Demultiple Methods Research Articles

Seismic de-multiple strategy in the submarine slope of Taiwan accretionary wedge

Reducing multiple contaminations in reflection seismic data remains one of the primary challenges in marine seismic data processing. Besides geological settings, its effectiveness is also dependent on the multiple removal methods. In this study, we undertook two legacy 2D multi-channel seismic data crossing the accretionary wedge off SW Taiwan to test the efficiency of various multiple-attenuation scenarios. The tectonic domain has resulted from the incipient arc-continent collision between the northern rifted margin of the South China Sea and the Luzon volcanic arc. The wedge extends from shallow water to deep water bathymetries, hence promoting both short-period and long-period multiples within the seismic records. A cascade of de-multiple methods was tested to attenuate multiple energy under various seafloor bathymetry and tectonic areas. The first step relies on the periodicity nature of multiples. Spatial dependent predictive deconvolution in the x-t domain was performed to attenuate reverberations and improve temporal resolution in the time domain. Wave-equation multiple attenuation (WEMA) was applied to suppress the water layer multiples based on a combination of numerical wave extrapolation in the shot domain through water layer and water bottom reflectivity. Surface-related multiple elimination (SRME) aimed to attenuate the residual water bottom multiple and peg-leg multiple by assuming surface-related multiples can be kinematically predicted via convolution of pre-stack seismic traces at possible surface multiple reflection locations. The second step exploits the spatial move-out difference behavior between primaries and multiples. Parabolic Radon transforms far-offset multiples by subtracting noise energy in the τ-p domain, whereas the frequency-wave number (F-K) filter aimed to eliminate any residual multiples energy in the F-K domain. Predictive deconvolution improved seismic resolution and suppressed sea-bottom reverberation energy in the continental and lower wedge slopes, but not in the upper wedge slope. WEMA, Radon filter, and F-K filter reduced multiples energy both at the continental slope and wedge slope; whereas SRME made minimal impact on both areas. Since the reflection seismic datasets stretch diverse tectonic environments and water depth, there was no single multiple attenuation method capable to suppress multiples in all tectonic environments and bathymetry.

Overburden-borne internal demultiple formula

Imaging targets buried under highly reflective overburdens might require removal of a substantial amount of internal multiples. However, existing data-driven internal demultiple methods are either not accurate, not very user friendly, or both, as they (1) (only) kinematically predict only some of the multiples, (2) allow for false predictions (multiples leakage), (3) require primary-harming adaptive filtering, (4) can be computationally expensive or require user intervention, (5) require other than reflection data nontrivial inputs, and/or (6) the output cannot be readily quality controlled. We have developed a unifying surface-related multiple estimation/elimination (SRME)-like macrolayer internal demultiple algorithm, which addresses these issues and accurately eliminates all overburden-borne internal multiples. We find that the leading order terms of our formula correspond to some well-known approaches. This strongly suggests that the main benefit of using the Marchenko-equation-based methods comes from higher order terms, which provide transmission amplitude correction and suppress unphysical events predicted by lower order terms. We illustrate the efficacy of our formula with a numerical example, use it to identify outstanding limitations, and suggest some practical solutions to them. We also show that this approach is sufficiently general to be relatively easily extended to include, e.g., viscoelastic effects.

A Fast Sparse Hyperbolic Radon Transform Based on Convolutional Neural Network and Its Demultiple Application

The hyperbolic Radon transform (RT) is a widely used demultiple method in the seismic data processing. But this transformation faces two major defects. The limited aperture of acquisition leads to the scissor-like diffusion in the Radon domain, which introduces separation difficulties between primaries and multiples. In addition, the computation of large matrices inversion involved in hyperbolic RT reduces the processing efficiency. In this letter, a specific Convolutional Neural Network (CNN) is designed to conduct a Fast Sparse Hyperbolic Radon Transform (FSHRT). Two techniques are incorporated into CNN to find the sparse solution. One is the coding-decoding structure, which captures the sparse feature of Radon parameters. The other is the soft threshold activation function followed by the end of neural networks, which suppresses the small parameters and further improves the sparsity. Thus, the network realizes the direct mapping between the adjoint solution and the sparse solution. Furthermore, synthetic and field demultiple experiments are carried out to demonstrate the rapidity and effectiveness of the proposed method.

