Pre-stack Data Research Articles

A fast reduced-rank interpolation method for prestack seismic volumes that depend on four spatial dimensions

Rank reduction strategies can be employed to attenuate noise and for prestack seismic data regularization. We present a fast version of Cadzow reduced-rank reconstruction method. Cadzow reconstruction is implemented by embedding 4D spatial data into a level-four block Toeplitz matrix. Rank reduction of this matrix via the Lanczos bidiagonalization algorithm is used to recover missing observations and to attenuate random noise. The computational cost of the Lanczos bidiagonalization is dominated by the cost of multiplying a level-four block Toeplitz matrix by a vector. This is efficiently implemented via the 4D fast Fourier transform. The proposed algorithm significantly decreases the computational cost of rank-reduction methods for multidimensional seismic data denoising and reconstruction. Synthetic and field prestack data examples are used to examine the effectiveness of the proposed method.

Optimizing CSG development: Quantitative estimation of lithological and geomechanical reservoir quality parameters from seismic data

SUMMARY Work over the last decade on seismic azimuthal anisotropy has identified a link between fracture density and orientation observed by well logs and the intensity and orientation of the observed anisotropy. Recent work has correlated these measurements to provide quantitative estimates of fracture density from 3D wide-azimuth seismic data for tight gas sands. The work highlights the impact of advanced seismic processing in successfully recovering reliable fracture estimates which correlate well with borehole observations. These kind of areal, quantitative estimates of fracture density provide a valuable tool to guide drilling and completion programs in tight reservoirs. Building upon this work and considering Coal Seam Gas plays in particular we need to consider some additional reservoir quality parameters whilst trying to impose the same quantitative approach on the interpretation of seismic data and correlation with borehole logging observations. The characterization of CSG plays involves the understanding of the reservoir matrix properties as well as the in-situ stresses and fracturing that will determine optimal producing zones. Pre-stack seismic data and azimuthal WAZ (wide azimuth) seismic processing can help in the identification of sweet spots in CSG resource plays through detailed reservoir-oriented gather conditioning followed by prestack seismic inversion and multi-attribute analysis. This analysis provides rock property estimates such Poisson’s ratio, and Young’s modulus, amongst others. These properties are in turn related to quantitative reservoir properties such as porosity and brittleness. In this presentation we show an integrated approach based on pre-stack azimuthal seismic data analysis and well log information to identify sweet spots, estimate geomechanical properties and in situ principal stresses.

Optimal stacking for multi-azimuth pre-stack seismic data

SummaryOffshore exploration for hydrocarbons in increasingly challenging environments often requires more advanced acquisition methods than conventional 3D narrowazimuth towed streamer to better image the sub-surface for AVO analysis and reservoir characterization. Multi- Azimuth (MAZ), Wide-Azimuth (WAZ) or Full-Azimuth (FAZ) seismic acquisition overcomes the limitations of the conventional acquisition in better illuminating the sub-surface, suppressing the multiple and enhancing signal to noise (S/N) ratio. Nevertheless, to realize the added value of multi-azimuth data, the data need to be combined in a way that will overcome the issues such as time-shift and amplitude difference due to varied illumination between the surveys.This paper describes a method that can be used for combining MAZ pre-stack data to generate AVOpreserving common image gathers (CIGs) in the presence of poor illumination. Based on the concept of crosscorrelation, MAZ CIGs are first flattened to account for any inaccuracy in the velocity model and imaging process so as to align the events to a pilot. Repeating the crosscorrelation process, weights are then derived from the correlation coefficients and applied to individual offsets that take into account the AVO behaviour. With this post-migration processing, any anomaly in AVO resulted from poor illumination can be mitigated. Applying it on MAZ post-stack data, the method can also provide optimal stacking for obtaining higher S/N images.We demonstrate, through synthetic and real data examples, that clearer images with high AVO fidelity can be obtained from MAZ data using our optimal stacking method.

