Wavefield Separation Research Articles

Artifact Suppression for the Joint Imaging of Primaries and Internal Multiples

Internal multiples bring great challenges for conventional seismic data imaging and interpretation, in which primaries only are regarded as signals. It is of great importance to suppress the internal multiples before conventional imaging, which is computationally expensive. Considering that internal multiples are also real reflections from the underground interfaces and reverse time migration (RTM) can realize the imaging of multiples. Instead of suppression before imaging, we propose to do the joint imaging of primaries and internal multiples based on RTM. However, the artifacts caused by the internal multiples should be suppressed to guarantee the precision. First, the imaging condition based on up-going and down-going extrapolated wavefield separation is introduced to improve the imaging accuracy. Then, the generation mechanism of image artifacts is analyzed, it concludes that nonphysical wave paths caused by the missing boundary wavefields under the surface result in the infidelity reversed-time backward-extrapolation, which is the main reason for the artifacts. Next, we propose to apply the modeling boundary wavefields with a match based on pseudomultichannel matching filter to compensate the missing boundary wavefields, which can avoid the image artifacts and realize high-precision joint imaging. Finally, the feasibility and effectiveness of the proposed method are verified by numerical examples and field data.

The P- and S-Wave Decomposition in a Multicomponent Elastic Wavefield Based on the Divergence and Curl Operators and Their Applications in Elastic Reverse Time Migration

The P- and S-wave separation is an important step in the elastic reverse-time migration (ERTM). It not only removes crosstalk artifacts in the image but also provides additional constrains to subsurface structures by providing images between different wave types. Traditional Helmholtz decomposition based on divergence and curl operations can separate the P and S wave modes but also modify the amplitude, phase and physical dimension of the original coupled wavefields. To recover the separated P- and S-waves to their original forms, we propose a method, in which the corrections to the distorted separated wavefields, including their phase, amplitude and vector polarizations, are organized into two spatial-time domain partial differential equations. Solving these equations can generate the separated P and S wavefields. The method can be conveniently applied in the time-space domain and consistent with most time-domain finite-difference based elastic reverse-time migration method. To verify the quality of the decomposed P- and S-wave components, we apply them to the ERTM using different elastic image conditions. For the PP image, we use the scalar imaging condition by crosscorrelating the corrected scalar Helmholtz potential wavefield from both source- and receiver-sides. For the PS image, we use the magnitude- and sign-based vector imaging condition by crosscorrelating the magnitudes of the source-side vector P wave and the receiver-side vector S wave, whereas the sign is determined through their dot-product results. Thus, accurate PP- and PS-reflectivity images can be obtained. Several numerical examples are used to verify our elastic wavefield separation method and the ERTM workflow.

Attenuation compensation for wavefield‐separation‐based least‐squares reverse time migration in viscoelastic media

ABSTRACTElastic least‐squares reverse time migration can image the multicomponent seismic recordings. However, on the one hand, without considering the intrinsic attenuation of the subsurface, it may produce blurred reflectivity images with incorrect positions of reflectors for seismic recordings with strong attenuation. On the other hand, the crosstalk artefacts created by the different wave modes may severely degrade the imaging quality. To alleviate these crosstalk artefacts and compensate for the attenuation effects, we have developed a time‐domain wavefield‐separation‐based least‐squares reverse time migration approach in viscoelastic media, which utilizes the viscoelastic wave equation on the basis of the standard linear solid model to simulate intrinsic subsurface attenuation. The key point of the proposed approach is that we utilize the separated gradient contribution of PP‐ and PS‐wave modes based on the wavefield separation technique in viscoelastic media to construct the P‐wave and S‐wave velocity images, respectively. Unlike the conventional wavefield‐separation‐based least‐squares reverse time migration approach, which generally uses the new stress–velocity equations to formulate, our proposed wavefield separation scheme fully depends on the conventional viscoelastic wave equation and its adjoint wave equation. By introducing the pure P‐wave stress in the forward and adjoint wavefields, the coupled wavefields can be well decomposed, which requires much less computational costs than the wavenumber‐domain wavefield separation scheme. Numerical examples using the layered and Marmousi‐2 models have shown that the proposed approach can improve the migration image quality under geological conditions with strong attenuation where elastic least‐squares reverse time migration may produce blurred and unfocused events. Meanwhile, the proposed least‐squares reverse time migration approach with the wavefield separation scheme has a better convergence rate and produces fewer crosstalk artefacts than that without the wavefield separation scheme.