Recent seismic reprocessing to revitalize research along the Atlantic Margin, offshore Ireland

In this study, we reveal what can be achieved from modern reprocessing of legacy regional 2D data using modern broadband and depth imaging techniques. We achieve a higher signal-to-noise ratio, improved event continuity and more reliable deeper Cretaceous and Jurassic images. Use of a single 3D velocity model-building method incorporating several 2D seismic lines simultaneously ensures line ties even for 2D regional data. The largest problems encountered were related to the 100 m to 4800 m range of water depths which required a variety of demultiple methods needed to ensure regionally consistent results in the presence of strong 3D effects. We illustrate regional quality control tools used and the removal of some spurious events associated with the original processing flow. We draw attention to some nuances involved in the interpretation of standard difference displays, show the improvements arising from the use of water column function velocities and quantify the additional value of Full Waveform Inversion (FWI) to improved imaging for the project. Improved imaging results will expand understanding of the basins’ geometry and sediment fill and should provide an excellent tool for future industry and academic research in the Irish offshore.

Robust estimation of primaries by sparse inversion and Marchenko equation-based workflow for multiple suppression in the case of a shallow water layer and a complex overburden: A 2D case study in the Arabian Gulf

Suppression of surface-related and internal multiples is an outstanding challenge in seismic data processing. The former is particularly difficult in shallow water, whereas the latter is problematic for targets buried under complex, highly scattering overburdens. We have developed a two-step, amplitude- and phase-preserving, inversion-based workflow that addresses these problems. We apply robust estimation of primaries by sparse inversion (R-EPSI) to solve simultaneously for the surface-related primaries Green’s function and the source wavelet. A significant advantage of the inversion approach of the R-EPSI method is that it does not rely on an adaptive subtraction step that typically limits other demultiple methods such as surface-related multiple elimination. The resulting Green’s function is used as the input to a Marchenko equation-based approach to predict the complex interference pattern of all overburden-generated internal multiples at once. In this approach, no a priori information about the subsurface is needed. In theory, the interbed multiples can be predicted with correct amplitude and phase and, again, no adaptive filters are required. We illustrate this workflow by applying it on an Arabian Gulf field data example. It is crucial that all preprocessing steps are performed in an amplitude-preserving way to restrict any impact on the accuracy of the multiple prediction. In practice, some minor inaccuracies in the processing flow may end up as prediction errors for which corrections will be needed. Hence, we conclude that the use of conservative adaptive filters were necessary to obtain the best results after interbed multiple removal. The obtained results indicate promising suppression of surface-related and interbed multiples.