Elliptical dip moveout (EMO) for 3D seismic imaging in the presence of azimuthal anisotropy

Seismic images of the earth?s interior can be significantly distorted by complex wave propagation effects arising from 3D structural velocity variations, combined with the presence of azimuthal velocity anisotropy within some of the rock layers. Most image processing techniques attempt to separate and compensate for both of these phenomena sequentially; they rarely address both simultaneously. These approaches implicitly assume that the effects of 3D structural velocity and azimuthal anisotropy are separable, when in fact both effects are coupled together in the seismic data. The presence of strong azimuthal velocity anisotropy can lead to significant errors in estimated seismic velocity and degraded quality of subsurface images, especially for large source-receiver offsets, wide azimuths and steep geologic dips. Such imaging errors can greatly increase the uncertainty associated with exploring, characterizing, developing and monitoring subsurface geologic features for hydrocarbons, geothermal energy, CO2 sequestration, and other important geophysical imaging applications. Our approach is to simultaneously address velocity structure and azimuthal anisotropy by development of an elliptic dip moveout (DMO) operator. We combine the structural-velocity insensitivity of isotropic DMO with elliptic moveout representative of azimuthal velocity anisotropy. Forward and adjoint elliptical DMO operators are then cascaded together to form a single elliptical moveout (EMO) operation, which has a skewed saddle-like impulse response that resembles an isotropic azimuthal moveout operator. The EMO operator can be used as a prestack data conditioner, to estimate azimuthal anisotropy in a domain that is relatively insensitive to 3D velocity structure, or to compensate and map the data back to its original prestack domain in its approximately equivalent isotropic wavefield form. We show that EMO can reduce structural dip image errors of 10-20o or more for realistic azimuthal anisotropy values at far offsets.

Seismic coherent and random noise attenuation using the undecimated discrete wavelet transform method with WDGA technique

One of the most crucial challenges in seismic data processing is the reduction of the noise in the data or improving the signal-to-noise ratio. In this study, the 1D undecimated discrete wavelet transform (UDWT) has been acquired to attenuate random noise and ground roll. Wavelet domain ground roll analysis (WDGA) is applied to find the ground roll energy in the wavelet domain. The WDGA will be a substitute method for thresholding in seismic data processing. To compare the effectiveness of the WDGA method, we apply the 1D double density discrete wavelet transform (DDDWT) using soft thresholding in the random noise reduction and ground roll attenuation processes. Seismic signals intersect with ground roll in the time and frequency domains. Random noise and ground roll have many undesirable effects on pre-stack seismic data, and result in an inaccurate velocity analysis for NMO correction. In this paper, the UDWT by using the WDGA technique and DDDWT (using the soft thresholding technique) and the regular Fourier based method as f?k transform will be used and compared for seismic denoising.

Tomographic velocity inversion for ADCIGs in areas with a rugged surface

Pre-stack depth migration velocity analysis is one of the key techniques influencing image quality. As for areas with a rugged surface and complex subsurface, conventional prestack depth migration velocity analysis corrects the rugged surface to a known datum or designed surface velocity model on which to perform migration and update the velocity. We propose a rugged surface tomographic velocity inversion method based on angle-domain common image gathers by which the velocity field can be updated directly from the rugged surface without static correction for pre-stack data and improve inversion precision and efficiency. First, we introduce a method to acquire angle-domain common image gathers (ADCIGs) in rugged surface areas and then perform rugged surface tomographic velocity inversion. Tests with model and field data prove the method to be correct and effective.

3D seismic imaging of volcanogenic massive sulfide deposits in the Flin Flon mining camp, Canada: Part 2 — Forward modeling