Fractured basement imaging using random-space-shift reverse time migration: A vertical seismic profile survey in the Bohai Bay Basin, China

Fractured basements are not only potential reservoirs for hydrocarbon resources, but they also provide significant storage space for carbon dioxide ([Formula: see text]) sequestration and radioactive waste disposal. However, fractured basements are challenging for seismic imaging methods due to the complexities in their fault and fracture networks, strong heterogeneity, highly variable structural dip, and strong impedance contrasts between the basement rocks and the surrounding sediments. We present a field case in which a walkaway vertical seismic profiling (VSP) survey was conducted at a fractured basement play located in the Bohai Bay Basin, China, to improve the resolution compared with a preexisting surface seismic profile. Using the advanced random-space-shift (RSS) reverse time migration (RTM), we obtain a high-resolution image with a clear delineation of the highly faulted dipping basement. From numerical and field examples, we find that the application of the RSS-RTM improves the final image by mitigating unavoidable errors in the migration velocity model, which would otherwise result in an unfocused image using the conventional RTM approach. In addition, we determine the importance of proper wavefield separation using 3C recordings, which is the key to ensuring the quality of the final image. With an optimized VSP imaging workflow, we provide an enhanced image for the fractured basement to support the geologic interpretations and development decisions.

First-order particle velocity equations of decoupled P- and S-wavefields and their application in elastic reverse time migration

The separation of P- and S-wavefields is considered to be an effective approach for eliminating wave-mode crosstalk in elastic reverse time migration (RTM). At present, the Helmholtz decomposition method is widely used for isotropic media. However, it tends to change the amplitudes and phases of the separated wavefields compared with the original wavefields. Other methods used to obtain pure P- and S-wavefields include the application of the elastic wave equations of the decoupled wavefields. To achieve high computational accuracy, staggered-grid finite-difference (FD) schemes are usually used to numerically solve the equations by introducing an additional stress variable. However, the computational cost of this method is high because a conventional hybrid wavefield (the P- and S-wavefields are mixed together) simulation must be created before the P- and S-wavefields can be calculated. We have developed the first-order particle velocity equations to reduce the computational cost. The equations can describe four types of particle-velocity wavefields: the vector P-wavefield, the scalar P-wavefield, the vector S-wavefield, and the vector S-wavefield rotated in the direction of the curl factor. Without introducing the stress variable, only the four types of particle velocity variables are used to construct the staggered-grid FD schemes; therefore, the computational cost is reduced. We also develop an algorithm to calculate the P and S propagation vectors using the four particle velocities, which is simpler than the Poynting vector. Finally, we apply the velocity equations and propagation vectors to elastic RTM and angle-domain common-image gather computations. These numerical examples illustrate the efficiency of our methods.

Extending subsurface imaging beyond ocean-bottom node coverage using multiples: A case study in deepwater Nigeria

We have evaluated the results of a receiver decimation study in a deepwater context using separated wavefield imaging (SWIM) algorithms to provide extended illumination for imaging without ocean-bottom node (OBN) positioning constraints. We carried out subsurface imaging using the SWIM imaging technique with a reduced OBN layout, and we compared the results with those from conventional one-way wave-equation migration. We found from the results that the SWIM algorithm makes it possible to reduce the OBN layout while obtaining a similar subsurface image with the same shot geometry, which allows a reduced receiver acquisition effort, offers more geometry flexibility without affecting the image quality, with a potentially significant reduction of acquisition cost and 4D processing turnaround time.