Sub-Basalt Imaging Reveals Deeper Plays Offshore India

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 30279, “Revealing Deeper Plays, Offshore Kutch, India: A Success Story of Sub-Basalt Imaging,” by S.K. Biswal, N.N.B. Naidu, and S. Basu, ONGC, et al., prepared for the 2020 Offshore Technology Conference Asia, originally scheduled to be held in Kuala Lumpur, 2-6 November. The paper has not been peer reviewed. Copyright 2020 Offshore Technology Conference. Reproduced by permission. The Deccan Traps volcanic province of India is considered one of the largest basalt-covered regions in the world but is essentially unexplored because of the limitations of conventional marine streamer P-wave seismic acquisition in imaging structures both intrabasalt and sub-basalt. In the complete paper, the authors demonstrate that, even with legacy marine streamer surveys, an appropriate work flow of combining suitable advanced technologies can help to overcome the long-standing challenges of sub-basalt imaging. The reprocessed data show clear uplift in sub-basalt imaging, and inversion results validate the quality of the new data in relation to the well logs. Introduction The Kutch offshore basin is characterized by the presence of the Deccan Traps, a large igneous province of up to 2000-m-thick basalt lava flows. These lava flows have hindered successful imaging of sub-basalt Mesozoic sediments for hydrocarbon exploration. To date, no single technique has been found to produce considerable improvements in deeper image quality. The solution lies in an appropriate combination of advanced technologies. The project consists of three legacy data sets acquired in 2004, 2010, and 2014 in the shallow-water area (water depth ranges from 25 to 50 m). Two of the surveys were shot in the north/south direction with six streamers having 100-m separation, 25-m shot spacing, 12.5-m receiver spacing, and 6-second record length. The third survey was acquired oblique to these with a similar acquisition geometry; however, it featured sparser 25-m receiver spacing and 8- second record length. These surveys were matched and merged before migration to ensure a seamless image across the surveys in the post-migration domain. A tailored processing work flow improved existing data quality significantly and provided new insights into the sub-basalt geology, thereby opening a new play to exploration and production. Challenges and Work Flow Sub-basalt imaging challenges include transmission losses, scattering, complex wave kinematics, prevalent multiples, interference effects, and variable illumination caused by high and variable acoustic impedance of thick heterogeneous basalt layers. The tertiary sedimentary sequences overlying the Deccan Trap consist predominantly of carbonates, shale, and fine-grained clastic sediments, accompanied by channels and nearly vertical faulting. The geological complexities from the water bottom to the base of the basalt present a substantial geophysical challenge to successful deeper imaging and require an appropriate work flow to mitigate them. Broadband processing, including de-ghosting, can increase the signal-to-noise ratio across the broad range of frequencies in the seismic bandwidth and can enhance the lower frequencies required to achieve enhanced imaging at sub-basalt targets. Demultiple methods can reduce the presence of surface-related and interbed multiples that prohibit reliable interpretation of Mesozoic sediment; imaging methods can focus the recorded data when used in conjunction with an accurate Earth model that captures the velocity complexities of carbonates, shale, basalt, channel, and faults.

Comparison of monotonicity challenges encountered by the inverse scattering series and the Marchenko demultiple method for elastic wavesElastodynamic Marchenko conditions

Comparison of monotonicity challenges encountered by the inverse scattering series and the Marchenko demultiple method for elastic wavesElastodynamic Marchenko conditions

Comparison of monotonicity challenges encountered by the inverse scattering series and the Marchenko demultiple method for elastic waves

The reflection response of strongly scattering media often contains complicated interferences between primaries and (internal) multiples, which can lead to imaging artifacts unless handled correctly. Internal multiples can be kinematically predicted, for example by the Jakubowicz method or by the inverse scattering series (ISS), as long as monotonicity, that is, “correct” temporal event ordering, is obeyed. Alternatively, the (conventional) Marchenko method removes all overburden-related wavefield interactions by formulating an inverse problem that can be solved if the Green’s and the so-called focusing functions are separable in the time domain, except for an overlap that must be predicted. For acoustic waves, the assumptions of the aforementioned methods are often satisfied within the recording regimes used for seismic imaging. However, elastic media support wave propagation via coupled modes that travel with distinct velocities. Compared to the acoustic case, not only does the multiple issue become significantly more severe, but also violation of monotonicity becomes much more likely. By quantifying the assumptions of the conventional Marchenko method and the ISS, unexpected similarities as well as differences between the requirements of the two methods come to light. Our analysis demonstrates that the conventional Marchenko method relies on a weaker form of monotonicity. However, this advantage must be compensated by providing more prior information, which in the elastic case is an outstanding challenge. Rewriting, or remixing, the conventional Marchenko scheme removes the need for prior information but leads to a stricter monotonicity condition, which is now almost as strict as for the ISS. Finally, we introduce two strategies on how the remixed Marchenko solutions can be used for imperfect, but achievable, demultiple purposes.

