanism offshore of Vesuvius Volcano in Naples Bay

 High-resolution seismic reflection data are used to identify structural features in Naples Bay near Vesuvius Volcano. Several buried seismic units with reflection-free interiors are probably volcanic deposits erupted during and since the formation of the breached crater of Monte Somma Volcano, which preceded the growth of Vesuvius. The presumed undersea volcanic deposits are limited in extent; thus, stratigraphic relationships cannot be established among them. Other features revealed by our data include (a) the warping of lowstand marine deposits by undersea cryptodomes located approximately 10 km from the summit of Vesuvius, (b) a succession of normal step faults that record seaward collapse of the volcano, and (c) a small undersea slump in the uppermost marine deposits of Naples Bay, which may be the result of nueé ardentes that entered the sea during a major eruption of Vesuvius in 1631. Detection of these undersea features illustrates some capabilities of making detailed seismic reflection profiles across undersea volcanoes. Received: 16 September 1997 / Accepted: 23 November 1997     

Separation of P- and S-wavefields from wide-angle multicomponent OBC data for a basalt model

Wide-angle multicomponent ocean-bottom cable (OBC) data should further enhance sub-basalt imaging by using both compressional and converted shear wavefields. The first step in analysing multicomponent OBC data is to decompose the recorded wavefields into pure P- and pure S-wavefields, and extract the upgoing P- and S-waves. This paper presents a new scheme to separate P- and S-wavefields from wide-angle multicomponent OBC data in the τ–p domain. By considering plane-wave components with a known horizontal slowness, the P- and S-wavefields are separated into the directions of observed P- and S-wave oscillations using the horizontal and vertical components of the data. The upgoing P- and S-waves are then extracted from the separated P- and S-wavefields. The parameters used in the separation are the seismic wave velocities and the density at the receiver location, which can be estimated from the first reflection phase observed on the horizontal and vertical components. Numerical tests on synthetic data for a plane-layered model show good performance and demonstrate the accuracy of the scheme. Separation of wavefields from a basalt model is performed using synthetic wide-angle multicomponent OBC data. The results show that both near-offset and wide-angle reflections and conversions from within and below basalt layers are enhanced and clearly identified on the separated wavefields.     

复杂地表的单程波动方程地震叠前正演

作者基于数学检波器和等时叠加原理,实现了复杂地表的单程波动方程地震叠前正演模拟。该方法采用虚拟的数学检波器接收地下的反射地震信号,灵活地将接收点布置在地表的任何地方,从而满足地表起伏的要求。此外,根据等时叠加原理, 该方法采用单程波动方程进行波场延拓和成像,计算简单快速。通过复杂正断层的数值模拟,得到了高信噪比的共炮集地震记录,并采用适用于起伏地形的深度偏移方法对该共炮集地震记录进行了叠前深度偏移,较好地实现了地震波的偏移归位,从而证明了这里提出的起伏地表的单程波动方程地震叠前正演方法是正确和有效的。     

Deghosting towed streamer data in τ/p domain based on rough sea surface reflectivity

Currently, the deghosting of towed streamer seismic data assumes a flat sea level and a sea-surface reflection coefficient of ?1; this decreases the precision of deghosting. A new method that considers the rough sea surface is proposed to suppress ghost reflections. The proposed deghosting method obtains the rough sea surface reflection coefficient using Gaussian statistics, and calculates the optimized deghosting operator in the τ/p domain. The proposed method is closer to the actual sea conditions, offers an improved deghosting operator, removes the ghost reflections from marine towed seismic data, widens the bandwidth and restores the low-frequency information, and finally improves the signal-tonoise ratio and resolution of the seismic data.     

Migration for velocity and attenuation perturbations

Migration maps seismic data to reflectors in the Earth. Reflections are not only caused by small‐scale variations of the velocity and density but also of the quality factor that describes attenuation. We investigated scattering due to velocity and attenuation perturbations by computing the resolution function or point‐spread function in a homogeneous background model. The resolution function is the migration image of seismic reflection data generated by a point scatterer. We found that the resolution function mixes velocity and attenuation parameter perturbations to the extent that they cannot be reconstructed independently. This is true for a typical seismic setting with sources and receivers at the surface and a buried scatterer. As a result, it will be impossible to simultaneously invert for velocity and attenuation perturbations in the scattering approach, also known as the Born approximation. We proceeded to investigate other acquisition geometries that may resolve the ambiguity between velocity and attenuation perturbations. With sources and receivers on a circle around the scatterer, in 2D, the ambiguity disappears. It still shows up in a cross‐well setting, although the mixing of velocity and attenuation parameters is less severe than in the surface‐to‐surface case. We also consider illumination of the target by diving waves in a background model that has velocity increasing linearly with depth. The improvement in illumination is, however, still insufficient to remove the ambiguity.     

LOW-FREQUENCY STANDARD HYDROPHONE * FOR CALIBRATING LINE HYDROPHONE ARRAYS (SEISMIC STREAMERS) AND MARINE SEISMIC SOURCES **

The design of a standard hydrophone with a maximally flat (Butterworth) response in the frequency range 8.0 Hz-1.0 kHz is described. The standard hydrophone has been developed primarily for calibrating line hydrophone arrays (seismic streamers) and marine seismic sources. The standard hydrophone has been used successfully during the past eight years for monitoring the output of a single air gun. It can be used for the calibration of a marine seismic streamer.