Controls on the architectural evolution of deep-water channel overbank sediment wave fields: insights from the Hikurangi Channel, offshore New Zealand

ABSTRACT Deep-water channels can be bound by overbank deposits, resulting from overspilling flows, which are often ornamented with sediment waves. Here, high-resolution bathymetry, backscatter, and 2D and 3D seismic data are integrated to discern the controls on flow processes on the overbank areas of the Hikurangi Channel. Qualitative seismic interpretation and quantitative analyses of sediment wave morphologies and distributions are conducted through the shallowest 600 m of stratigraphy up to the seafloor. Four outer-bend wave fields are present throughout the studied stratigraphy on the landward margin (left margin looking down-channel) only. Originally closely spaced or combined, these fields evolved to become spatially separated; two of the separate wave fields became further subdivided into distinct outer- and inner-bend fields, whose constituent waves developed distinct differences in morphology and distribution. Sediment wave character is used to interpret the direction and strength of overbank flow. Nine controls on such flow and associated deposition are identified: flow versus conduit size, overbank gradient, flow tuning, Coriolis forcing, contour current activity, flow reflection, centrifugal forcing, interaction with externally derived flows, and interaction of overspill from different locations. Their relative importance may vary across parts of overbank areas, both spatially and temporally, controlling wave field development such that: (1) outer-bend wave fields only develop on the landward margin; (2) the influence of centrifugal force on outer-bend overbanks has increased through time, accompanying a general increase in channel sinuosity; (3) inner-bend wave fields on the landward margin form by the interaction of Coriolis-enhanced inner-bend overbank flow, and outer-bank flow from up-channel bends; (4) inner-bend fields on the oceanward margin form by the interaction of axial flow through wave troughs, and a transverse, toward-channel flow component. This work has implications for interpreting overbank flow from seafloor and seismic data, and for palaeogeographic reconstructions from outcrop data.

A review of OBN processing: challenges and solutions

Abstract In the last decade, a significant shift in the marine seismic acquisition business has been made where ocean bottom nodes gained a substantial market share from streamer cable configurations. Ocean bottom node acquisition (OBN) can acquire wide azimuth seismic data over geographical areas with challenging deep and shallow bathymetries and complex subsurface regimes. When the water bottom is rugose and has significant elevation differences, OBN data processing faces a number of challenges, such as denoising of the vertical geophone, accurate wavefield separation, redatuming the sparse receiver nodes from ocean bottom to sea level and multiple attenuation. In this work, we review a number of challenges using real OBN data illustrations. We demonstrate corresponding solutions using processing workflows comprising denoising the vertical geophones by using all four recorded nodal components, cross-ghosting the data or using direct wave to design calibration filters for up- and down-going wavefield separation, performing one-dimensional reversible redatuming for stacking QC and multiple prediction, and designing cascaded model and data-driven multiple elimination applications. The optimum combination of the mentioned technologies produced cleaner and high-resolution migration images mitigating the risk of false interpretations.

FD-injection-based elastic wavefield separation for open and closed configurations

SUMMARYAn important step in the processing of seismic data that are recorded at the free surface is the isolation of the primary incident wavefield from the total recorded wavefield (which is contaminated with the immediate reflections off the free surface). We present a 3-D wavefield reconstruction technique, based on numerical wavefield injection along a closed boundary, that allows us to isolate this primary wavefield from measurements at the free surface. The technique consists of injecting only the three-component particle velocity recordings acquired at the free surface into a numerical wavefield simulation, and additionally requires information about the medium properties. The result of our proposed procedure is the separation of elastic waves into their first-order incident and reflected constituents, even when the recording or injection surface has sharp corners. With the use of synthetic data it is shown that the method achieves close to numerically exact wavefield separation, provided that the true elastic model in the interior is used. In practice, the parameters for a homogeneous elastic model can be determined efficiently from the surface data itself using an optimization scheme. Finally, the wavefield separation technique is successfully applied to experimental data, with particle velocity recordings acquired along five faces of a cubic granite rock volume. In addition to characterizing materials in laboratories, the presented technique has applications in numerical modelling and in so-called immersive experimentation, where the incident field is required to immerse an elastic object in an arbitrary larger, virtual elastic environment.