Limited off-set consideration in de-multiple process of seismic Lake Towuti

Multiple attenuation effect study in Lake Towuti seismic data is based on limited offset recording. The goal of this study is to better understand the sediments structure of Lake Towuti that layered above East Sulawesi ophiolitic and metamorphic rocks for stratigraphic interpretation. Surface-related multiple and internal multiple are a common noise signal that recorded in this seismic data, alongside the primary signal from the sediments layer. In order to get a representative subsurface image of the lake, a combination of some de-multiple method to suppress this noise signal has been applied in previous study. However, the line seismic that used in this seismic recording is less than 100 m. From our perspective, this limited offset line fact have to be considered carefully. It’s because some application of de-multiple method in this case will not effectively suppress the noise. In this study, we give an alternative combination method in function to put out the surface-related multiple of the case. First, we enhance the S/N ratio and velocity analysis of the data by Common Reflection Surface (CRS) method. From this enhancement, CRS can give a direct advantage when applying some de-multiple method, especially in Predictive Deconvolution and SRME method. Moreover, we combine the application of de-multiple theory with the strict arrangement of Predictive Deconvolution, Surface-Related Multiple Elimination (SRME), and the last is F-K filter. Those combination methods precisely give a better imaging of Lake Towuti sediments layer in this seismic processing study.

Accelerated parabolic Radon domain 2D adaptive multiple subtraction with fast iterative shrinkage thresholding algorithm and its application in parabolic Radon domain hybrid demultiple method

Adaptive multiple subtraction is an important step for successfully conducting surface-related multiple elimination in marine seismic exploration. 2D adaptive multiple subtraction conducted in the parabolic Radon domain has been proposed to better separate primaries and multiples than 2D adaptive multiple subtraction conducted in the time-offset domain. Additionally, the parabolic Radon domain hybrid demultiple method combining parabolic Radon filtering and parabolic Radon domain 2D adaptive multiple subtraction can better remove multiples than the cascaded demultiple method using time-offset domain 2D adaptive multiple subtraction and the parabolic Radon transform method sequentially. To solve the matching filter in the optimization problem with L1 norm minimization constraint of primaries, traditional parabolic Radon domain 2D adaptive multiple subtraction uses the iterative reweighted least squares (IRLS) algorithm, which is computationally expensive for solving a weighted LS inversion in each iteration. In this paper we introduce the fast iterative shrinkage thresholding algorithm (FISTA) as a faster alternative to the IRLS algorithm for parabolic Radon domain 2D adaptive multiple subtraction. FISTA uses the shrinkage-thresholding operator to promote the sparsity of estimated primaries and solves the 2D matching filter with iterative steps. FISTA based parabolic Radon domain 2D adaptive multiple subtraction reduces the computation time effectively while achieving similar accuracy compared with IRLS based parabolic Radon domain 2D adaptive multiple subtraction. Additionally, the provided examples show that FISTA based parabolic Radon domain 2D adaptive multiple subtraction can better separate primaries and multiples than FISTA based time-offset domain 2D adaptive multiple subtraction. Furthermore, we introduce FISTA based parabolic Radon domain 2D adaptive multiple subtraction into the parabolic Radon domain hybrid demultiple method to improve its computation efficiency while maintaining its accuracy. Synthetic and field data examples demonstrate the effectiveness of the proposed FISTA based parabolic Radon domain 2D adaptive multiple subtraction and its application in the parabolic Radon domain hybrid demultiple method.

Elastic internal multiple analysis and attenuation using Marchenko and interferometric methods

Conventional seismic processing aims to create data that contain only primary reflections, whereas real seismic recordings also contain multiples. As such, it is desirable to predict, identify, and attenuate multiples in seismic data. This task is more difficult in elastic (solid) media because mode conversions create families of internal multiples not present in the acoustic case. We have developed a method to predict prestack internal multiples in general elastic media based on the Marchenko method and convolutional interferometry. It can be used to identify multiples directly in prestack data or migrated sections, as well as to attenuate internal multiples by adaptively subtracting them from the original data set. We developed the method on two synthetic data sets, the first composed of horizontal density layers and constant velocities, and the second containing horizontal and vertical density and velocity variations. The full-elastic method is computationally expensive and ideally uses data components that are not usually recorded. We therefore tested an acoustic approximation to the method on the synthetic elastic data from the second model and find that although the spatial resolution of the resulting image is reduced by this approximation, it provides images with relatively fewer artifacts. We conclude that in most cases where cost is a factor and we are willing to sacrifice some resolution, it may be sufficient to apply the acoustic version of this demultiple method.