We applied seismic modeling for a detailed 3D geologic model of the Flin Flon mining camp (Canada) to address some imaging and interpretation issues related to a [Formula: see text] 3D survey acquired in the camp and described in a complementary paper (part 1). A 3D geologic volumetric model of the camp was created based on a compilation of geologic data constraints from drillholes, surface geologic mapping, interpretation of 2D seismic profiles, and 3D surface and grid geostatistical modeling techniques. The 3D modeling methodology was based on a hierarchical approach to account for the heterogeneous spatial distribution of geologic constraints. Elastic parameters were assigned within the model based on core sample measurements and correlation with the different lithologies. The phase-screen algorithm used for seismic modeling was validated against analytic and finite-difference solutions to ensure that it provided accurate amplitude-variation-with-offset behavior for dipping strata. Synthetic data were generated to form zero-offset (stack) volume and also a complete prestack data set using the geometry of the real 3D survey. We found that the ability to detect a clear signature of the volcanogenic massive sulfide with ore deposits is dependent on the mineralization type (pyrite versus pyrrhotite rich ore), especially when ore-host rock interaction is considered. In the presence of an increasing fraction of the host rhyolite rock within the model volume, the response from the lower impedance pyrrhotite ore is masked by that of the rhyolite. Migration tests showed that poststack migration effectively enhances noisy 3D DMO data and provides comparable results to more computationally expensive prestack time migration. Amplitude anomalies identified in the original 3D data, which were not predicted by our modeling, could represent potential exploration targets in an undeveloped part of the camp, assuming that our a priori earth model is sufficiently accurate.

A hybrid imaging solution for a complex near surface

Leon Hu, Saleh M. Al-Saleh and Krzysztof (Kris) Sliz show how the imaging of the well known challenges of some Middle East near-surface conditions can be achieved with a hybrid datuming technique, based on space-frequency explicit wavefield extrapolation, that can datum prestack data directly from topography and also handle strong lateral velocity variations.

Part II — CRS-beam PSDM: Kirchhoff-beam prestack depth migration using the 2D CRS stacking operator

Beam-migration methods efficiently extend the benefits of Kirchhoff migration and enhance its imaging accuracy, producing high-fidelity images of complex geological structures . The common-reflection-surface (CRS) stack method produces zero-offset (ZO) sections with a high signal-to-noise ratio and better continuity of reflection events, particularly for dipping structures, from multi-coverage data. The CRS stacking operator depends on three kinematic attributes determined from prestack data with automatic optimization procedures. These attributes have several applications: velocity model determination via tomography , the determination of projected Fresnel zones, and time and depth migration. Following the concepts of the beam-migration methods, we present a new depth migration procedure, called CRS-beam prestack depth migration (CRS-beam PSDM), that combines Kirchhoff depth migration and the CRS stacking method. This new migration approach is based on the CRS stacking operator's ability to collect paraxial contributions around each reference trace amplitude that is to be migrated through the Huygens curve. The present migration algorithm was tested on a synthetic 2D seismic dataset containing reflections from the steeply dipping interfaces. The results obtained show significant improvements when compared to conventional Kirchhoff migration. The algorithm was also applied to prestack migrate a real, low-fold, 2D seismic line acquired in the Tacutu basin, Brazil. The comparison of our two final results demonstrates improvements to image resolution and quality; thus, our new method leads to more accurate geological interpretations. ► We present a new beam type prestack depth migration based on the CRS stack method. ► The CRS traveltime is used for the beam stack on the fly traces prior to migration. ► The aperture for the beam stack is estimated from the CRS attributes. ► The migrated energy is concentrated based on the dip and curvature information.

Frequency-dependent AVO attribute: theory and example

Fluid-saturated rocks generally have seismic velocities that depend upon frequency. Exploring this property may help us discriminate different fluids from seismic data. In this paper, we introduce a scheme to calculate a frequency-dependent AVO attribute in order to estimate seismic dispersion from pre-stack data, and apply it to North Sea data. The scheme essentially combines the two-term approximation of Smith and Gidlow (1987) with the method of spectral decomposition based on the Wigner-Ville distribution, which is used to achieve high resolution. The result suggests the potential of this method for detection of seismic dispersion due to fluid saturation.