Vector elastic deconvolution migration with dual wavefield decomposition

Elastic-wave imaging using multicomponent data can provide more useful subsurface information than acoustic-wave imaging, but it is usually algorithmically challenging. We have developed a vector elastic deconvolution migration method for high-resolution imaging of subsurface structures in isotropic and anisotropic elastic media. Our new method uses a vector deconvolution imaging condition based on dual wavefield decomposition, including an explicit directional wavefield separation using the Hilbert transform and a P/S vector wavefield decomposition using the low-rank decomposition method. Using three elastic models, we numerically determine that our new method produces notably higher resolution and more amplitude-balanced elastic images compared with a crosscorrelation-based vector elastic reverse time migration method.

Imaging complex three‐dimensional structures, applying three‐component vector migration to synthetic vertical seismic profiling data

ABSTRACTBy applying a ray‐based three‐dimensional vector migration process to fully elastic three‐component synthetic Vertical Seismic Profiling data, we are able to demonstrate that the process gives an accurate compressional reflection image of the complex formation without much visible artefacts from spatial aliasing, or from ‘migration smiles’ caused by the limited aperture of the receiver array. We demonstrate that the process reconstructs images of both steeply dipping salt flanks, and the sediments that tend to be dragged along by the moving salt and truncated against the sides of the salt body. The process uses raw unseparated data as input, and the wavefield separation of compressional and shear waves are done by projection operators inside the kernel of the migration process. The compressional reflection image appear to have no visible artefacts from the abundance of converted shear.

A joint imaging method of vertical seismic profiling data by generalized radon transform migration

ABSTRACTThe images produced by conventional vertical seismic profile imaging methods often suffer from the problems of illumination limitation and uncompleted wavefield separation. In this paper, we give a deep insight into the illumination behaviours of the upgoing primary and downgoing multiple signals and illustrate their merits and demerits in illuminating the subsurface structure around the well trajectory. Based on this, we develop an efficient joint imaging method of acoustic vertical seismic profile data based on the generalized Radon transform migration inversion theory. By analysing the interference characteristics of the primary and multiple reflections, we verify the capability of this method that can handle the joint imaging directly. The proposed method combines the advantages of primary and multiple contributions to the subsurface image without additional wavefield separation. The numerical results of synthetic data and field data demonstrate high‐quality construction of the structure in the vicinity of the borehole, also away from the well trajectory.

Addressing the crosstalk issue in imaging using seismic multiple wavefields

Surface-related-multiple wavefields constitute redundant information in conventional migration and can often be difficult to attenuate. However, when used for migration, multiple wavefields can improve subsurface illumination. Unfortunately, the process of imaging using multiples involves the management of crosstalk, which largely restricts its application. Crosstalk causes phantom images formed by spurious correlation of unrelated events in a migration process. These events can be unrelated orders of multiples in the source and receiver wavefields; they can also be one event associated with a reflector in the source wavefield and another event generated by a different reflector in the receiver wavefield. We have first examined crosstalk by explicitly investigating its generation mechanisms in a migration process and classifying it into different categories based on causality. Following this analysis, crosstalk can be predicted in a migration process and subtracted in the image domain; however, this method is usually difficult to apply due to the complexity of wavefield separation and adaptive subtraction. Furthermore, we develop different algorithms to attenuate the crosstalk, including a deconvolution imaging condition, a least-squares migration (LSM) method, and an advanced algorithm combining LSM with a deconvolution imaging condition. We evaluate these different strategies with synthetic examples. A deconvolution imaging condition can attenuate some crosstalk, but it is less effective at suppressing strong coherent crosstalk events. However, the LSM method can fundamentally address the crosstalk issue, and this approach is further optimized when combined with a deconvolution imaging condition.

Closed-aperture unbounded acoustics experimentation using multidimensional deconvolution.