Evolution of Marine Acquisition Technology after Wide Azimuth

Seismic exploration in the deep-water Gulf of Mexico was based for many years on the 3D acquisition method and, as a result, significant oil discoveries were made and most of the plays were found below salt or in intra-salt-body basins. The quality of the seismic data acquired in deep-water subsalt environments was occasionally satisfactory for exploration purposes, but, in most cases, it was not good enough to support an accurate earth model for reservoir development. The main challenges for data interpretation are: incomplete reservoir illumination, poor signal-to-noise ratio of the subsalt events and poor seismic resolution. Developments in the last decade in marine seismic acquisition and data processing were driven to solve these challenges. One reference point in the evolution of marine seismic technology in the last decade was the introduction of wide azimuth acquisition (WAZ). Introduced in 2006 by British Petroleum in the Gulf of Mexico, the method was quickly adopted by the industry as a seismic technology to explore the complex subsalt geologic structures where improved subsurface illumination and signal-to-noise ratio are required. The introduction of WAZ started a period of several innovations in the seismic industry: dual-sensor streamer acquisition, full-azimuth towed streamer acquisition, broadband seismic measurements on both the source and receiver sides, long-offset marine acquisition, simultaneous shooting, and multimeasurement streamers. Challenges in processing wide-azimuth data lead to new developments in velocity model building based on tomography and full-waveform inversion, 3D demultiple methods, 3D anisotropic imaging with reverse time migration, and other improvements in computational methods. The presentation will review the latest innovations in marine seismic acquisitions with examples of applications, and will discuss the geophysical benefits and limitations, as well as specific survey design and processing aspects related to each method.

Complex structural imaging of transition zones in Bohai Bay, China, by OBC technology

A series of ocean-bottom cable (OBC) surveys has been conducted in the Bohai Sea in China in recent years to overcome difficulties experienced with streamer surveys in shallow water, such as strong currents, missing near offsets, and obstacles. The main challenges in OBC data imaging include steeply dipping structures, serious multiples in the shallow-water environment, large lateral velocity variations, fault shadow effects, and low signal-to-noise ratio (S/N). To obtain optimal images, advanced processing technologies have been developed and applied to OBC data which involve effective PZ summation and shallow-water demultiple, a high-fidelity beam migration in the wide-azimuth domain, and accurate velocity-model building in 3D tilted-transverse-isotropy (TTI) media. The PZ summation and shallow-water demultiple methods aim to effectively eliminate shallow-water ghosts to achieve broadband seismic data. Furthermore, high-fidelity controlled-beam migration (CBM) and TTI velocity-model updates greatly enhance steep dip imaging, improve S/N, and reduce turnaround time. Through the combination of these technologies, OBC data processing provides high-quality images with well-defined steeply dipping structures to reduce exploration risk in the Bohai area.

Automatic velocity analysis with reverse-time migration

We apply a method to automatically estimate the background velocities using reverse-time migration. The method uses a combination of differential semblance and similarity-index (a.k.a., “semblance” or “stacking-power”) to measure the focusing error in imaging and a nonlinear optimization procedure to obtain the background velocities. A challenge in this procedure is that, for media consisting of complex and strongly refracting velocities, artifacts in the reverse-time migrated image (low-frequency noise) can cause the velocity analysis to diverge. We successfully overcome this issue by applying a simple vertical derivative filter to the image that is input to velocity analysis. The resultant velocity analysis method is tested in two 2D synthetic examples and one 2D field data example. Due to the assumptions inherent to prestack depth migration, the data that are input to velocity analysis must be singly scattered. To apply the method to multiple-rich data, we propose an image-based demultiple method. The method consists of muting events in the subsurface offset common image point gathers constructed with reverse-time migration, and remodeling the data using a kinematic demigration. A field data example shows how the image-based demultiple of the data helps to improve the velocity analysis in the presence of multiple scattering.