Pre-stack seismic inversion based on a genetic algorithm: A case from the llanos basin (Colombia) in the absence of well information

The Llanos basin is the most prolific of the Colombian basins; however few stratigraphic plays have been explored due to the uncertainty in determining the lithology of the channels. Inside a migrated 2D section, a wide channel was identified inside a prospective sandy unit of the Carbonera Formation, composed by intercalations of sand and shale levels, and considered a main reservoir in this part of the basin. However, the lithology filling the channel was unknown due to the absence of wells. To infer the channel lithology, and diminish the prospective risk a model based pre-stack seismic inversion was proposed.However, without well logs available along the line, the uncertain initial model diminishes reliance on the inversion. To circumvent this impasse, a seismic inversion with a genetic algorithm was proposed. The algorithm was tested on synthetic seismograms and real data from an area of the basin, where well logs were available. The error analysis between the expected and the inverted results, in both scenarios, pointed out a good algorithmic performance. Then, the algorithm was applied to the pre stack data of the 2D line where the channel had been identified.According to the inverted results and rock physics analysis of wells near the seismic line with comparative geology, classified the channel was described as to be filled by silt, shale and probably some levels of shaly sands, increasing the exploratory risk because this lithology has low porosity and permeability, contrary to the producing reservoirs in neighbor fields, characterized by clean sands of high porosity. The algorithm is useful in areas with few or no borehole logs.

A tensor higher-order singular value decomposition for prestack seismic data noise reduction and interpolation

A patch of prestack data depends on four spatial dimensions ([Formula: see text], [Formula: see text] midpoints and [Formula: see text], [Formula: see text] offsets) and frequency. The spatial data at one temporal frequency can be represented by a fourth-order tensor. In ideal conditions of high signal-to-noise ratio and complete sampling, one can assume that the seismic data can be approximated via a low-rank fourth-order tensor. Missing samples were recovered by reinserting data obtained by approximating the original noisy and incomplete data volume with new observations obtained via the rank-reduction process. The higher-order singular value decompostion was used to reduce the rank of the prestack seismic tensor. Synthetic data demonstrated the ability of the proposed seismic data completion algorithm to reconstruct events with curvature. The synthetic example allowed to quantify the quality of the reconstruction for different levels of noise and survey sparsity. We also provided a real data example from the Western Canadian sedimentary basin.

Kinematic time migration and demigration of reflections in pre-stack seismic data

SUMMARY In kinematic time migration one maps the time, slope and curvature characteristics of seismic reflection events, referred to as reflection-time parameters, from the recording domain of the seismic data to the time-migration domain. The inverse process is kinematic time demigration. We generalize kinematic time migration and demigration in several respects: the reflection-time parameters may belong to arbitrary source–receiver offsets; local heterogeneity of the time-migration velocity model is accounted for; the mapping operations do not depend specifically on the type of diffraction-time function and the parametrization of the velocity model. Time-migration and time-demigration spreading matrices are obtained as byproducts of the mapping operations. These matrices yield a paraxial expression for the connection between midpoint and image-point gather locations of mapped reflection events. Also, we obtain the time-migration counterpart of the so-called second duality theorem in Kirchhoff depth migration. Diffractions and reflections are assumed to be without conversion, and sources and receivers are located along the same measurement surface. Our framework enables the identification of a full set of first- and second-order reflection-time parameters from time-migrated seismic data followed by a kinematic demigration to the recording domain. The idea of this route is to ‘undo’ eventual errors introduced by time migration and result in reliable estimation of recording-domain invariants, that is, parameters insensitive to the time-migration velocity model. The developed concepts associated with time migration are of interest in reflection seismic and global earth applications. Two numerical examples demonstrate the potential of kinematic time migration and demigration techniques in seismic time imaging and velocity-model building.

Heat flow derived from BSR and its implications for gas hydrate stability zone in Shenhu Area of northern South China Sea

Bottom simulating reflectors (BSRs), known as the base of gas hydrate stability zone, have been recognized and mapped using good quality three-dimensional (3D) pre-stack migration seismic data in Shenhu Area of northern South China Sea. Additionally, seismic attribute technique has been applied to better constrain on the distribution of gas hydrate. The results demonstrate that gas hydrate is characterized by “blank” zone (low amplitude) in instantaneous amplitude attribute. The thickness of gas hydrate stability zone inferred from BSR ranges from 125 to 355 m with an average of 240 m at sea water depth from 950 to 1,600 m in this new gas hydrate province. The volume of gas in-place bound in hydrate is estimated from 1.7 × 109 to 4.8 × 109 m3, with the most likely value of around 3.3 × 109 m3, using Monte Carlo simulation. Furthermore, geothermal gradient and heat flow are derived from the depths of BSRs using a conductive heat transfer model. The geothermal gradient varies from 35 to 95°C km−1 with an average of 54°C km−1. Corresponding heat flow values range from 43 to 105 mW m−2 with an average of 64 mW m−2. By comparison with geological characteristics, we suggest that the distribution of gas hydrate and heat flow are largely associated with gas chimneys and faults, which are extensively distributed in Shenhu Area, providing easy pathways for fluids migrating into the gas hydrate stability zone for the formation of gas hydrate. This study can place useful constraints for modeling gas hydrate stability zone from measured heat flow data and understanding the mechanism of gas hydrate formation in Shenhu Area.