In physical acoustic laboratories, wave propagation experiments often suffer from unwanted reflections at the boundaries of the experimental setup. We propose using multidimensional deconvolution (MDD) to post-process recorded experimental data such that the scattering imprint related to the domain boundary is completely removed and only the Green's functions associated with a scattering object of interest are obtained. The application of the MDD method requires in/out wavefield separation of data recorded along a closed surface surrounding the object of interest, and we propose a decomposition method to separate such data for arbitrary curved surfaces. The MDD results consist of the Green's functions between any pair of points on the closed recording surface, fully sampling the scattered field. We apply the MDD algorithm to post-process laboratory data acquired in a two-dimensional acoustic waveguide to characterize the wavefield scattering related to a rigid steel block while removing the scattering imprint of the domain boundary. The experimental results are validated with synthetic simulations, corroborating that MDD is an effective and general method to obtain the experimentally desired Green's functions for arbitrary inhomogeneous scatterers.

Seismological Processing of Six Degree-of-Freedom Ground-Motion Data.

Recent progress in rotational sensor technology has made it possible to directly measure rotational ground-motion induced by seismic waves. When combined with conventional inertial seismometer recordings, the new sensors allow one to locally observe six degrees of freedom (6DOF) of ground-motion, composed of three orthogonal components of translational motion and three orthogonal components of rotational motion. The applications of such 6DOF measurements are manifold—ranging from wavefield characterization, separation, and reconstruction to the reduction of non-uniqueness in seismic inverse problems—and have the potential to revolutionize the way seismic data are acquired and processed. However, the seismological community has yet to embrace rotational ground-motion as a new observable. The aim of this paper is to give a high-level introduction into the field of 6DOF seismology using illustrative examples and to summarize recent progress made in this relatively young field. It is intended for readers with a general background in seismology. In order to illustrate the seismological value of rotational ground-motion data, we provide the first-ever 6DOF processing example of a teleseismic earthquake recorded on a multicomponent ring laser observatory and demonstrate how wave parameters (phase velocity, propagation direction, and ellipticity angle) and wave types of multiple phases can be automatically estimated using single-station 6DOF processing tools. Python codes to reproduce this processing example are provided in an accompanying Jupyter notebook.

Depth Migration Based on Two-Way Wave Equation to Image OBS Multiples: A Case Study in the South Shetland Margin (Antarctica)

With the development of marine seismic exploration, the ocean bottom seismometer (OBS) as a new seismic acquisition technology has been widely concerned. Although multiple waves are frequently viewed as noises, they may carry a wealth of subsurface information and produce a broader illumination than primary waves. To perform multiple wave imaging, we propose to utilize a two-way wave equation depth wavefield extrapolation method which is rarely used in this field. A simple dipping model is imaged by using primary and multiple waves, which proves the superiority of multiple waves in imaging over the primary waves and lays a foundation for practical application. Moreover, the comparison of multiple imaging results by reverse time migration and those by our proposed method demonstrates that our proposed method requires less storage space. In this study, we apply this migration method to actual OBS data collected in the South Shetland margin (Antarctica), where gas hydrates have been well documented. Firstly, the wavefield separation method is adopted to process the OBS data, so as to produce reliable primary and multiples waves; secondly, the ray-tracing method is used to derive the velocity field; and finally, the depth wavefield extrapolation method based on the two-way wave equation is applied to image primary and multiple waves. Migration results show that multiple waves provide a broader illumination and a clearer sediment structure than primary waves, especially for the highly shallow reflections.

Right-Hand Rule 3C VSP Wavefield Separation Method

Wavefield separation is one critical step in the three-component (3C) vertical seismic profiling (VSP) data processing. The conventional 3C VSP wavefield separation method exhibits low precision polarization angle and cannot obtain desirable results. This letter proposed the right-hand rule 3C VSP wavefield separation method which has many advantages over the conventional 3C VSP wavefield separation method—particularly the polarization angle precision and wavefields continuity. Analysis of real 3C VSP data demonstrates that the right-hand rule 3C VSP wavefield separation method can enhance polarization angle precision and show a very desirable ability to produce high-continuity wavefields.