Application of Inverse Scattering Series Method for Internal Multiple Attenuation – a case study

Internal multiples due to a series of subsurface impedance contrasts are commonly observed in seismic data acquired in various places such as the Gippsland Basin of Australia and the Santos Basin of Brazil. The origin of these impedance contrasts can be due to coal seams as in the case of the Gippsland Basin or geological unconformities as in the Santos Basin. Regardless of how they are generated, internal multiple reflections often pose problems to the interpretation of geological structures. They are not easily handled by conventional de-multiple methods such as Radon Transform because internal multiples often have little difference in move out than their corresponding primary events, or predictive deconvolution because the multiple generators, and hence the multiple period, are often not known. In this paper, we present a case study of applying inverse scattering series (ISS) based method for attenuating internal multiples. The ISS method is a data-driven approach that can predict all internal multiples of a given order without any priori knowledge of the multiple generators. Moreover, it does not require any information about the subsurface velocity field. We discuss the application of this method for handling internal multiples in Santos Basin data, and present results on both the synthetic and field data from the Tupi oilfield. The results show that the internal multiples are well predicted and, with a suitably constrained subtraction technique, the migration artifacts caused by these multiples are greatly suppressed resulting in much clearer migration image for interpretation.

The free surface assumption for marine data-driven demultiple methods*

In the past, integral formulations for marine data-driven demultiple methods have been derived from reciprocity theorems. Two fundamental assumptions in these derivations were that the sea-surface is flat and has a known reflection coefficient, often taken to be minus one. In this paper, we show that for dual sensor data these assumptions can be relaxed. The sea-surface has to obey the same conditions as any other reflecting boundary in the subsurface: it must be constant in time but shape and reflection strength can vary in space. For both surface-related multiple elimination, and multiple attenuation by multi-dimensional deconvolution, we derive integral equations that depend only on the measured pressure and particle velocity fields. Finally, we show there is an intimate connection between the integral equations for the methods.

Deep water prestack multiple suppression on reflection data from ionian sea (offshore western greece)

Multiple attenuation comprises a very important phase of seismic reflection processing because it removes undesir- able effects occurring during propagation of seismic signal in the Earth. The present work aims to present the effec- tiveness of modern multiple suppression methods applied on streamer seismic reflection data. The seismic dataset represents an area with moderately dipping seafloor. The specific dataset was chosen to present the elimination of the water-column reverberations and internal multiples in dipping layers. Strong seafloor and internal multiples, lat- eral reflections and coherent noise impose difficulties in discriminating primary reflections in the seismic image of the present work. The prestack demultiple scheme combines wave equation method and deconvolution to predict and attenuate water-column reverberations and internal multiple reflections respectively. In particular, the region of mod- erately dipping seafloor chosen for the application of the proposed scheme images the Frontal Bulge in the outer Hel- lenic arc. The prestack demultiple method improved the quality of the stacked and migrated sections by attenuating the majority of multiple reflections and extracting information on the structural units of the study area.

