Determination of porosity and saturation using seismic waveform inversion

Characterization of a reservoir model requires determination of its petrophysical parameters, such as porosity and saturation. We propose a new method to determine these parameters directly from seismic data. The method consists of the computation and inversion of seismic waveforms. A high frequency method is presented to model wave propagation through an attenuative and dispersive poroelastic medium. The high frequency approximation makes it possible to efficiently compute sensitivity functions. This enables the inversion of seismic waveforms for porosity and saturation. The waveform inversion algorithm is applied to two laboratory crosswell datasets of a water saturated sand. The starting models were obtained using travel time tomography. The first dataset is inverted for porosity. The misfit reduction for this dataset is approximately 50%. The second dataset was obtained after injection of a nonaqueous-phase liquid (NAPL), possibly with some air, which made the medium more heterogeneous. This dataset was inverted for NAPL and air saturation using the porosity model obtained from the first inversion. The misfit reduction of the second experiment was 70%. Regions of high NAPL and high air saturation were found at the same location. These areas correlate well with the position of one of the injection points as well as regions of higher NAPL concentrations found after excavation of the sand. It is therefore possible to directly invert waveforms for pore fluid saturation by taking into account the attenuation and dispersion caused by the poroelasticity.     

Implications for focused fluid transport at the northern Cascadia accretionary prism from a correlation between BSR occurrence a d near-sea-floor reflectivity anomalies imaged in a multi-frequency seismic data set

 A high-resolution seismic survey was carried out at the accretionary prism on the continental slope off Vancouver Island, Canada. Two GI-Gun data sets with different source frequency ranges of 50–150 and 100–500 Hz were combined with 4 kHz narrow-beam echosounding data (Parasound). The data allow spatial correlation between a gas hydrate bottom simulating reflector (BSR) and distinct areas of high near-sea-floor reflectivity. An integrated interpretation of the multi-frequency data set provides insight into the regional distribution of tectonically induced fluid migration and gas hydrate formation in the vicinity of ODP Leg 146 Sites 889 and 890. The BSR at the base of the gas hydrate stability field is observed within accreted and deformed sediments, but appears to be absent within bedded slope basin deposits. It is suggested that these basin deposits inhibit vertical fluid flow and prevent the formation of a BSR, whereas the hydraulic conductivity of the accreted sediments is sufficiently high to allow for pervasive gas migration. An elevation of the BSR beneath the flanks of a topographic high is interpreted as an indicator for local upflow of warm fluids along permeable pathways within outcropping accreted sediments. Parasound data reveal discontinuous zones of high reflectivity at or directly beneath the sea floor, which may indicate local cementation of surface sediments. In combination with GI-Gun data, the occurrence of these reflective areas can be related to the location of slope sedimentary basins acting as hydraulic seals. It is proposed that the seals sometimes fail along faults extending beneath the BSR, leading to focused upflow of methane-bearing fluid and the formation of carbonate pavements at the sea floor. Received: 9 November 1998 / Accepted: 6 April 1999     

Ray and finite-difference modelling of CDP seismic sections for shallow lignite deposits

A numerical experiment carried out to investigate the structural model of the Domenico lignite site is discussed. The model is a 2D structure containing several lignite layers at different depths, and a low-velocity layer at the top of the model. The experiment consists in simulating a measured CDP section by two independent techniques, based on completely different concepts: the finite-difference method and the ray method. Due to the incompleteness of the ray synthetic wave field, as well as to numerical problems of the finite differences at higher frequencies, the agreement between the synthetic seismogram sections for the individual shot points is poor. However, the CDP stacked sections modelled by the ray and finite-difference methods agree rather well. This is because the main differences between the wave fields computed by the two methods are due to the presence of the low-velocity layer (ground roll, head waves, etc.), and just these parts of the wave field can be suppressed by routine data processing such as f–k filtration. Synthetic ray and finite-difference CDP stacks agree relatively well with the observed data. They confirm three lignite seams and a fault in the shallower one. The synthetic data also indicate that many apparent horizons of the measured section may be due to the multiple reflections within the subsurface low-velocity layer.     

Crustal structure of the ultra-slow spreading Knipovich Ridge,North Atlantic,along a presumed amagmatic portion of oceanic crustal formation