Separation of fluid and solid shear wave fields and quantification of coupling density by magnetic resonance poroelastography

Biological soft tissues often have a porous architecture comprising fluid and solid compartments. Upon displacement through physiological or externally induced motion, the relative motion of these compartments depends on poroelastic parameters, such as coupling density ( ) and tissue porosity. This study introduces inversion recovery MR elastography (IR-MRE) (1) to quantify porosity defined as fluid volume over total volume, (2) to separate externally induced shear strain fields of fluid and solid compartments, and (3) to quantify coupling density assuming a biphasic behavior of in vivo brain tissue. Porosity was measured in eight tofu phantoms and gray matter (GM) and white matter (WM) of 21 healthy volunteers. Porosity of tofu was compared to values obtained by fluid draining and microscopy. Solid and fluid shear-strain amplitudes and were estimated both in phantoms and in in vivo brain. T1 -based measurement of tofu porosity agreed well with reference values (R = 0.99, P < .01). Brain tissue porosity was 0.14 ± 0.02 in GM and 0.05 ± 0.01 in WM (P < .001). Fluid shear strain was found to be phase-locked with solid shear strain but had lower amplitudes in both tofu phantoms and brain tissue (P < .05). In accordance with theory, tofu and brain were negative. IR-MRE allowed for the first time separation of shear strain fields of solid and fluid compartments for measuring coupling density according to the biphasic theory of poroelasticity. Thus, IR-MRE opens horizons for poroelastography-derived imaging markers that can be used in basic research and diagnostic applications.

Wave-mode separation in inhomogeneous anisotropic media based on adaptive sampling

Abstract The reverse time migration of an elastic wave is an advanced method for an imaging velocity model, and the reverse time migration of an elastic inhomogeneous TTI medium has become a research hotspot. The amount of calculation and storage is an important factor restricting its development. To solve the storage problem, we improve the wave field separation algorithm using adaptive sampling in the process of reverse time migration. This resampling algorithm can retain the core region and resample the marginal area of a quasi-differential operator. After sampling we only need to save the sampling point and the sampling location. It is proved by model calculation that the storage capacity of the algorithm can reduce a large part of the computational time of a traditional algorithm without reducing calculation accuracy. The new algorithm is more suitable for GPU because the storage is greatly reduced. In the process of reverse time migration, we can store the quasi-differential operator in the GPU so as to avoid a lot of I/O of GPU to CPU and improve calculation efficiency.

Data- and model-domain up/down wave separation for reverse-time migration with free-surface multiples

SUMMARYSeismic data recorded using a marine acquisition geometry contain both upgoing reflections from subsurface structures and downgoing ghost waves reflected back from the free surface. In addition to the ambiguity of propagation directions in the data, using the two-way wave equation for wavefield extrapolation of seismic imaging generates backscattered/turned waves when there are strong velocity contrasts/gradients in the model, which further increases the wavefield complexity. For reverse-time migration (RTM) of free-surface multiples, apart from unwanted crosstalk between inconsistent orders of reflections, image artefacts can also be formed along with the true reflector images from the overlapping of up/downgoing waves in the data and in the extrapolated wavefield. We present a wave-equation-based, hybrid (data- and model-domain) wave separation workflow, with vector seismic data containing pressure- and vertical-component particle velocity from dual-sensor seismic acquisition, for removing image artefacts produced by the mixture of up/downgoing waves. For imaging with free-surface multiples, the wavefield extrapolated from downgoing ghost events (reflected from the free surface) in the recorded data act as an effective source wavefield for one-order-higher free-surface multiples. Therefore, only the downgoing waves in the data should be used as the source wavefield for RTM with multiples; the recorded upgoing waves in the seismograms will be used for extrapolation of the time-reversed receiver wavefield. We use finite-difference (FD) injection for up/down separation in the data domain, to extrapolate the down- and upgoing waves of the common-source gathers for source and receiver wavefield propagation, respectively. The model-domain separation decomposes the extrapolated wavefield into upgoing (backscattered) and downgoing (transmitted) components at each subsurface grid location, to remove false images produced by cross-correlating backscattered waves along unphysical paths. We combine FD injection with the model-domain wavefield separation, for separating the wavefield into up- and downgoing components for the recorded data and for the extrapolated wavefields. Numerical examples using a simple model, and the Sigsbee 2B model, demonstrate that the hybrid up/down separation approach can effectively produce seismic images of free-surface multiples with better resolution and fewer artefacts.