Introduction to this special section: Carbonate research in China

Carbonate reservoirs in western China have tremendous potential and are the key area for oil and gas exploration in that country. Compared to conventional carbonate reservoirs, discovering and characterizing this kind of reservoir is much more difficult and presents significant challenges. Solutions to the major geophysical problems began with a detailed analysis of exploration issues for such complex carbonate reservoirs and, subsequently, a set of techniques for complex carbonate reservoir prediction was established. These include highly accurate seismic acquisition, prestack amplitude-preserved processing including prestack depth migration, new rock physics models, prestack elastic inversion, seismic multiattribute optimization and integrated prediction, fracture detection using prestack data with limited azimuthal distribution, and fluid identification based on prestack inversion. Additionally, to solve some special problems, diffraction wavefield separation and imaging algorithms were developed for better imaging of caves. Q-migration and nonstationary deconvolution algorithms were also developed for resolution enhancement. Application of these techniques to field data has proven effective, and drilling results show good agreement with our research. Nevertheless, the field is still in its infancy.

Optimising CSG development: quantitative estimation of lithological and geomechanical reservoir quality parameters from seismic data

Within the past decade, new developments in seismic azimuthal anisotropy have identified a link between fracture density and orientation observed in well logs and the intensity and orientation of the actual anisotropy. Recent studies have shown a correlation between these measurements that provide quantitative estimations of fracture density from 3D wide-azimuth seismic data in tight-gas sand reservoirs. Recent research shows the significance of advanced seismic processing in the successful recovery of reliable fracture estimations, which directly correlates to borehole observations. These quantitative estimations of fracture density provide valuable insight that helps optimise drilling and completion programs, particularly in tight reservoirs. Extending this analysis to CSG reservoirs needs to consider additional reservoir quality parameters while implementing a similar quantitative approach on the interpretation of seismic data and correlation with borehole logging observations. The characterisation of CSG plays involves the understanding of the reservoir matrix properties as well as the in-situ stresses and fracturing that will determine optimal production zones. Pre-stack seismic data can assist with identifying the sweet spots—productive areas—in CSG resource plays by detailed reservoir-oriented gather conditioning followed by pre-stack seismic inversion and multi-attribute analysis. This analysis provides rock property estimations such as Poisson's ratio and Young's modulus, among others, which in turn relate to quantitative reservoir properties such as porosity and brittleness. This study shows an integrated workflow based on pre-stack azimuthal seismic data analysis and well log information to identify sweet spots, estimate geo-mechanical properties, and quantify in-situ principal stresses.

A Study of the Three‐Term Non‐Gaussian Pre‐Stack Inversion Method

AbstractIn view of the wide use of the “Gaussian distribution hypothesis of noise” in geophysical inversion problems, we study the non‐Gaussian distribution characteristics of the noise in pre‐stack seismic data. Upon this analysis, the concept of non‐Gaussian inversion of pre‐stack three‐term inversion is put forward, in which the mixed‐norm is proposed for suppressing both the Gaussian and non‐Gaussian noises. In view of the non‐derivable feature of the objective function, the Powell algorithm is used to solve the objective function. Model test and real seismic data inversion results show the correctness and reliability of the algorithm.