Subsonic near-surface P-velocity and low S-velocity observations using propagator inversion

Detailed knowledge of near-surface P- and S-wave velocities is important for processing and interpreting multicomponent land seismic data because (1) the entire wavefield passes through and is influenced by the near-surface soil conditions, (2) both source repeatability and receiver coupling also depend on these conditions, and (3) near-surface P- and S-wave velocities are required for wavefield decomposition and demultiple methods. However, it is often difficult to measure these velocities with conventional techniques because sensitivity to shallow-wave velocities is low and because of the presence of sharp velocity contrasts or gradients close to the earth's free surface. We demonstrate that these near-surface P- and S-wave velocities can be obtained using a propagator inversion. This approach requires data recorded by at least one multicomponent geophone at the surface and an additional multicomponent geophone at depth. The propagator between them then contains all information on the medium parameters governing wave propagation between the geophones at the surface and at depth. Hence, inverting the propagator gives local estimates for these parameters. This technique has been applied to data acquired in Zeist, the Netherlands. The near-surface sediments at this site are unconsolidated sands with a thin vegetation soil on top, and the sediments considered are located above the groundwater table. A buried geophone was positioned 1.05 m beneath receivers on the surface. Propagator inversion yielded low near-surface velocities, namely, 270 ± 15 m/s for the compressional-wave velocity, which is well below the sound velocity in air, and 150 ± 9 m/s for the shear velocity. Existing methods designed for imaging deeper structures cannot resolve these shallow material properties. Furthermore, velocities usually increase rapidly with depth close to the earth's surface because of increasing confining pressure. We suspect that for this reason, subsonic near-surface P-wave velocities are not commonly observed.

Attenuation of multiple diffractions using a cascaded noise removal sequence

John Brittan and Andy Wrench, PGS Geophysical, UK, describe a seismic processing sequence to suppress diffracted multiple energy. Multiple diffractions are a significant noise problem on seismic data from many of the world’s most important hydrocarbon-rich provinces. These noises are most prevalent in the case of deep water and a complex near-surface reflection sequence. In such regimes the multiples of diffracted energy by the near surface geology are often coincident in time with primary reflections within the subsurface. The energy composing the primary reflections will have a considerably reduced amplitude and frequency content due to the long travel-paths through the Earth. By contrast, the multiples from the near-surface reflectors have travelled most of their propagation path through the very weakly absorbing sea-layer. Thus the deeper parts of the seismic section are dominated, particularly at high frequencies, by the incoherent, high amplitude diffracted multiple arrivals. Due to their aliased, non-hyperbolic nature these arrivals are difficult to suppress using standard demultiple methods, e.g. 2D SRME (Surface Related Multiple Elimination), parabolic Radon demultiple. It has been shown that the use of high-fold, multiazimuth data acquisition can considerably increase the effectiveness of CMP stacking in removing diffracted multiple energy (Widmaier et al., 2002). However, for standard marine acquisition, a rigorous noise attenuation methodology must be adopted. Indeed, while techniques such as 3D SRME hold great promise for multiple diffraction attenuatio (van Borselen et al., 2004), we discuss in this paper how for typical field data a cascaded sequence of (up to) four different processes can be used to attenuate the problem events (Figure 1).

Removal of water‐layer multiples from multicomponent sea‐bottom data

A method for suppressing water‐layer multiples in multicomponent sea‐floor measurements is presented. The multiple suppression technique utilizes the concept of wavefield separation into upgoing and downgoing modes just below the sea floor for eliminating the sea‐floor ghost, the sea‐surface ghost, and the accompanying water‐layer reverberations. The theory applies to each of the recorded components: pressure, vertical velocity, and horizontal velocities. The fundamental physical principle for the multiple suppression technique rests on identifying these multiples as downgoing waves just below the sea floor, while the primaries of interest arriving from the subsurface are upgoing waves. White presented this realization for the pressure component three decades ago; hence, the theory for the velocity field is an extension of the theory. In this paper, the theory is derived for an experiment with a marine source in the water layer above a locally flat, elastic sea floor with known elastic parameters. The method is otherwise multidimensional and operates on a shot‐to‐shot basis; hence, it is computationally fast. Aside from this, we show that this demultiple method removes the strongest multiples in sea‐floor data without knowledge of the source wavelet. Synthetic and real data examples are provided to illustrate the application of the algorithms to the pressure, in‐line velocity, and vertical velocity components. The numerical tests show that strong multiples have been attenuated on the pressure and the velocity recordings, producing promising results.