The ultra-slow, asymmetrically-spreading Knipovich Ridge is the northernmost part of the Mid Atlantic ridge system. In the autumn of 2002 a combined ocean-bottom seismometer multichannel seismic (OBS/MCS) and gravity survey along the spreading direction of the Knipovich Ridge was carried out. The main objective of the study was to gain an insight into the crustal structure and composition of what is assumed to be an amagmatic segment of oceanic crust. P-wave velocity and Vp/Vs models were built and complemented by a gravity model. The 190 km long transect reveals a much more complex crustal structure than anticipated. The magmatic crust is thinner than the global average of 7.1 ± 1.0 km. The young fractured portion of Oceanic Layer 2 has low seismic velocities while the older part has normal seismic velocities and is broken into several rotated fault blocks seen as thickness variations of Layer 2. The youngest part of Oceanic Layer 3 is also dominated by low velocities, indicative of fracturing, seawater circulation and thermal expansion. The remaining portion of Layer 3 exhibits inverse variations in thickness and seismic velocity. This is explained by a sequence of periods of faster spreading (estimated to be up to 8 mm/year from interpretation of magnetic anomalies) when more normal gabbroic crust was being generated and periods of slower spreading (5.5 mm/year) when amagmatic stretching and serpentinization of the upper mantle occurred, and crust composed of mixed gabbro and serpentinized mantle was generated. The volumetric changes and upward fluid migration, associated with the process of serpentinization in this part of the crust, caused disruption to the overlying sedimentary layers.     

华南大陆东部壳幔结构的宽频带地震学探测

华南大陆是新元古代以来全球地质演化历史最复杂的地区之一,也是欧亚板块东南缘地壳生长和大陆增生最活跃,大规模构造变形、岩浆活动和多金属矿产资源最丰富的地区。揭示该区浅表构造与岩浆活动和成矿作用机制离不开对深部壳幔结构的研究。宽频带地震学是开展深部壳幔结构探测的重要手段,基于宽频带地震学数据可以刻画地壳-岩石圈-上地幔-地幔过渡带不同深度和尺度的深部结构,为深入理解研究区的深部构造、动力学过程、岩浆活动与成矿作用提供有效约束。本文较全面地总结了近二十年来在华南大陆东部地区开展的宽频带流动地震探测工作,并对研究区的地壳厚度、Vp/Vs比值、岩石圈底界(LAB)深度、上地幔速度结构与各向异性等进行了分析与讨论。本文旨在为相关研究人员和团队提供未来在该区新布设地震探测台站时的参考,也可为后续深入研究该区的深部结构与成矿过程提供一些深部要素约束。     

槽波地震勘探在煤矿大型工作面的应用

以陕北某煤矿大型工作面槽波地震工程为例,开展了超大型工作面内断层的槽波地震探测技术研究。槽波探测采用全排列接收,最大限度地保障了槽波信息的获得。根据得到的槽波记录数据以及CT成像结果,解释了工作面内断层的发育情况,与后期巷道揭露情况基本吻合。槽波地震勘探在大型工作面的成功应用,可为矿井实现盘区勘探提供技术支持。      

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.     

Geomorphological and seismostratigraphic evidence for multidirectional polyphase glaciation of the northern Celtic Sea

High-resolution seismic and bathymetric data offshore southeast Ireland and LIDaR data in County Waterford are presented that partially overlap previous studies. The observed Quaternary stratigraphic succession offshore southeast Ireland (between Dungarvan and Kilmore Quay) records a sequence of depositional and erosional events that supports regional glacial models derived from nearby coastal sediment stratigraphies and landforms. A regionally widespread, acoustically massive facies interpreted as the ‘Irish Sea Till’ infills an uneven, channelized bedrock surface overlying irregular mounds and deposits in bedrock lows that are probably earlier Pleistocene diamicts. The till is truncated and overlain by a thin, stratified facies, suggesting the development of a regional palaeolake following ice recession of the Irish Sea Ice Stream. A north–south oriented seabed ridge to the north is interpreted as an esker, representing southward flowing subglacial drainage associated with a restricted ice sheet advance of the Irish Ice Sheet onto the Celtic Sea shelf. Onshore topographic data reveal streamlined bedforms that corroborate a southerly advance of ice onto the shelf across County Waterford. The combined evidence supports previous palaeoglaciological models. Significantly, for the first time, this study defines a southern limit for a Late Midlandian Irish Ice Sheet advance onto the Celtic Sea shelf. © 2020 John Wiley & Sons, Ltd.     

An estimation method for effective stress changes in a reservoir from 4D seismics data

Advances in seismics acquisition and processing and the widespread use of 4D seismics have made available reliable production‐induced subsurface deformation data in the form of overburden time‐shifts. Inversion of these data is now beginning to be used as an aid to the monitoring of a reservoir's effective stress. Past solutions to this inversion problem have relied upon analytic calculations for an unrealistically simplified subsurface, which can lead to uncertainties. To enhance the accuracy of this approach, a method based on transfer functions is proposed in which the function itself is calibrated using numerically generated overburden strain deformation calculated for a small select group of reference sources. This technique proves to be a good compromise between the faster but more accurate history match of the overburden strain using a geomechanical simulator and the slower, less accurate analytic method. Synthetic tests using a coupled geomechanical and fluid flow simulator for the South Arne field confirm the efficacy of the method. Application to measured time‐shifts from observed 4D seismics indicates compartmentalization in the Tor reservoir, more heterogeneity than is currently considered in the simulation model and moderate connectivity with the overlying Ekofisk formation.