Elliptical dip moveout for 3D seismic imaging in the presence of azimuthal anisotropy

Seismic images of the earth’s interior can be significantly distorted by complex wave propagation effects arising from 3D structural velocity variations, combined with the presence of azimuthal velocity anisotropy within some of the rock layers. Most image-processing techniques attempt to separate and compensate for both of these phenomena sequentially; they rarely address both simultaneously. These approaches implicitly assume that the effects of 3D structural velocity and azimuthal anisotropy are separable, whereas in fact, both effects are coupled together in the seismic data. In the presence of strong azimuthal velocity anisotropy, this can lead to significant errors in seismic velocity estimation and degraded quality of subsurface images, especially for large source-receiver offsets, wide azimuths, and steep geologic dips. Such imaging errors can greatly increase the uncertainty associated with exploring, characterizing, developing and monitoring subsurface geologic features for hydrocarbons, geothermal energy, [Formula: see text] sequestration, and other important geophysical imaging applications. Our approach simultaneously addressed velocity structure and azimuthal anisotropy by development of an elliptic dip moveout (DMO) operator. We combined the structural-velocity insensitivity of isotropic DMO with elliptic moveout representative of azimuthal velocity anisotropy. Forward and adjoint elliptical DMO operators were then cascaded together to form a single elliptical moveout (EMO) operation, which had a skewed saddle-like impulse response that resembles an isotropic azimuthal moveout operator. The EMO operator can be used as a prestack data conditioner, to estimate azimuthal anisotropy in a domain that is relatively insensitive to 3D velocity structure, or to compensate and map the data back to its original prestack domain in its approximately equivalent isotropic wavefield form. We demonstrated that EMO can reduce structural dip image errors of 10°–20° or more for realistic azimuthal velocity anisotropy values at far offsets.

P-wave fracture prediction algorithm using pre-stack data with limited azimuth distribution: A case study in the TZ45 area, Tarim Basin, China

Fractured reservoirs always show anisotropic amplitude features, i.e. the reflection amplitude of seismic waves varies with offset and azimuth (AVOZ). A noise attenuation fracture inversion algorithm is presented for fracture detection based on P-wave AVOZ. The conventional inversion method always fails when applied to limited azimuth data because of the existence of noise. In our inversion algorithm, special attention is paid to suppressing the noise during inversion, to overcome the limitation of the conventional inversion method on limited azimuth data. Numerical models are employed to illustrate the effectiveness of the method. The inversion algorithm is then applied to Tazhong 45 area field data which is acquired under limited azimuth distribution. Compared with cores and fullbore formation microimage (FMI), the inverted results (fracture density and orientation) are reasonable, suggesting that the inversion algorithm is feasible for fracture prediction in the Tarim Basin.

Estimating saturation of gas hydrates using conventional 3D seismic data, Gulf of Mexico Joint Industry Project Leg II

We present a methodology for generating pre-drill estimations of hydrate saturations using conventional 3D seismic data. These seismic-based estimates will be compared with well log derived saturations from the subsequently drilled wells of the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II (JIP Leg II) expedition.Predicting saturation of gas hydrates (Sh-seismic) combines pre-stack seismic inversion, rock physics modeling and stratigraphic interpretation. Before the wells were drilled, no nearby sonic and density logs were available to define and calibrate the elastic property trends for the shallow target interval containing the gas hydrate stability zone. Therefore, rock property trends were established by applying principles of rock physics and shallow sediment compaction, constrained by known regional geological parameters. Sh-seismic volumes were generated by inverting pre-stack data to acoustic impedance (PI) and shear impedance (SI) volumes, and then analyzing deviations from modeled impedance trends. To enhance the quality of the inversion, the signal-to-noise ratio of the offset data was maximized by conditioning the seismic prior to inversion. Seismic stratigraphic interpretation plays an important role by identifying the more promising strata and structures for the presence of gas hydrates.The pre-drill Sh-seismic results are compared with saturations calculated from log data, Sh-log, of the wells drilled in the JIP Leg II campaign. Due to weaker seismic reflections, predictions may be less accurate for low concentrations, such as saturations less than 40%, and for thin intervals below the vertical resolution of the seismic data (about 15m). However, the integrated geophysical workflow is very effective for identifying and quantifying significant hydrate concentrations, making the method a promising prospecting technique.